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.
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
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.
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?
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.
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! https://www.utterpower.com/todays-email-how-to-wire-a-generator-head-for-rv-use/
I HOPE THIS GIVES YOU A PLACE TO START…scroll all the way down for comments..