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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.

RustyShackleford
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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 08 '19 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 08 '19 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 08 '19 at 23:05
  • Yes, certainly. – RustyShackleford Sep 09 '19 at 03:49
  • 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 09 '19 at 22:18
  • If my panel have separate ground and neutral bus bars, and a ready means of de-bonding them, then I could do that. – RustyShackleford Oct 09 '19 at 22:20
  • Perhaps it'd be ok to ground the inverter separately (e.g. a dedicated ground bar); it has a grounding lug, wouldn't even have to do it from the outle – RustyShackleford Oct 09 '19 at 23:40

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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".

Harper - Reinstate Monica
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  • 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 09 '19 at 04:00
  • 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 09 '19 at 04:04
  • I like subpanel idea. I think they actually make load centers with a transfer switch instead of a main breaker (such as Cummins RA112L1 that @ThreePhaseEel recommended in solar discussion). But I guess a simple no-lug with interlock is way cheaper; but I thought interlocks were designed to interlock a main breaker and a double-pole breaker at the top of the stack; are there ones that will interlock two side-by-side double pole breakers ? – RustyShackleford Sep 09 '19 at 04:15
  • @RustyShackleford Yes there are, and they're blushingly cheap. Siemens ECSBPK01 is like $25. Square D QO2DTI is also in that ballpark. Add two $9 breakers and you're off to the races. If you don't have any MWBCs, that settles that. My life is full of em :) I honestly see no problem running 120/240V appliances with both legs at the same potential, unless they are doing something MWBCish internally - so you'd have to check their schematics for that. . The 240V loads just won't work. – Harper - Reinstate Monica Sep 09 '19 at 04:25
  • Why so much cheaper than this I wonder: https://www.interlockkit.com/ProductDetails.asp?ProductCode=K%2D9010 – RustyShackleford Sep 09 '19 at 04:35
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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 09 '19 at 15:49
  • One wonders how to safely feed the interlocked breaker in the subpanel from the inverter, if the inverter has regular outlets (actually GFCI ones) instead of being hard-wired. I think a short cord coming from the interlocked breaker, with a male plug on it, is actually safe. Because the interlocking insures that the male plug cannot be energized if the subpanel is connected to the main panel. Then run an ordinary cord (male at the inverter end, female at the other end) from inverter to the cord hanging out of the subpanel. – RustyShackleford Sep 12 '19 at 19:32
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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 '19 at 20:15
  • Can you explain cordless with an inlet ? Concealed Romex or conduit to an ordinary outlet, then a double-male adapter to go between that and the cord from inverter ? If so, seems dangerous, as someone could plug that adapter into any outlet. – RustyShackleford Sep 12 '19 at 23:25
  • @RustyShackleford I mean wiring to a junction box like it was an ordinary outlet, but then using a suitable one of these... – Harper - Reinstate Monica Sep 13 '19 at 03:22
  • Thinking I don't want to mess with a subpanel right now, I'm just going to feed the main panel (just have to be sure to turn off the heavy loads when in backup mode). It's so old that Siemens didn't make a generator interlock. But this outfit does: https://www.geninterlock.com/product/siemens-ite-generator-interlock-kit-transfer-switch-150-or-200-amp-panels-3/ I was wondering about UL listing, and it contains this language: "Classified to UL67 with respect to installation and mechanical operation only". What does this mean ? – RustyShackleford Sep 16 '19 at 20:38
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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 '19 at 20:51
  • Ah, that's why the instructions have you zip-tie (no foolin') the backfed breaker to the neighboring one (in the right column). – RustyShackleford Sep 16 '19 at 23:37
  • BTW, how do you size the backfed breaker, since it's not protecting anything ? – RustyShackleford Sep 16 '19 at 23:38
  • @RustyShackleford ... And UL listed the instructions, so there 'ya go. Presumably UL tried it and found it met all their parameters. Anytime you have a breaker that isn't needed for overcurrent protection, just size it big enough to be not an impediment. I'm a fan of 60A because it's the largest breaker at the $10 price point. However, if this goes to an inlet of some kind, size it to protect the wires and inlet. – Harper - Reinstate Monica Sep 17 '19 at 00:43
  • I tried a quick experiment, just back-feeding one pole of my panel with the inverter. All but one of my critical-loads were on that "side", and I moved the one that wasn't (so basically, fridge, great-room lights, outlets with modem & phone). Worked fine. – RustyShackleford Oct 18 '19 at 20:30
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    Better still, when I switch on the double-pole 240vac breaker for the range, the gas burners functioned normally. – RustyShackleford Oct 18 '19 at 20:31