Todays Email, How to wire a Generator Head for RV use.


Randy's Arc

DIYer RV Wiring

You’ll notice I didn’t put up an April Fools Joke on April 1, sorry, I wasn’t thinking, but the article I put up about the page is an everyday joke, read that.



So it’s April 2nd, and my Grandson Zach who’s not three yet is waiting for his Sister to Arrive this month, being fascinated by all things Garbage or Recycle, he’s picked out a     name for her, “Trash”. 

Today’s Email:

Name: Dan B

Subject: Generator

Message: Hi, I was hoping I might ask a bit of advice if you have time? I read your posting about rewiring the generator head to get full power on 11o volts. When I bought my generator it said the front plug would provide 30amp service, so with an adapter for my RV plug I thought I would be all set. Turns out they meant 30amp at 22o only.

15amps will not run my RV. My question is, rather than taking the generator all apart, do you think it might be possible that I could get a twist-lock plug for the 240 twist lock socket and combine the wiring at that point into a small fuse box with an RV socket coming out of that. My goal would be that I could easily return the generator to normal function when others barrow it or if I were to sell it. Any advice you can give me on the subject is greatly appreciated as my wife is ready to go spend thousands on a bigger unit if I can’t get this one to handle the job. I should mention it is a 5000 watt unit.

Thanks, Dan

As I sit here, I attempt to frame the question, and look at the data provided.

30 amps at 220 volts = 6600 watts,  so if his generator manufacturer says it’s a 5000 watt generator, how they do that?

I’m totally  ignorant of how many ways RVs are wired to accept 120 VAC, and Dan’s statement that 15 amps doesn’t do it raises questions.  The first question is what is the typical source of AC power used to power an RV? At least a good percentage of the supply comes from a KOA campground plug, or one from the home.

In most new houses, the wiring is now 14 gauge, and they are often fitted with 15 amp breakers. With the understanding that breakers are only rated to carry 80% of the rating on the face of the breaker on a continuous basis, so we know that figure is 12 amps we might be able to pull without blowing the breaker.

If we are lucky, we might find a dedicated  120 VAC plug-in the garage that’s fed by a 20 amp breaker, but we have another safety concern, and that is the rating of the wall plug itself,many are not rated for a full 20 amps if you could carry it,  and we should note our 20 amp breaker is only rated to carry 16 amps continuous without tripping. Yes, your 20 amp breaker might trip closer to 20 amps, BUT the manufacturer is allowed a little leeway so he doesn’t have to sell you a precision breaker at high dollars to you.

For those following along, I’m thinking  out loud here, we need study the entire path of current flow from the typical power supply to the RV.  And one place I see real trouble is extension cords, some people want to set up their generator in the next zip code over, and run it to the camp site, so the noise is a lot lower. We need be VERY careful to note the size of the wire (gauge) and know that a cheap cord of small gauge (higher numerical number) is a BAD idea. you’ll need a good cord, and if it’s any length, you should consider 10 gauge, and even a moderate cord in length should be 12 gauge by my measure WHEN you think you are pulling near the limits of a (typical) 120VAC circuit.

So how do we answer Dan’s question? I was ready to show him how to install a switch to go from 120VAC only, at full Amperage, but perhaps all we do is set him up to start a fire in the RV wiring as we deliver more current through a box or plug not rated to carry it.

There’s an additional question, and that’s the real capacity of this generator, many are rated 6600 watts peak, and 5000 continuous. if this is the case, the unit should deliver 20 amps per stator winding 20 amps at 120 VAC, per side. If this is the case, we have the an outlet from the generator at 120VAC that will carry as much current as what we find in any standard home outlet.

So before we proceed with a solution, we need to hear from the RV experts that visit utterpower, I know there are several who know far more than I do about how they are wired. We also need to know what is drawing all this current? Perhaps it’s the fridge that runs on either propane or electric, and we can just make sure we are on propane? Perhaps a switch to isolate that one load that draws so much, and run a second extension cord to run it directly?

And Dan, at this point, you are using half the power your generator can deliver, you wife needs to know a bigger generator may not deliver any more current at 120 VAC than your present one.  A complete schematic of the RV wiring would be helpful, if the RV typically needs more than 15 real amps, I’d expect it to be wired different, but then again, it’s not clear that your wife’s adornments aren’t pulling 12 amps all by themselves 🙂

So as not to frustrate Dan too much, I’ll anser this much.. It’s not often the generator manufacturer makes it easy to run in the 120VAC only mode, and as we touch on above, it would require a plug rated to carry the current that would be delivered in that situation (on the generator).  Some Honda generators I’ve seen have a switch to go from 120 only to 120/240VAC,  it’s easy to wire that in on many generators if you have access to both ends of the two stator windings.  When we create a special non standard situation, we can catch things on fire, and create a liability for ourselves. RVs are dangerous enough without our help in increasing those dangers. I remember a Racer Friend of mine getting ready to go to a race in another state, the motor home and race trailer were in the drive, and at about 4AM the morning they were to leave, a loud explosion, and the back yard filled with burning rubble.  I saw it on the morning news on a Seattle TV station.

DIYers.. Scroll down to comments help me! let us know what you’d do, Dan, try and identify why you need this much juice. what circuits pull this kind of current?

Added  Comment 4/3/12 :

I forgot we had so much info on this page, thanks for the reminder Mike L.





George B.    


This entry was posted in DIYer Skills, Generator Realities, Generators, How Tos, Legal, Off Grid Power, Questions & Answers and tagged , , . Bookmark the permalink.

25 Responses to Todays Email, How to wire a Generator Head for RV use.

  1. Dan B says:

    The RV uses 30amp service. The entire camper runs on a dedicated plug intended for RV’s, which then goes into a service panel to be distributed in the same way a house is. The A/C unit pulls too many amps at start-up, which kicks the breaker on the unit due to it only supplying half the rated power when you are using a 110 plug to convert to RV use.
    So basically, I need both sides of the generator supplying power to achieve the 30amp/110volt service I thought I was buying when I got this unit.
    Thanks, DAN

    • George B. says:

      OK Dan, forgive me for being on the slow side here, there is no standard 30 amp 120 VAC Service that I know of, so maybe you have a 220 VAC 30 amp service and only need the proper cord between the 220 VAC outlet and the RV? I am confused. Maybe a picture of the generaor panel, and the RV’s power plug? How many prongs are in that RV plug in?

      • Alan Larson says:

        George, 120 volt / 30 amp service is provided to an RV from a NEMA TT-30 connection. This is a 30 amp 120 volt connection. Some newer RVs also use L5-30P on the RV, and L5-30R on the cord. Marine shore power cables also frequently use this configuration.
        Most generators for small to medium sized RVs are 120 volt only.

        • George B. says:

          Alan, this is a great comment, I keep forgetting that RVs have made this important standard, and now you tell me they are making others… Thank you for your comment…

  2. Libo Gomez-Maier says:

    RVs do indeed have 30 amp rated plugs for hook ups at 120 volts as crazy as it sounds, and even 50 amps, but those are based off the the standard 4 wire dryer plug that modern homes would have. With the 50 amp plug, they balance the loads on each leg, very few have 240 volt devices I would imagine, but I’m sure there out there.

    Here’s a link showing the difference:

    The only way you would be able to get 30 amp at 120 out of the generator is to rewire it in the head, you can’t rewire a 30 amp 240 socket and get 30 amps 120. But there is another option if your up for it.

    If you don’t want to rewire the gen head, you would to have to make a custom cord to run into the RV and run the A/C off of one of the legs or side, will call it “A”. Then all the other loads in the camper would run off of “B”.

    Do note that you would need to run a 4 wire cable from the generator to the RV, not a 3. You want your grounds and neutrals separate otherwise if your netural was ever to break (has happen to people) the power will be hunting for ground and electrify anything metal on or in the the RV! Very important.

    The easier way, at least in my opinion, is to rewire the gen head for 120 only if at all possible, then either attach another electrical box with a 30 amp RV socket (they do sell them) or mount it into the existing electrical box on the gen head. Then you just be able to plug in the RV directly with not having to mess with any of the wiring on the RV.

    Otherwise, you’ll have to hunt around for another gen head with the outputs you need, I’m sure there are ones that have a 30 amp twist loc at 120v that you just jack into and then have a jack for the RV plug, or you could easily make your own.

    • George B. says:

      OK! There we go, this is a suprise to me, as I’ve never paid attention to this type of service.

      Seems Libo has the options laid out in his comment.

      I’d be tempted to get the generator schemetics, and wire in double pole double throw switch that will allow any generator that has all four stator windings brought out to a terminal, where you can wire into it, and then direct the output of the two stator windings to that plug shared in the link above.

      With the informaiton shared, it’s easy to see how we could snap in 30 amp breaker and wire in an outlet to feed our RV at home, or set up a plug for our visiting friends.

      As for doing the mods on the gen set, fairly easy for a lifetime DIYer, a good project for an electrician, but not so good for a guy with no solder skills, and one challenged by electrics. Of course we DIYers new or old can do anything when time or money is not a concern.

      This all makes me real curious as to how many small generators are set up for 120V only operation, as I said some Hondas are, but I haven’t seen that many selector switches on small generators!

      The key to the MOD is developing a truth table and distilling the logic down, I’ve done that for you.. and then implementing your selector with as few contacts as possible, and still being safe.
      Don’t forget the 30 amp breaker! If you do this mod.. you will use the highest quality switch rated for the current you’ll switch, and as a precaution, you will only switch when the generator is off and nothing plugged into it. Go back to the bottom of this article and see the DPDT switch and how it gets wired in, the X makes, and the / breaks when the switch is operated. I’d consider a new box for the 30 amp plug as an adjunct.

      I learn something new every day 🙂 thanks Dan, thanks Libo.. I really talked a big circle, but there will be others that might attempt to wire it wrong . my ramblings may work as a guide to what we don’t do.


  3. Dan B says:

    Thanks to all for the info, I will give it a try this weekend if possible.

  4. Harold says:

    Since we don’t know the make / model of the generator in question the following info may be useless but it is an example of what may be.

    For a while I repaired motor home generators as a side line and I found the Master brand (portable generator) pretty expensive but well made and supported on line about the best I have seen. I have a 2500 / 3000 watt model I bought on the cheep as a box of parts. Most, if indeed not all the motor home specific units I have seen, such as Onan, Generac, etc, are configured as 110 volt only units. Some portable units can be used either way by choosing the appropriate plug / cord.

    A PDF version of a Master portable generator manual is found at this link that can be downloaded or read on line:

    Its 32 pages are a little over one megabyte.

    Page 11 of this on line manual has some pretty good info on the 30 amp configuration of one generator, pretty typical of the ones I am familiar with. Schematics and wiring diagrams are on page 16 to 18.

    I hope this helps.

  5. Michael Lawton says:

    To the OP, yes it depends on the make and model of gen you have if it can be rewired easily or not.

    If you look here you will see the last picture is my gen. That is the molex type connector that goes to the generator outlets. As you can see all 4 wires are there and a ground. This is what you need to look for.

    A side note, since I rewired my gen for 120v ( because I have no 220 loads) I would do it different and wire a switch instead. This would require adding a 30 amp 120v outlet to the current gen. This would retain the 220 outlet. You see I am the only one that knows this gen is rewired. And if someone else used it in my absense they would have no clue. Its easier to label a new outlet and switch then modify the current gen outlet labels.

    Here is my gen wiring schematic in original configuration.

    Here is my modification. Note that I cheated and still used the 120/220 outlet at the gen. This is just something I need to address down the road.

    This is my little 3250 watt work horse. I need to watch my use but it powers well, lighting, refridge, a/c if needed ( window unit ).

    I also did this mod for future RV use. They sell adapters for different outlets at the gen to convert for travel trailers.

    Now my opinion at this time would be use one of these and power your RV. But just dont use the A/C. Run the refridge on propane. It uses very very little propane to run the refridge.

    If you are able to change your gen to a 30 amp 120v outlet you would get a adapter like this for the gen.

    If you want to power the RV from the 120/220 outlet this outlet will use one side at 120v only.

    Also for a standard outlet.

    Hope this helps some.

    Mike NJ

  6. Pingback: Synchronous Generator Basics, Simple Guide to rewire your HEAD. |

  7. Joe T says:

    M. Lawton,

    Yes, take care of that 20A connector, as now you can pull 3250/120 = 27Amps through it and it will get hotter than its designed for.
    I am going to do the same mod, but I think I will wire to a separate 30Amp RV connector so that I dont overload the 4 prong 120/240 connector.
    I convinced myself that the 2nd winding is protected by the existing 20 amp breaker in the neutral line [I wonder if that is to code ?]

  8. The Oracle says:

    “For those following along”

    That’s what I was trying to do, but when you got to

    “but we have other insures”

    I lost it – couldn’t “follow along” anymore. What’s an “insures”?

    • George B. says:

      I was expecting the Oracle to know it was a typo 🙂 this should have been “We have other issues” perhaps a poor way of saying ‘other concerns’. I edited it to read more clearly I hope.. and thank you for the op to correct same.


  9. David A says:

    To give some clarification as I understand it.
    A RV with 50AMP cord is 220V but nothing in the RV is 220V usage. A RV setup to have a washer dryer and or a 2nd AC would have this setup. Basically they divide up the loads between the 2 available 120 legs coming from the plug. In a scenario of camping 50 amp service would allow you to run everything. but if 30amp is the only option an adapter is available and everything is on the 1 leg of 120V, and everything works … you just can’t run every thing at once. So 50amp = 2 25 amp 120 services to the RV. They even make adapter to go from a 50 amp camp ground service to 2 30 amp RV. And I believe I have seen an adapter that allows you to pulg in a 20 amp and 30 amp from the camp ground to a 50 amp plug for the RV, but this assumes the 20 amp and 30 amp plugs at the camp ground are separate 110 breakers and lines from the main source, and not just a 20 amp daisy chained off of the 30 amp.

  10. Asghar Ijaz says:

    I have a Hyundai generator of 2.5 KW and running on gas for the last one year. Now it drops its voltage onload 190 out 220 and a frequency of 48. Is it dangerous for the generator and please guide at your earliest.

    • George B. says:

      Without more info, I must make the assumption your generator is typical in design for a generator of this size and price range.

      A big load WILL cause droop, (voltage to go down) I must assume you have a load less than 1600 watts for testing..

      The first thing I’d do is remove the float bowl and check for dirt, gum, varnish etc. Remove the main jet and clean if you can. This should be done every few years o assure your generator will run properly when needed.

      Next, start the generator with clean fuel and a new fuel filer if you can get one.

      Adjust speed to 51hz (no Load) check that voltage isn’t too high, redo your test with moderate load.. You may have to adjust the speed to keep gen slightly below 240 volts?

  11. Jim Melton says:

    Do you still sell bushed pulleys?

  12. jdrahlgreen says:

    so i have a question .. i recently picked up an older generator that has 2 110v outlets and 2 220v outlets that look similar to the 120v outlets … the only real difference if one wasnt paying attention to which i which is the color of them .. anyways .. my question is how do i hook up my 30amp rv plug up to it to get full power and not have to use a 30amp to 110 dogbone .. is there a way to make a plug that will allow me to use both 110 outlets to get full 30amp power

  13. Dustin says:

    The Harbor Freight Predator 4000 generator is not working properly in 120v only config. Measuring the outlet, I am only getting about 21v. What would cause this? Any ideas? I have it wired properly… I just have no idea how two legs of 120v output can turn into 21v. Could it be the AVR? I’ve already spent the money on the DPDT switch, the breakers, and the upgraded outlets. I’d like to get it working. Any ideas?? Thanks!

  14. Dustin says:

    What about the wiring diagram at the bottom of this page? Would that solve my issue with the Harbor Freight generator?

    • George B. says:

      I answer in haste… before you make changes, you need understand the source of your excitation voltage! If it is derived from 240 VAC, OR is somehow altered when you re-arrange your stator leads, then your gen head may not be a candidate for the MOD.

  15. Nicholas Johnson says:

    I have an Onan CMM 5500 that is 120/240vac, 22.9 amps, 60 Hz. It does not have any plug ins on it and only has the 4 wires coming out. I know the two black wires are hot, white is neutral, and green is ground. How do I wire those up to get a 30 amp RV receptacle, and if possible one more 110 plug

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