I'll start off by saying that I am familiar enough with the electric code to know that either an interlock or manual transfer switch panel is required so as not to backfeed the electric lines.

I have a portable generator that I would like to connect to my home electrical panel. My generator, as with other generators I have seen, has 2 output plugs, one is a 4 prong twist lock 120/240v rated at 21 amps and the other is a 3 prong twist lock 120v rated at 20 amps. The total output from my generator is 5000 watts continuous or 41 amps total. The surge wattage is higher.

As I understand it I can not connect both output plugs from the generator to my electrical panel to get the full 41 amps due to load sharing not being equal (which could trip one of the generator breakers prematurely) and possible out of phase ac power from the different plugs on the generator.

So my first question is is that correct? It make sense to me.

Second, if I can not connect both gen outputs directly to my panel how do I get the full 5000 watts to my electric panel? I believe I understand the math here. If I only run 120v equipment, which is what I forsee, no need for 240v during a power outage, then 20 amps is 2,400 watts at 120v. So one of the generator plugs will not get the job done. I'll only have half of the gen output available.

Is my math and thinking above correct?

The only way I can see to get the full generator output into the house is by using both a backfeed breaker in my panel with the appropriate manual interlock per the electric code and also installing a manual switch panel.

I would run the generator output from the 120/240v 21 amp plug to a backfeed circuit breaker in my main panel (with appropriate interlock). After a power outage I can turn off all the main electric panel CB's, go off and on with the main and gen breakers via the interlock, power up the generator and have 21 amps (about 2500 watts) available to use as I see fit via the breakers on the main house electric panel.

Now, for the other output from the generator, the 120v 20 amp output I would connect to a 10 circuit manual transfer switch. I would only connect 120v loads to this panel because I would only have 120v coming into it. Now, flipping the rocker switches on the manual transfer switch panel from line to gen would disconnect the chosen circuit from the main panel power (wherever that power is coming from, the line or the other generator plug) and connect it to the second 120v generator output. This would make the other 20 amps (about 2400 watts) from the generator available to the manuel transfer switchs 10 circuits.

So after all is said an done I would have 120/240v with 21 amps from the generator available to all circuits of my main panel (yes I know I would not be able to run all circuits, I plan to install a watt meter so I can see how much power I am pulling on the main panel and I would not be powering the 10 circuits connected to the manual transfer switch once those switches are turned to gen). Second I would have the 120v 20 amp plug connected to a 10 circuit manuel transfer switch panel. During a power outage those 10 circuits would be disconnected from the main and connected to the generator.

Settings things up this way I would have 2 separate plugs from the generator powering two basically separate electrical systems which would in turn power the house. The parts for this setup including the cabling to the generator would be about $700 to $800.

Does anyone see a problem with this setup? Is there anything that would not be to NEC in doing this?

Am I over complicating this? Does anyone know of a better or more efficient way?

Any input on problems with this setup or better ways would be appreciated.

Thanks, Pete

  • 1
    Running a generator at full rated power (in my experiance) shortens the generator life significantly. The peak or surge level is only for motor loads like refrigerators and well pumps that only draw the peak or surge level for a few seconds while the motor is starting. I have seen Honda's only last 2 seasons and larger generac only last 1 big storm when sized close to 100% my best comparison is do you run your car at even 50% from the time it starts ? Get a generator with a larger capacity if you want it to last because the cost of the generator will be much more if you have to replace it.
    – Ed Beal
    Sep 18, 2017 at 0:59
  • What loads are you trying to run on the generator? Sep 18, 2017 at 1:22

1 Answer 1


Ok you wrote a book. Proposing all manner of third rate hackery. And what does it boil down to? You want to get 5000W out of your 5000W generator. Quick question.

What is 240 x 21 ?

By my math, it's 5040. There's your 5000W. You do get it out of the big NEMA L14-20 connector.

I have no idea where you got 41A. I'm pretty sure you made that up, probably by dividing 5000 by 120. I seriously doubt it was on the generator spec. There's a way if you really really want that, but as you get educated, you will realize you do not.

What is it you're missing? The odd idiom of North American 2-pole service. I don't blame you for not getting it... It's weird.

Your house is served by +120V, neutral (0V), and -120V. I just described an instant in time, they're AC so they will reverse position 120 times a second. The poles are called L1 and L2 and the middle is Neutral.

240V loads grab L1 and L2. 120V loads grab either pole and neutral. Which pole they grab is nearly random and that's the idea, to make them average out so loads are balanced.

For you, with 21A on each pole, balancing is a big deal. You'll have a problem if you put 30A of load on one pole. So you'll need to get into the gory details of what is on which pole, and manage accordingly.

Step 1: Control MWBCs so they don't kill you

I don't recommend rearranging things on a panel because you can break a type of wiring called a multi-wire branch circuit. Find an electrician and tell him to do exactly this:

find every multi-wire branch circuit in my home, and make sure both its hot wires are served from the same 2-pole breaker.

Step 2: get rid of double-stuff breakers

If your panel is stuffed, and has lots of breakers that have 2 breakers in 1 space, those will drive you absolutely bat crazy. ack... You know what, to heck with all that.

Let's just get you a new subpanel with the appropriate interlocks, and move the loads you want the generator to power into this new subpanel. Make this subpanel quite large (at least 20 space) realizing you'll use 4 spaces just for the interlock.

In a perfect world, your new panel will have ammeters which will tell you how close to 21A each pole is getting. Even better get one of those new fangled whole house monitoring systems. Ask a new question on how to get one to work in a generator interlocked panel.

Step 3: rearrange your loads in the panel

Now finally, it's time to learn the gory details of how poles are assigned in a panel. Read my posting here. Your panel may differ, but probably not by much.

Move your loads into the new panel, and consciously and carefully balance the loads. For instance if your table saw is on L1, put your dust collector on L2. Stuff like that.

  • I knew I was screwing something up because all the generators I looked at are setup the same as mine. Thanks for the answer. Sep 18, 2017 at 3:42

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