Synchronous Generator Basics, Simple Guide to rewire your HEAD.

Keep it simple

Zach Says.. let’s keep this page simple!

Notice: for legal reasons, all that follows is for discussion sake only, drawings may contain errors,…. mine or someone elses. I am not licensed to instruct you, and only a licensed electrician, PE,  your city, country, state, or federal authorities count for a thing! If you find any of the drawings or comments in this article contain errors, I’ll be grateful if you scroll down and make a comment on what and where, or any other concern you might have. The goal of the article is to help make people more self-sufficient, the cost of taking a generator to a shop to have simple repairs made can be expensive, and if it’s a 200 mile trip to town, it can be expensive just to get it to repair and back.

Attempting to write an article that answers every question for every reader is not something I’d care to take on. I write for the DIYer, and many have basic electrical skills. I think this article is in demand, as there seems to be a lot of confusion about basic generator head wiring.  I haven’t found the topic very well covered elsewhere, and my article on ‘Zach’s Generator and Transfer Switch install has generated questions as to how you wire the generator.  Zach’s house has a lot of natural gas appliances, it allows us to run in 120VAC only mode, and this can have some real advantages. BUT.. If your Generator is 120/240VAC, how do you do that? and why? We’ll cover that here.

Perhaps the place to start is explaining that a lot of generators are very much alike, Synchronous generators make up around 99% of generators ever sold, and so many of the big box store generators are near identical electrically .  I use the example of the Coleman generator for several reasons, one.. I bought one about 20 more years ago, and it still runs well. It’s 1850 watts, green in color, and has been used for small projects, hunting, camping and more. A reader has a larger one and wants to hook it up to a transfer switch, he sent the drawing of the Coleman so we’ll use it!

Why 120VAC only? We’ll get into this more later, but simply stated, a 120/240VAC generator typically can only deliver one half of the generator’s rated capacity into a single load. Let’s say you have a 3000 watt generator, that is capable of supplying 120 and 240 volts. If you set it up in your drive way,  plugged in a heavy extension cord and drug the far end into the living room, you might assume you’re ready to go.. you grab a power strip, and plug it in, and then power up the TV, cable box, maybe even one of those tiny little ceramic heaters because your kids toes are cold. With this small load you may have already overloaded your generator!  If you had a Kill-A-Watt like I suggest people get and use, you might see the voltage is well south of 110 volts already! Why? because your loads could be running on one half of the generator’s stator windings, and already overloaded. Fact is, some generators don’t quite carry their advertised capacity, so you need know what it will carry, and monitoring the voltage is one way we can do that on the cheap.  With the typical hardware store generator,  we expect excessive voltage droop when we overload the generator.  The Kill-A-Watt will also remotely monitor frequency, and if the generator is well loaded, it may fall well below frequency as well, this is all so easy to monitor with the Kill-A-Watt, and they’re cheap as dirt.

So let’s look at a drawing of a Synchronous Generator and look at the typical parts.

Coleman Gen-main-Parts

Coleman Gen-main-Parts

We need focus on these two stator windings circled in Red, these two windings are like independent power sources, they can work together in parallel, work together in series to provide a higher voltage than either can provide by itself, OR they can work autonomously kind of like two flashlight batteries.

Before we discuss the stator further, notice the field winding, in most cases the field is the rotating part in the generator, and it’s attached to the shaft. In this generator, there’s power provided by some exciter winding(s) inside the stator housing, this is a separate winding and you can see that it goes to a box marked BRB here. Normally, this is nothing more than a full wave bridge rectifier. In almost all of these big box store gen sets, the radio shack 30 amp full wave bridge rectifier is more than adequate as a replacement if yours goes bad. The part has an AC input, and plus and minus for DC out to the brushes and onto the field, IF you replace a bad rectifier and accidentally mix up the plus and minus output, the generator will not put out voltage unless you (flash) the field with a battery, so take time to note polarity and match it!

Most problems with big box store generators are found in the brushes, or there’s a bad rectifier, both are easy to check.

Back to our story about 120 VAC only

Flashlight Analogy

Flashlight Analogy

Look at the flashlight batteries, pretend they’re rechargeable NMHs, that means they are 1.25 volts each..    In this case the batteries represent the stator windings, and if we go across the batteries in series we’d measure from terminal (1) to terminal (7). We’d read 2.50 volts. Notice that there is no way of getting both batteries to work together to support a larger single 1.25 volt load, we’d need to rewire to do that, and this is very much like the stock 120/240 VAC generator.

Coleman was nice to bring out both ends of their Stator winding to a terminal block, this means we CAN rewire the generator for 120VAC only, and use the output on a heavy extension cord and power a number of loads AND share the load across both Stator windings. In this configuration, what ever the load is, it will be shared equally by both stator windings. This is normally a more ideal setup if you are going to power a transfer switch and have the luxury of powering 120 volt loads only (no need to power a 240VAC load like a well pump.

But.. with a little effort, we can add a DPDT (double pole, double throw) switch, and set the generator up for 120/240 operation, AND 120 volt only, you will find some generators have this feature, Honda construction generators of 5KW and less have been supplied with this feature (a switch) to mention just one.

Here’s a method to accomplish a switch between 120/240 and 120 only, remember, the switch should only be thrown when the generator is off.

Using a DPDT to selector 120VAC only or 120/240 operation

Looking at the above drawing, you’ll see six switch contacts, using a DPDT switch.

You might understand the drawing better in the following drawing,  when  (|) contacts are closed the (X) is open, and when the switch is thrown to close the (X) contact, the (I) contacts are open.













Added Note: 4/16/12  As a friend tells me, 80% of your work remains AFTER your write an article, you need catch all those mistakes and typos. Read comments, and see if this makes the selector switch modification more clear?

120-240VAC -or- 120VAC only Selector Switch 2

120-240VAC -or- 120VAC only Selector Switch 2



Now, there’s a bunch of assumptions I make with this example, you’ll know to provide your grounds, you’ll understand that ground and neutral are not the same thing.  You’ll  consult your areas electrical codes as a final step.

If you are checking grounds with a clamp on amp meter, and you find a ground carrying current, you likely have a wiring problem. you want grounds free to carry fault current to ground and hopefully away from you, using a ground as a neutral means it’s working already, and might be too tired or too busy to handle an emergency request. I once saw a rather ingenious fix in a barn, the owner lifted a ground at both ends, and used it as a switched neutral to turn on an off a light, he was pretty pleased with the work around, but likely didn’t fully appreciate that he had violated

One more mention… persons new to AC power often assume that the two 120VAC hots are similar to having two of three phases of three phase power, and sometimes they’ll even (wrongly) call it two phase  power or 240VAC.  Don’t get this bad picture in your head!!! Both 120VAC and 240VAC are single phase power. Here’s an old article I wrote about single phase .

A hint about modifications, many troubles are created when poor connections are made, consumer generators have various connections, some use quality spade connectors,and if you use same, a dab of silicone cement or similar can assure the spade connection doesn’t vibrate off. Any connection that can be soldered is one that won’t cause you trouble later, but you need to know how to make a clean tight mechanical connection at the joint before you solder, and you need to know that rosin core is the right stuff, never use an acid core solder. As for wiring in a DPDT switch as I have shown above, I personally would never use a screw terminal switch, I’d only use a switch with solder connections, and I’d use the proper wattage of soldering iron or gun to make those connections.

Commercial and Military generators often have a ton of screw terminal connections, but the panels where these connections are made are normally we isolated from vibration and they correctly use higher grades of stranded wire (many fine strands) that are far better at holding up against months or years of vibration.

Do not confuse the stranded wire you find the commercial electricians using for buildings with the stuff you need use in a portable generator like those found in a big box store. To get an idea of the difference, look at how fine the wire strands are in an extension cord, then look at the six and eight strand stuff at home depot at 14 and 12 gauge.. big difference!

The wire you use should be rated for temperature, and the insulation should be adequate as well. some times you’ll see added insulation added, like a secondary sheath in an area where vibration could occur, you’ll also see nylon ties used to keep wires clear of rotating parts, or aways from parts that get hot. A lot of this is plain common sense to an older DIYer, but in the new modern world or throw away everything, younger folks have had less opportunity to fix or repair things and gain basic practical experience.

If your read this far, you are likely rare indeed! and for you I have a question:

Q: If you have your two stator generator wired for 120VAC only, and one stator coil opened up, or a connection between one coil and the other opened, what would the symptoms be as you attempted to power loads? Select the best Answer:

A:  The voltage would be off

B:  The frequency would be only 30hz

C:  You’d only see half the sine wave, your loads wouldn’t run.

D:  All would look normal until you exceeded the load a single stator winding can carry.

Answer= D.  When you reach only half the rated capacity (Amps), the voltage will droop rapidally, and you are likely to see less than 100 volts if you keep adding load. this is a bad situation, and it’s a typical failure that shows up right in the middle of an outage! It is why you use the correct wire, and make the best connections possible, it’s why you buy the proper rated quality switch and pass on the cheaperjunk, you want this to work when you need it!

Here’s a picture from Mike on his Coleman, note this plug contains the two stator windings and a ground, it’s always smart to run grounds between your gen head, frame, electrical boxes, all the stuff you might touch.

Coleman plug from generator head

Coleman plug from generator head







There’s information about specific RV wiring here, it was a suprise to me that they have 30 and 50 amp 120 vac only RV hook up services!

I HOPE THIS GIVES YOU A PLACE TO START…scroll all the way down for comments..


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92 Responses to Synchronous Generator Basics, Simple Guide to rewire your HEAD.

  1. Ralph G. says:

    good primer, and well written, as usual.
    Keep up the good work.
    Ralph G.

  2. Harold says:

    George, you threw me for a loop this time.

    With the switch set so that the center terminals are connected to the bottom ones the generator is indeed in the 220V configuration.

    With the switch set so the center terminals are connected to the top ones, stator terminals 1 and 3 are connected. So far, so good but stator terminal 2 is disconnected and stator winding A is removed from the circuit.

    What am I missing here?

    • George B. says:

      Harold, perhaps I need to add a drawing of the current flow in either mode. I won’t assume I understand your concern, but when you’re in 120 VAC only mode, we route all stator power in parallel to two connections, one we wrongly call neutral, the other is our hot wire. As discussed in another article, we best route this 120 VAC only to a connector or plug rated to carry the current we’ll draw and make sure our conductors are rated for same as well. Go back up to the article and see the two drawings I added.. if this doesn’t clear up your concern, please let me know, and thanks for your interest in helping me get this right, and understandable 🙂

  3. Harold says:

    O.K. I’m awake now. We need one more wire from the top right terminal of the switch to terminal 4 on Stator Winding B.

    • Art M. says:

      I believe Harold is right with his comment. I agree you must bridge number 2 on winding A to number 4 on winding B to have the 2 windings in parallel. (Refering to the DPDT switch schematic.) I am hopeful someone can confirm this based on their success. Great article and well written.

  4. Michael L says:

    OK since I have the same gen I am rewiring today. I just got the kill a watt and tested the gen as is now.

    Both windings read about 130- 128 volts. Hz is about 63-62.8

    Testing on the 240v hots I get 260 volts.

    I am opting on wiring all outlets 120 only. I have no use for 240.

    Also I am making all connections after the breakers and will use the 4 prong as my 120 only and not use one hot prong. This is to save money. No need for a switch.

    Harold the switch makes it from series center tapped 1—2/3—-4 to parallel. 1/3 – 2/4.

    OK here it goes…I will comment later.

    Your friend in NJ,

    Mike L

  5. Michael L says:

    OK job complete. Tested power from all outlets. All good. 120v everywhere.

    Am going to tackle the transfer switch now.

    The only thing I am not sure is I get 50-60 volts from hot to ground and 50-60 volts nuetral to ground.?? Going to test my homes outlets to see if its normal.

    Mike L. NJ

  6. Michael L says:

    Wish I could post pics? I took a few.

    Anyway on to the transfer switch.

    I have tied the two hots of the transfer switch togther with the hot side of the wiring coming from the gen inlet.

    The nuetral went to the neutral.

    I have not tried it yet hooked to the home. I am checking info on that 50v to ground issue, that I read was normal, and want to make sure that correct.

    I am assuming once I plug the gen into the inlet that 50 volt may go away since I am bonded at the panal.

    This is all assumption in my head. I got this far and tested a skill saw on the gen itself. ( Works good ) So I did not blow up the gen.. Yeah!!!!!

    You see the thing now is I dont want to get electricuted. Its my fear.

    I also wonder if when I use the gen for standalone, or such as a travel trailer, I should of added a switch where I can bond and unbond the neutral.

    Ok. More to come.

    Mike NJ

  7. Michael L says:

    Test the gen today with one of those plug in type testers. I have two of them. They both showed open ground. I would assume this is normal because the ground is just screwed to the frame.

    I am still working on the transfer switch. I need a larger wirenut.

    I will update my findings.

    Mike NJ

  8. Michael L says:

    OK 100% complete. Tested all outlets at gen and home on gen power. Tested with plug in polarity tester to insure hot,neutral, and grounds were correct. And they are. Ran some test loads and I can run all 6 loads at one time. Running off both windings feeding 120v to both sides of the panal.

    Just remember there are alot of variables you have to consider. The equiptment I bought was all wired and purchased for 30 amps already, and thats all my gen can produce. If you do this with a bigger gen you may need to upgrage some stuff.

    Among the electricians there is big debate on grounding issues. I followed code in that the gen is not bonded but bonded at the panal when connected to the home wiring. Here is a good link that may help people with this issue, but of corse follow all state and local codes at all times.

    I feel you can size a gen more properly running 120v if you have no 240v loads. This gives me the smallest possible gen with increased fuel savings. Also my well pump barley ran off one winding. Why buy a bigger gen. This one is plenty. Remember its emergency power. Your not living at the Hilton.

    I did cross the hot and neutral on the gen duplex outlets only. The L14 was wired correct in regards to how it connects to the main panel hot/neutral. This was because I did not follow through on my wiring and thinking in terms of putting every aspect on paper.

    I suggest whoever attempts this mod to take thier time and draw out all schematics on how your going to wire it. It will save alot of trouble if you cross something.

    I will be putting together something alittle more detailed in regards to what and why I did this, and possibly you may see it posted if George allows.

    Next may be Natural gas conversion.

    Here are some pics of my rewire




    Mike L. NJ

    • George B. says:

      Yes, there is quite a bit of discussion about the necessity of a grounding rod placed at the generator, and the pros and cons. The answer is you do not need that ground rod in most cases, and as shared in the above OSHA document there is a case that you could increase the chance of getting a shock with certain defects. Since members of our AE community have lost equipment to lightening strikes, and they often locate their generator in a shed out back with other mixes of equipment, they may choose to drive a GROUND ROD, or make use of one already present, they may have steel in the air, a wind turbine, maybe a long wire antenna, hame gear etc, our community often does, and this may cause us to bond to the grounds near our generator in ADDITION to the ground wire going back to the service panel. It’s likely the argument for a ground rod at the generator as well as the ground wire running to the service panel and cross connected there would be fought from an equipment protection standpoint. A few people have smoked a room full of expensive equipment, inverters, charge controllers, etc. Would an additional ground rod or two have kept it out of the power room where the equipment fried? I guess a complete survey would improve our guess. As I have always said, and as you say above.. “It doesn’t really mean a darned thing what we think, it’s what your local authroities say.” What is going to be the most compatible with your home owner’s policy working for you should something go wrong? Mike, all of your above links should work here.. I greatly appreciate your contributions, and glad your transfer switch and gen install works well. You have captured the essence of our back up power strategy… it should be done with a reasonable investment, and the smaller machines prove to have many advantages over a machine larger than required. Your efforts to convert to 120 volts only will pay dividends, and I bet you are one of a handfull of people who modifed a Coleman to work best with your transfer switch.. Zach and Elmo say “Hats off to Mike!.” Keep it simple!

      As you read this.. There is likely some guy… some where in the western hemisphere staring at the voltagemeter and wondering why it’s gone so low? It’s what happens when you have one stator winding over loaded… a potential problem you have removed from your list 🙂

      • Mark C. says:

        They are trying to get across the concept that there should
        be only a single ground point in the entire local system. If you
        have more than one ground, currents can flow in in places
        not expected during fault conditions. Apparently in recreational vehicles they take this so seriously that the have a jumper
        or relay on the generator that can unground the generator
        when the RV is plugged into an external source of utility

        The other thing to know is that a “digital” inverter generator
        works entirely differently from these synchronous ones. Digital
        inverter frees up the engine to run at any speed as the
        frequency determining component and the energy
        determining one work separately, while these two functions
        are coupled and will interact in a synchronous generator.

  9. Michael L says:

    George I was not sure that I could actually post the image. hich it did not show but the links worked. You can delete the three image lines and leave the links if you can. makes it look nice.

    Anyway on my thoughts of grounding for fault protection on one hand I am not sure it will always work against lightning. I feel if you have expensive stuff to protect another route should be looked at.

    I would think here is where one would start and then look into all the equipment suggested toward the end.

    Its a good read if you read it through.

    Application guide. Lightning protection.

    With this in mind I do have some issues with UL listings. Maybe isolated but I will give an example of one.

    Flexible dryer vents you buy at the local home store are UL listed and advertized as dryer vents. You have seen those white plastic accordian style vents. Some have a fake foil that makes it look metal….LOL . THEY CATCH ON FIRE!!! Its plastic, come on!!!

    These are not to code. Plumbing Code states dryer vents must be rigid metal with a smooth interior. So you use a thinner gauge smoke pipe or they do make a semi rigid dryer vent that is somewhat flexible. Thats what needs to be used.

    Anyway, so the flexibles are UL listed and they burn and catch on fire from lint…

    Go figure…

    Although I really like this lightning application guide in the link above.

    Mike NJ

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  11. Ernie Menard says:

    I’m a dummy. I’d wish you’d taken the first schematic that shows the coleman generator wiring and then reproduced the same schematic with the necessary changes to get full power at 120v.

    Looking at your second drawing with the coleman schematic next to the batteries my presumption is that I connect the two black terminals together and then use the two whites to provide full 120v power. Is this correct?

    • George B. says:

      It’s a good move to print out the schemetics and mark them up the way you’ll wire it.. see the added drawings I added in the body of the article.. Maybe it will be more clear what you need do..

  12. Perry Ostroski says:

    George, this is not about the subject at hand,but I have a question about portable generators. I bought one that was remanufactured,which was my mistake. I bought it a few years ago so no warranty applies. I just finally decided to check it out. I have 120 volts to the receptacles,but do not have 240 at its receptacle. I do have 120 volts at each respective lug on my 240 volt outlet when I check each by touching the ground,but I do not have 240 volts when checking across the two 120 lugs. Do generators produce 120 and 240 volts independently of each other? If so could the 240 volt portion be shorted? I am no expert on this subject, I am just confused as to why I have 120 volts on each respective lug on the 240 volt outlet,but not 240 volts when I check them together. Thanks for any input.

    • George B. says:

      The more typical wiring error is finding one of the stator windings reversed! Just swap the leads on one stator and try again.. just visualize two flash light batteries with one in the flash light backwards.. reading across each winding (battery) you get 1.25 volts, but across both (in series) you’d get nothing with a battery (winding in backwards). Make sense?

  13. Perry Ostroski says:

    George, I checked out the wiring schematic first and it compares to the way the generator is wired. I took off the cover at the end of the generator and checked the wiring when attached to the terminal block and also with the wiring removed with the same result as at my 240 volt outlet. Should I still switch the wires and try them? Thanks for having patience with me on this.

  14. Perry Ostroski says:

    George,does this generator produce 120 volt on a seperate portion of the generator,and 240 volts also seperately. If so could this be the problem,120 is working but not 240? I thought I would ask this even though I do not feel it does.

    • George B. says:

      you need visualize two flash light batteries with their positive ends touching.. what do you read with a volt meter across each battery? what do you read across both batteries? Across both batteries should be zero.. how will you fix it? so you get the voltage of a battery times two? I hope I got your question right, but the battereis and the stator windings are the same situation.. both in series makes the higher voltage.

      • Perry Ostroski says:

        George, I swapped the leads on one stator and everything is still,the same as I mentioned in my original post,no change,anything else to try. Thanks.

        • George B. says:


          What I do in these cases, is make up test leads and bring them out to an area where I can measure them. If you can read each individual stator lead at around 110-120 Volts, and you can arrange them both in series, and even turn one winding around.. then something is very wrong. You needs all four leads brought out..

          prove you can read each winding.. then move on the readign both of them in series..

  15. Michael Lawton says:

    Perry here is a before and after of a gen mod that I performed.

    Before 120/240 gen

    After 120 only gen.

    Hope this helps

    Mike NJ

    • doug says:

      Wondering if you can give any advice on wiring a generator in parallel. HF 3200/4000. Mine is similar to what you show but I have a voltage regulator. I do have a 4 wire setup. I’ve tried several configurations and don’t seem to work. Any advice would be greatly appreciated. Thanks

  16. Dave says:

    So help me out here. I have a Harbor Freight 120 volt generator. That is all it is suppose to be, not 240.

    It has been sitting for a long time and this time when I got ready to use it I noticed something was not right, I measured the voltage and it says 240. It is a pretty simple generator. It runs 3600 RPM just like it is suppose to. I did notice when you try to put any kind of load on it the 240 volts drops way down. Understandable, it is not suppose to be a 240 generator. I think it is a brushless type. I do see a small capacitor on it and if you unplug it nothing comes out. I contacted Harbor Freight and they are worthless when it come to a remedy of thing to check.

    • George B. says:

      Dave, I’m not that familiar with this generator, having 240V when you should have 120V, and knowing the rpm is right.. I’d want to see a drawing, or have more info before I commented. If this is a typical rig, you have an exciter winding in the stator that controls the field in the brushless rotor. Twice the voltage in a 120VAC only head..hmmm, can they send you electrical drawings?

      I’m wondering who all Harbor frieght sells too, and if there are options for 50hz 220V operation? Or maybe a supplier to HF miss branded a 50hz 220V unit? I only mention such a thing because if you rule it out.. it will be the problem.

      Many of the problems people have with the brushless designs are bad capacitors, and burned out diodes, neither of these faults produces a problem like yours that I know of..

      maybe we have a DIYer here familiar with the gen head?

  17. David says:

    If you look at the manual for the Harbor Freight it only shows you a picture of the system broken down. No real wiring diagram. It does have a running cap and some diodes and it mentions something about a voltage dependant resister. What is crazy is this thing worked last time I used it.

  18. Kelley pack says:

    Hello I have onan marquis 7000 its a 120unit its has 2 breakers cb1 and cb2 around about 28 amps apiece . In my trailer I have two qo square d breaker boxes they are seperate 1 box for cb1 2 20 amp breakers and same kind of box for cb2 have them loaded evenly as possible . I want to connect those 2 boxes together and get the full 56 amps manual is very vague on cb1 and cb2 testing them thy have continuity and they show 0 volts between them not 220 . I don’t understand if voltage reg , windings, and other components will sence this right . This is my service manual b4 would be my wiring schematic . thank you my name is Kelley very interesting reading I’m trying to learn more about the electrical end of single ph rv and standby generators anything or reading material any where would be appreciated.

  19. bob g says:

    if it were me i would call cummins on this one

    reason being, the wiring diagram is an “as wired” diagram and not a schematic, so there is really no way to know for sure what might happen if you parallel the two 120 output legs

    failing that, this is what i would do to test connections

    parallel the two commons, tie them together

    connect the two hots (after the circuit breakers) with a small value fuse. start it up without load, if the fuse blows then you must reverse the connections because they are 180 degree’s out of phase with each other. if the fuse does not blow then it is connected in phase, so shut it down and remove the fuse and connect both hot lines together.

    then power up and start applying load and see if the regulation is the same as it was with the two legs separate. my bet is it will be fine, but i hate to state this with 100% without a schematic. while it is hard for me to see how or why they would build the generator so that the two legs could not be parallel connected, almost anything is possible for some odd reason.

    this fuse method i have used many times before to connect in parallel two windings i am unsure of the phase of, beit generators or transformer primaries and secondaries. its quick and dirty, sometimes blows a fuse, but is far less destructive than just blindly making connections. one would think that it would be a 50/50 proposition, however my luck is somewhat less than this when it comes to things that make smoke.

    bob g

    • Kelley pack says:

      This is my first time to read this site looks very good and you sound like you know good bit about the gen end . I appreciate your advice I have talked to 3 onan techs each in a different large city and emailed head quarters for a engineer they are about 50-50 on it to that’s why I am searching around. Something you mentioned is there would be a differance between a schematic and wiring diagram if I understand you correct. All the manuals I have found are pretty vague on this subject know where does it say if you can cobble the breakers or outputs together the breakers have holes in the flip switchs where you could run a rod though them and if one kicked of it would kick both. I searched the Internet and a lot of people want to do this some say you can and some say you can’t . But nobody is sure . I tried it one time and it didnt hurt anything but when I got up to 28 amps the gen started bogging down like it was overloading I rewired it back the other way and I am still searching if I was doing it right or just go with the way I have it . I really appreciate you time and will keep reading your forum I’m impressed with your knowledge.

  20. bob g says:

    i would not recommend interconnecting this genhead until
    a proper schematic an be secured.

    i am fighting the flu and feel horrible, so i don’t want to suggest doing
    anything else until we know more about how it is built.

    bob g

  21. Kelley pack says:

    Morning George when you get to feeling better lets talk about how to get the schematic I don’t know if onan will let me have that. Would that be a build schematic or what would it be called so I kinda sound like I know what I’m talking when asking for it.

    • George B. says:


      My Cummins/Onan dealer in Renton, Washington is amazing, if yours is half as good, you’ll have no trouble. I think what you want is a clear diagram of the Stator windings, where they are brought out, where they terminate, and how you might re-arrange them. Some where on the utterpower blog is an article about adding a switch to go from 120/240, to 120 VAC only.. Hondas, and some other Japanese built generators have this feature. As noted in other articles here, when you are in 120VAC only mode, assure than any plug connector you use IS rated for the full current you will pull.

      I’d go to the dealer, present the counter man with a starbucks, and ask for help..

  22. bob g says:

    i a bob, aka “mobile_bob”

    you will want a schematic

    not a wiring diagram, not an engineering schematic, not a working drawing, but a true schematic.

    and there should be no reason for them to refuse giving you one. should an rv breakdown in outer mongolia the repair shop would need a schematic in order to repair it efficiently.

    years ago, i had the opportunity to buy a small appliance store here in my home town, the owner sold and serviced maytag, frigidaire and zenith tv’s/stereos.

    his advise was this, “bob never take on a repair for any product that you do not have a real schematic for” “if you choose to do otherwise you will lose your butt” there is no way to efficiently repair something if you first have to reverse engineer the damn thing before you start to do diagnostics.

    as far as i am concerned generators are no exception to this rule. while some generators are simple enough to sort out without a schematic there are others that clearly are much more complicated with the addition of AVR’s and other controls.

    additionally when it comes to an older unit, who knows who might have got in there and moved some wires around? this would greatly complicate things because we are tempted to accept the way it is wired as being correct when the reality might be quite the contrary.

    in this regard i don’t even trust a new product, they can be wired incorrectly from the factory, not common but it does happen.

    when you call cummins, you will need to call one of the larger dealers that do a lot of generator sales and service. ask to speak to their technical department and tell them you have a question. tell them you cannot bring the generator in because it is 500 miles away from the nearest cummins service center and you have a shop that can diagnose and repair the unit, however they won’t touch it without a schematic. under those conditions they will email you a copy no problem.

    lets face it, your unit is small taters to what they make their money on to start with, and them knowing you have a guy smart enough to demand a schematic before he starts diagnostics will provide assurance that he likely is smart enough to repair the unit.

    if the first cummins dealer is no help, go online and call another one! be persistent.

    bob g

  23. George B. says:

    Wow.. I’m glad I wasn’t the only one that thought that drawing was far less than I needed….. to know what was going on.

    We all appreciate detached drawings showing the area of interest, the actual stator windings, the source of excitation voltage, the field, and the connections between. And of course, how the one we are looking at was wired and optioned.

    One of my customers had an interesting experience, he took his RV into an RV repair place with a generator probem. He was presented with a generator in pieces, a drawing of his generator, and a bill for six hours of shop time. The shop reccomended he replace the unit with a new generator, and they offered to order same, and of course install it for him. Their diagnosis… bad stator windings.

    I told the owner to take the stator to a rewind shop and pay them to test it.. he did so, and they said A-OK, and mentioned, they knew this product to be a good unit with low failure rates.

    A few more questions, and we determined that the RV repair guy had been working off drawings for a far different gen head! The owner with a little advice found an inexpensive rectifier at fault.. and was able to replace it and re-install his unit. First time he hit the switch, he had full power 🙂

    I got the feeling that this guy might not have owned a pair of coveralls, nor had his hands dirty for a while, but plenty smart and not having a problem getting his hands dirty to expedite a proper repair to get his toy back into working order for a summer trip.

    I wouldn’t own an RV generator unless I could order a repair manual for it,, having that manual in the Coach… where it belongs. The rectifier would have been the first thing checked by most people who had the right drawing, and the unit could have been easily reached when installed in the coach (as per the smart design). An hour of labor and the cost of the part would be the charge in a good shop… especially if you loaned them your manual 🙂

    I got the feeling this was more the result of haivng the shop boy shoot the trouble, VS a shop owned and operated by Hiway Men. It reminds me of the RV Electrician who wired the light in my canopy. He put the fuse in-line with the wire between the lamp and ground 🙂

    • Kelley pack says:

      I will talk with cummins Monday thanks George hope you get to feeling better

      • Kelley pack says:

        Morning George I talked with cummins yesterday and they were suppose to send the schematic but haven’t seen it yet . I was looking in the service manual at the wring diagram and it looks like the one on the right is called a schematic and the one one left is a wiring diagram. Am suppose to be asking for a different schematic than that one before I call them back. Kelley Pack

        • Kelley pack says:

          Hello George I kept looking at schematic and cb1 and cb2 go into terminal block and come out as one tb1 so I ran the hots together and tried it again I got it to work this time I ran it for 30 minutes with 46 amps on it nothing bad happened so I guess it will work thanks. I am not going to run over 50 amps that’s enough.

          • Kelley pack says:

            Oh yes each output lead was pulling about half the load wih a clamp on meter at 10amp load each lead was about 1amp difference at 46 amps it was about 3amps difference.

          • George B. says:

            Sound like you have it, now the job of makign sure you have adequate gauge wire and connectors rated to carry it all to pints of use.. I learned from a customer that RVs have high amperage 120VAC services, and have had for years! Here’s a source of the proper connectors and more..

  24. Lee says:

    What about converting a 120v only generator to 240v? It’s an old Coleman 4000w, and I need to be able to run a well pump. ( I realize a smart person could figure it out from your diagram, but that isn’t quite enough for me) Thanks.

    • George B. says:

      Most 120VAC only generators do not have leads brought out of the stator that allow you to wire for 120 or 120/240, they will have less than four power leads out of the stator housing. It’s why you get the wiring info before you make an attempt..

    • bob g says:

      most of the 4kwatt class generators are 2 pole 3600rpm units
      and if they are 120 volt only even if you could separate the poles you would only get about 60vac from each stator group.

      so i don’t think it would do any good to open the stator up and separate the windings for reconnection, the two poles are likely already connected in series to put out the 120vac and you just can’t get to 240 that way.

      bob g

  25. Art M. says:

    Hi George, Very informative on the 240 to 120 volt modification. Just the information I needed to modify a new 5100 watt generator. I am hoping you can confirm Harold’s earlier comment regarding the 2 parallel windings and the DPDT switch. Harold states there is a missing connection on your DPDT diagram between stator winding A, wire number 2 and winding B, wire number 4. Can you please confirm this is needed to wire the switch correctly for parallel operation. Thank you!

    • George B. says:

      Art, no doubt you and Harold are right! some how that neccessary lead got over looked, and didn’t get placed in the drawing.. check out the update and see if I missed anything else? This is a simple modification, but one best use a very high quality DPDT switch.. beware of the Chinese stuff that only pretends to meet the spec.

  26. N3GLV says:

    Hi guys, I am looking at the 900w peak 2cycle genny from Harbor Freight.
    Anyone had one of these apart? My question is, how many phase coils it has.
    I need 24-30v DC output, at 800-900w if I can get it.
    I have an electric bike that consumes about 750w on a steep hill @ 24v
    and would like to be able to mount this gen on a trailer to be pulled behind
    me for long distance trips. It makes sense to me that if they have several windings
    that are in series to make 120v, if I could break that down and fold them to make 24 or
    48v, and avoid some of the loss of going from 120 to 24v, I can handle the diodes, regulators etc, mid range electronics background here. Thanks in advance.

  27. Curtis Hofmann says:

    Hi George…Congrats on the best info I can find on DIY portable generator mods 🙂

    My situation: Have need to use 110 vac power tools up to 15 A. in a remote location in Andes Mountains. A friend from Germany has offered me to buy (cheap) his used 220/400 vac portable generator. It is a “Priz” brand, model “PEG 8800”. I can’t find much information about this model in English. It IS nearly 9 Kw with a 12 Hp. motor.

    My assumption before reading your article: That I can split the “phase” and “inverted phase” that supply 220 v into two 110 v. circuits. I could remove 2 of the 3 220v receptacles from the panel, and replace them with 4 110 v. receptacles, and still have a 220 v receptacle in case I want to use my arc welder, or whatever.

    My fear after reading your article: This generator has a 440 v. winding that is center tapped for 220 v. outlets, and I cannot get access to any 120 v. tap points, as they would be inside the windings.

    Please help me by clearing this up. Maybe you understand German, and the online manuals would be of help.

    My nagging little thought that keeps bugging me, but I keep thinking it is not a problem: Frequency. I guess that this generator is 50 Hz. If I can get it to output 120v, and my tools are a little low on power, then can I increase the carburetor/governor settings to increase rpm of the motor until I get the proper 60 Hz.

    Thank you very much for your help.


    • George B. says:

      Curtis, no need for German, a good drawing will do. I’d expect all 50hz units to be void of any 120 VAC taps or windings, BUT that doesn’t stop you from using a 2:1 transformer.

      Power tools are mostly series wound, and there’s less concerns about frequency, you need know your load, and what type of motor it is. Frequency and RPM are related in induction motors, not so in series wound motors.

      BTW, a 12 horse ‘ENGINE’ will typically produce a max of 6KW in a typical generator, you may know this, but I mention it anyway.

      • Wow, thanks George, you are amazeing!!!!!

        I especially like the way you tell me it is possible and give me just enough information to lead me in the right direction with the research I will have to do to understand this at the level you do 😀

        I will probably have another question in a few weeks.

        Happy Travels,


  28. Thierry Jeannier says:

    Hello All

    I am trying to convert my Harbor Freight Predator 4000 generator to 120/240.
    I will vary rarely ever need 240 but it would be nice to be able to switch if needed.
    I confirmed that the coils are not bonded internally, I have 4 wire coming out from the coils ( red & white, black and white) and tested with meter to verify they a separate. I tried making it straight 120 no switch I will add the switch later if I can get it to work.
    This generator has an AVR and it apparently does not like having the 2 coils in parallel. It only produces about 20-30 volts when in parallel. Any suggestions?
    Thanks in advance for your replies.

    • Brandon G says:

      I tried to do the exact same thing and got the exact same output as you. I would love for my HF predator 4k to be able to run the AC in my trailer, or a small welder. It has the power, but only half of it is available.

      It seemed to me like the bridge rectifier was a more modern design that was not just a simple rectifier as in this circuit. It would never output the correct field voltage to get the 120V output.

      It is slightly adjustable and I could turn it up a little, but not more than 50V output. I stopped here, but it might be possible to replace that rectifier and get the output needed.

    • George B. says:

      Did you attempt to turn one coil around? I didn’t reference a diagram, but if field power is seperate, there’s little reason to think you CAN’T make 120VAC only work.

      • Thierry says:

        Yes I tried changing the polarity of one of the coils.

        I wonder if an AVR from a Honda generator that is setup for 120/240 would fix the problem.

        • George B. says:

          It’s almost always best to put your effort into understanding why it is, VS spending money to try somethign else. The key is to know all elements of what makes field power, what all does proper field voltage and current depending on?

          • justin says:

            i noticed the larger 8k harbor freight generators had the 120 vac outlet when i was there today . wonder whats different on that bigger generator because it has the 120 vac and the 120/240 4 prong

          • George B. says:

            wired for 120/240 as I would expect…

  29. Howard says:

    Hi, anyone? Doesn’t the 120/240 switch need to have contacts rated for the full generator output? 80 amp contacts for a ST 7.5. Thanks, H.

    • George B. says:

      Kiss answer? 7500 watt gen head, so you need figure how you’ll use it and what loads you’ll need carry. Most will breaker it at 30 amps for each coil, as for any switching, most would consider switches with the same or higher rating, and few would dare switching same unless the power plant is dead.

      • Howard says:

        Thanks G. The reason for switching 120/240 is, full power at 120, and because I have read that STs don’t like unbalanced loads. Correct?

        • George B. says:

          I don’t think any generator likes to run unbalanced, and there’s really nothing special about an ST. We need remember a few things, The chinese HAVE indeed made some very different machines and quoted the same Government ‘spec’. I’ve heard that this violation could get you shot at sunrise if the product was being made for the home market. With that said….

          Bill Rogers and I once visited an off grid site, the owner quite happy with his power, but together we couldn’t account for some ‘growls’ we heard coming from the ST gen head. A simple measure across the two windings that were in series showed WAY less than expected voltage, and one coil was wired in backwards! The owner had no use for 240VAC, and each side was putting out 120VAC. It was some years ago, I forget the details, but we laughed hard, turned the one winding around, and the Generator quit growling.

          My major concern with 120/240 operation, is having an owner FORGET he has only half the capacity per side, if it’s inadequately fused, it could all go up in smoke. Do run wired (120VAC only) when you have no 240 VAC loads.. it’s a good practive WHEN you are working with small generators. That’s my opinion, and there’s always others out there.

          • Howard says:

            OK. Sorry to be so far behind the curve here.Maybe a sanity check is in order. I THINK: At 240 , that is series wired,each leg is 120, with the junction forming neutral, and each 120 leg is providing half the rated power, for the total of, say, 7.5 kw. If the two windings are parallel wired, the output is 120, at the full rated power (in theory, anyway). Also, wouldn’t the parallel wiring eliminate the imbalance/noise at 120?
            I now have Facebook access, is there any help there? Thanks George.

          • George B. says:

            In 120 VAC only operation (stator coils parallel) any load is shared equally by the coils, no imbalance. Don’t assume that a load on one side in a 120/240 config WILL cause a growl or noise. I’ve heard that, but I haven’t had the problem in the many generator sets I’ve built and played with. MY concern steps in when people are using small generators, no properly breakered, and over loaded on one side. Do note, this can happen when coils are in parallel and you have a bad connection, this problem reduced or eliminated when each coild has it’s own breaker.

          • Howard says:

            Right, I would always do the breaker stuff, rather than destroying a ,now hard to get. machine. If barack ebola destroys our country, my needs would be ,mostly, 120. However, I would occasionally need to run a 240 compressor, or welder. Both will run on less than 7.5 kw, or 3.5k per leg, which is what I have. If I wire for 240, without a switch arrangement of some sort, and just hang two 120 sockets above the 240 one, I lose capacity/add stress on the 120 system because the windings are in series at the 240 socket, right? So, is switching for maximum capacity frowned upon for some reason? I’m looking forward to getting the engine running this winter, and tinkering with the gen head. H

          • George B. says:

            Switching anything WHEN it’s not necessary is not the best idea. We need study switches, and understand we need early breaks before makes in contacts WHEN we perform certain operations, if you avoid switching hot, (like in reconfiguring the stator windings), then you need not worry about problems. And.. few would argue that contacts that are not switcched hot and under loads last a lot longer. If I recall correctly, some Honda construction generators warn you NOT to move the switch from one stator config to the other when the gen is running, not because it can’t be done, BUT because it’s the best and safest plan.

  30. Brian B says:

    Thanks for taking the time to write this up. I intuitively expected to be able to turn by 110/220v generator into a straight 110 30a, but my first attempt ended up with half-genny half-motor (!). Your article gave me the confirmation I needed to keep going and I now have a fully functioning 30a system.


  31. eric says:

    Thanks for the info George, but my Gen head only has 3 wires coming from the stator. Is it possible to wire this for 120 v full amperage? I tried connecting the 2 120 volt leads together but that created a full load on generator at start up (short I assume).

    • George B. says:

      eric, you’ll need to carefully make your way into the stator wiring, and find that connection where the ends of the two stator windings meet and are center tapped. You then need break that connection and bring four wires out to your connection box. This can be tedious, or easy depending on the gen head..

      • Eric says:

        Thanks..this is a Chinese 5.2 kw head and i will see if i can find the connection. I didnt see one on the front side. I can find generators with bad engines cheap and it may be easier to swap the entire head. I dont think it will run my 30 amp 120 volt camper using just half of the windings.
        thanks again..

        • George B. says:

          You are smart to wonder, there’s generally two stator windings in there, and the ends often tied together and brought out like a center tap. They do this to save a penny? If the completed assembly is dipped after the center tap is made, it’s a big job to make a mod.

          • Eric says:

            I’ll look more closely. One would think all portable generators would be made with the capability to utilize both windings together or separate (with a switch like in your diagram ). I have a kawasaki ge2900 that does. But it’s only 2900 watts and if I load it heavy it really puts a load on the engine because it is using the full potential. It also has a voltage regulator and volt gauge that stays constant until the engine almost stops. Big difference in quality.

          • George B. says:

            A person need look at consumer generators from a manufacturers perspective, and that is.. you need sell them; the market is already flooded with low end values, so exactly where will you compete? IF you bring all the stator ends out to a terminal block where they can be reconfigured, you add cost, if you add a switch to move from 120/240VAC to 120 VAC only, you add more cost.

            We who worship KISS recognize the ANSI standards, and attempt to meet them, but we know that a voltage regulator comes at cost and can be the element that destroys the generator, and makes it more costly to rebuild than buying a new replacement. In an unregulated machine, loading down one stator winding causes the voltage to droop, you can provide some protection with a proper circuit breaker per winding. When adding a voltage regulator, you need be certain you don’t over drive the field winding to destruction; and this does happen with unattended generators that run out of fuel unless you have a current limiter for the field. Generators that go through cycles of stalling and then finding a bit more fuel can really toast a field winding when things are wrong.

            The best voltage regulators can cost more than commercial generators, and you need optimize it for the specific application. You need to protect the field and not overdrive it to destruction. I often think of that play written log ago…”Death of a Salesman”, what a classic. The death of a field winding might make a book or at least a long chapter. That one event where the VR overdrives the field long enough where the hot spot cooks the varnish to that richer brown many of us have seen. That can be the beginning of the Insulation failure, and it can take years for the insulation to fully break down and short X number of turns, or produce another fault.

            A classic is putting a generator away after it ran out of fuel, and discovering it doesn’t make juice at the next use, and that next use is nearly always an emergency, at least in one’s mind.

            We always need keep in mind, we are one percenters, and only fools and Government are likely to build equipment to our spec.

    • George B. says:

      If you can’t remove the center tap and bring both ends of the stator windings (four wires), then you will only supply your 120 VAC loads with 1/2 the power the head is rated for.

  32. John c says:

    I tried this with my new HF 4000w generator. It was real easy to do but it didn’t work. I swapped the wires like the schematic shows. For some reason I only got 22 volts. Hooked up a angle grinder just to see if the power would come up with a load and no. It barely turned the grinder. I reversed one of the outputs of one and when I started the Gen it didn’t sound good and I had 0 volts. Only ran for maybe 5 seconds and I turned it off.
    Put unit back to original config and unit works same as out of box.
    I verified the wiring and when I had the 22 volts it is correct per the diagram. I didn’t do the switch because I wanted to see if it would work and I have no use for 220 v.
    Any suggestions / ideas would be much appreciated.

  33. John says:

    No. But ther terminal block is numbered 1red-2white-3white-4black. Circuit 1 – terminal 1 Hot. 2 N. Circuit 2 – 3 N. 4 Hot
    These 4 wires come right out the back of Gen and go directly to the outlets.
    I moved 1 to 3. 2 to 4. Getting 22 volts
    I put 1 to 4 and 2 to 3 and that’s when the Gen sounded like it was fully loaded and 0 volts.
    How do I attach a pict of the wiring?

    • George B. says:

      IF you have a harbor freight drawing, you can attach it to email and send it to me at

      If there are two stator windings, it should be possible to make a 120/240 or 120 only gen head. I looked at a Harbor Freight Ad, the so called 10,000 Watt head didn’t mention voltage… and unlike many HF ads, there was no button for more info..

  34. Jason says:

    I’ve read this article plus all the comments a few times and have also read through a lot of the other posts on this site but I still have a question. My Craftsman 5600 Watt Gen comes with 2 20A outlets as well as the typical 4 prong twist lock 30A 120/240 outlet. I understand that if I rewire the gen head per the terrific instructions above, I can get 120v with probably in excess of 40 amps (5600/120=46). How is this all possible with the 2 20 amp circuit breakers that come on the unit? I understand protecting the stators so that if one becomes disconnected you’re not overloading and burning the other one out. Will the 20A breakers not pop if the dpdt switch is installed downstream of the breakers?

  35. Dustin says:

    Take a look! Thoughts??

    • DP says:

      A while back I tested mine several ways and basically came to the conclusion of exactly what your saying. The AVR was the issue. You have explained that in the technical way. Thank you very much. Would be interested in knowing if you ever got yours wired up with a transformer and if you did, what did ya use and how did ya do it. Would be nice if there was a different AVR that could be swapped in. ?? Maybe one that was looking for 100 volts instead of 20. Thank you sir. Nice video..

  36. Brandon Garrison says:

    That video is a nice find. Good detective work. I too got the 24V, and even with adjustment could only hit +-4V or so. This makes absolute sense, as the conclusion I came to was that I would need a different AVR to accomplish what I wanted.

    Found a 115V to 20V Transformer on Ebay for $8, I think I’m going to order one in.

  37. Dustin says:

    Awesome find on the transformer. I’m going to get it going soon.

  38. Ty- says:

    The 3BELTWESTY video is great. Really saved me from a lot of leg work figuring out the head.

    I’m curious how well the transformer will work that Brandon found. I did not consider one that small because I was worried about the voltage swing. At no load my reference wires had a draw of about 0.125 amps and went almost as high as 0.500 amps under a full load.

    I just bought this one …. The spec sheet indicates that I should only expect about a half volt swing over no matter how the genny is loaded down

  39. john kell says:

    Hi George,

    Whet do you do if your two field winding’s four terminals are not marked with black and white insulation? My Onan Pro 5000E only has one terminal (T2)That is white which is also grounded directly to the generator’s frame and then three black wires T1, T3 and T4.

    Generator Terminals (Wire Label/Generator Label): T1/R, T2/W, T3/Br and T4/L

  40. SpringerPop says:

    Some time ago I wired the stator coils in parallel, without voltage switching, and all is good. My Coleman Powermate 5000 operates just fine.

    However, my field excitation is the simple four-diode and capacitor type, with a separate excitation supply winding and there’s virtually no voltage regulation.

    Is there a real AVR that can be suggested from another application that I can sub in to get “real” regulation?

  41. Tom says:


    Dustin, DP, and Brandon posted 9 months ago that they had a solution to the Predator Generator AVR problem by using a stepdown transformer to change the AVR’s voltage input. But never posted whether it worked or not?

    Did anyone ever hear back from them?

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