# Can I power a two-phase home service with a single-phase generator?

It is hurricane season here in Florida, so naturally I pulled out the generator to check it and had a question.

The generator I have says it can output 30amp single phase.

Let's say you wanted to add an interlock to the breaker panel to feed in power when the mains goes out. But, the refrigerator is on 1 phase, and the tv is on another. Since these 2 (or more) devices need to be powered by the generator, and since it is single phase, is it possible/safe to get it to work on both phases?

Was thinking of a double pole breaker interlock with the hot wire split in to each pole.

Note: this is out of pure curiosity, I will not be attempting to do this.

Edit: I just checked the manual. You generator produces 120/240 volt single phase which is the same thing as your split-phase, two-leg service that your electric company provides. Feel free to feed it directly into a transfer switch to power loads on both legs, within the generator’s current capability, of course.

Yes, in theory, you can connect the generator output hot to both hot inputs to the transfer switch. This will put 120 volts on both legs but feed zero volts to any 240 volt load.

However, if you have any multi-wire branch circuits (MWBC), where a neutral is shared between two 120 volt circuits, that neutral could be overloaded and potentially catch on fire. This is because the two hots are on the same phase so that the return current would add together on the neutral instead of cancelling.

This is a real good reason not to do it. Not to mention the fact that if your transfer switch were to fail, your bridge would end up being a short circuit across the power company legs.

A better idea is to rearrange the breakers so that all of the generator loads are on one leg. Just be careful not to overload that leg.

• Emphasize that " into a transfer switch..." part. Commented Aug 29, 2019 at 15:56

## Phases and poles

The question of "phases" comes up when dealing with 3-phase power. The 3 phases are equally spaced: like runways at an airport arranged in a star or triangle pattern, one aiming north (12:00), southeast (4:00) and southwest (8:00).

What if 2-phase power existed? That would be runways aiming due north (12:00) and due south (6:00). Wait a minute, that would be the same runway!

So 2-phase power isn't really a thing; it's just single-phase power stacked on top of each other for at twice the voltage. With a common middle (at neutral), it can be tapped end-middle for 120V or end-end for 240V. And now you understand our 120V/240V split-phase system!

Note that your generator has a NEMA 14 receptacle which is designed to power a split-phase system.

## You need a generator interlock.

Your setup must make it mechanically impossible to backfeed onto the electric grid. Of course you'd have no reason to want to do that; so some people think "Oh, I can make it a checklist procedure", -- nope, not allowed. Because that never works in a stressful emergency.

Several companies make a variety of interlock kits with sliders or pivots that block the main breaker from being on at the same time as a double breaker in a particular position.

You may notice that most regular breakers flip inward to be on. The simplest interlock puts two breakers opposite each other, and inserts a spacer block so they can't both be inward (on). These two breakers are then back-fed, either from the utility or from the generator. Siemens/Murray makes a \$23 kit for their panels that also straps down those breakers, a Code requirement, and works with the covers removed, a Code requirement in some areas. Square D also makes a very inexpensive one for their "QO" (not Homeline) panel line, but I'm not sure it provides all that. However this only works on smaller panels where the utility side can be back-fed, e.g. Subpanels.

## Maybe put it in a subpanel

One easy way to provide the generator/interlock solution is to put a subpanel right next to your main panel, with one of the Siemens/Murray style interlocks on the top 2 breakers. Then relocate each of your loads you want the generator to support into this panel. There is no limit to the physical size of the subpanel; feel free to use a 42-space and put every circuit here. This trick can even be used to solve a separate "my panel is full" problem! That is why we are big fans of big panels.

All in all, such a subpanel is far, far cheaper than a "Reliance" style multi-circuit transfer switch, which allows you to switch up to 6 or 8 circuits between line and generator individually (why would you want that?). It also place nicely with AFCI and GFCI.

• Great answer; one small nitpick -- true two-phase power, with a phase angle of 90 degrees, does exist in a few places: en.wikipedia.org/wiki/Two-phase_electric_power. But you're correct that most people (including OP) actually mean split-phase when they say two-phase. Commented Aug 29, 2019 at 19:58
• Would you mind elaborating on the benefits of a subpanel vs just putting an interlock in the main panel? Is it simply to exclude the possibility of providing 120V generator power to MWBCs and 240V appliances? How exactly does it relate to AFCI and GFCI? Thanks!
– mrog
Commented Mar 2, 2021 at 21:47
• @mrog Yes, that would let you exclude MWBCs altogether and either a) not bring over 240V appliances, or b) bring over 240V appliances which can do useful things on 120V only (gas range w/ electric oven; dryer you can get useful use out of on tumble only). AFCIs and GFCIs would be happy. Note that the 6-8-10 circuit "transfer switches" are incompatible with GFCI, AFCI and MWBC, and generally handle neutral the way a carjacker handles an overlooked baby. Commented Mar 3, 2021 at 19:21

sure you can do that. thats how i am set up. i have a single phase 4000 watt , 30 amp generator, and i hook power up to each leg in the breaker cabinet through a 30 amp breaker.

the important thing to remember is that you cannot power two phase loads, like the dryer, the electric stove, or 220V AC system. so they have to be isolated.

i do not have a generator interlock so I have to manually isolate the house from the street before i connect the generator.

• Please remember to isolate the house - linesmen's lives depend on you remembering to do so! Commented Sep 15, 2022 at 17:57
• In some areas it's illegal to connect to your house panel without a transfer switch. If you isolate your panel, depending on where you do it, the power company could pull your meter and require an inspection before installing it back.
– JACK
Commented Sep 15, 2022 at 19:26
• -1 for the dangerous suggestion to attempt to backfeed the house without an interlock Commented Dec 23, 2022 at 15:02