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In a situation of temporary backup power (short-term grid outages), is it ok to feed both "sides" of a standard split-phase main load center from a single 120vac backup source (e.g. an inverter driven from batteries and/or solar PV) ? Assuming that the main breakers and any double-pole breakers are turned off.

Secondary question: What happens if any double-pole breakers are turned on ? For a 240vac load with no neutral connection, it seems the load would see zero voltage, since the two "hot" wires are in phase; of course both hots would show 120vac relative to ground, but that's the case normally. For a 240vac load with a neutral connection, I imagine it would be ok too; I think that's generally the case if the 240vac load is expected to contain 120vac loads (e.g. the control circuitry in a cooking range), and again it seems ok, since the 120vac loads would still see their 120vac, and it'd only be a problem if the load contained separate 120vac loads powered off the two hots and it was somehow necessary for them to be out of phase, conceivable I suppose, but far-fetched.

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    Yes, people do this but no, it's not OK to backfeed your home with a generator. Get a transfer switch and do it right. The cost is minimal compared to the risk of doing it the wrong way. – jwh20 Sep 8 at 22:36
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    @jwh20 he's talking about using an interlock or transfer switch but with both generator-side incoming hot legs paralleled with each other – ThreePhaseEel Sep 8 at 23:04
  • You'd be using a generator interlock to keep the main breaker and the inverter breaker from being on at the same time, right? – ThreePhaseEel Sep 8 at 23:05
  • Yes, certainly. – RustyShackleford Sep 9 at 3:49
  • Actually, I'm not sure this is going to work. I did a quick test, and the GFCI outlet on the inverter tripped. The problem is that when the inverter output is connected to the panel, ground and the GFCI outlet's neutral are connected (since neutral and ground are bonded in the panel) and the GFCI does not like this. It would probably work if I connected only the hot and neutral from the GFCI outlet to the panel; but then I'd have to leave the ground of the GFCI outlet disconnected, and that seems wrong. May simply have to use extension cords. – RustyShackleford Oct 9 at 22:18
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No "checklists"! Hard-built interlocks only.

There is no such thing as "making sure your main breaker and 2-pole breakers are turned off". You must not do generator interlocking via a checklist or procedure.

There are only three legitimate ways to switch from mains to backup power:

  • a generator-style interlock or transfer switch
  • "permanently" rewire the panel or circuits so they hardwired to power off the alternate source only, and no longer are able to power from mains. When power returns, "pemanently" wire them back.
  • switch the loads via cord-and-plug connection.

The cheapest and simplest way to implement a generator interlock is, if an interlock kit isn't made for your main panel, to get a no-lug subpanel with an inexpensive interlock (Murray or QO) and have two backfeeding breakers back to back interlocked, and then put the loads you want to switch into this panel.

Go ahead and feed both poles, except...

Once you have done this, go for it. Go ahead and split your 120V supply so it feeds both legs of the interlocked 240V "generator" breaker.

... except Multi-Wire Branch Circuits

However, this will be very bad for multi-wire branch circuits (MWBC). These are circuits which share a neutral. The hot wires must be on opposing poles so the neutral only handles differential currents. Putting them on the same pole will mean neutral handles the sum of both hots' currents, and that can overload it.

How do you avert this? Ideally, you put your to-be-gen-powered circuits in a separate subpanel (again, the cheap way to provide that gen interlock)... and simply don't migrate MWBCs to that panel.

Or go on a crusade to eliminate all multi-wire branch circuits,

At the very least, you identify in advance your MWBCs, by going through your panel, finding them, and placing them on 2-pole breakers.

Notably, a "make turning off MWBCs part of your checklist" apparently violates the "no checklists" rule. However the main reason for that reg is so you don't kill linemen, who are innocent outsiders. I am not sure if Code has taken a stance on this entirely local consequence.

What will happen if you power a 240V appliance?

A 240V appliance (water heater, A/C): Nothing. Every conductor in the appliance will be at the same potential, so no electrons will move. This will happen to be 120V from safety ground, but who cares.

A 120/240V appliance (dryer, range): The 120V parts of the appliance will power up (tumbler, timer, clock, oven light). The 240V parts, see above. So for instance your dryer will work, but only on "fluff/no heat".

  • I have no MWBCs; I wonder though, if code would prohibit my OP (120vac feeds both poles of panel), since although interlock saves linemen, my plan could overload a neural conductor of an MWBC as you describe, and the NEC is typically pretty concerned about over-loaded conductors. – RustyShackleford Sep 9 at 4:00
  • As far as 240vac appliances with neutrals, I have an ulterior motive for wanting that to be ok, which is that my dual-fuel range won't less gas flow unless it's got electricity. Although I won't have near the juice to operate the oven it would be nice to use the burners. Assuming that valve runs off the 120vac, I'd be good to go. Sounds sketchy gas safety-wise, but a left-on-but-unlit burner is just as dangerous under normal operating conditions. – RustyShackleford Sep 9 at 4:04
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    @RustyShackleford insurance, I'm sure, along with being a more complex, one-sIze-fits-all solution that is low volume, so, having to spread UL approval costs across fewer units. Siemens/Sq.D.are the OEMs so that has its advantages. – Harper - Reinstate Monica Sep 9 at 15:49
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    @RustyShackleford. Yeah. That would actually work. Modulo the rules for flexible cordage, of course, which don't come to mind right now. The cordless way to do that is an inlet. – Harper - Reinstate Monica Sep 12 at 20:15
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    @RustyShackleford "Classified" in panels means it's a brand X product UL listed for a brand Y panel. That's harmless. The rest tells you that it does not satisfy code requirements for strapping down backfed breakers (so they can't rock out like a normal breaker can) and does not satisfy some location's Code requirements that the interlock still work with the cover removed. – Harper - Reinstate Monica Sep 16 at 20:51

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