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I have seen a variety of postings on similar questions online, but there seem to be different people replying with the exact opposite answers. It is, ahem, apparently contentious.

So I am going to ask again, hoping for a clearer answer and ideally from someone with real world direct experience.

If you have actually done this yourself, or seen it done, and seen the outcome, that would be the best of all.

What will actually happen / does actually happen? Are there issues because the current on both sides of the panel are in phase? Are there issues because they probably actually won't be perfectly in phase, although maybe close?

Is this safe for 120V appliances, eg computers, that are on one or the other side of the panel?

Is this safe for 240V equipment (which will not be running, since the input is only 120V)?

The electrician who recommended this plan to me (yes, an electrician was the architect of this plan, but left me to actually do it, myself, after taking off with the $$) said that since both sides of the 120V panel are in phase, any 240V appliances will receive near 0V, will have no power, no current will flow, so nothing bad will happen. He said that he does this same thing at his own home.

To be clear, the question regards using a switch that disconnects both sides of the panel from main power, and instead connects both sides of the panel to the hot side of a 120V generator, and the neutral to the neutral of the generator, and ground to earth. Let me ask you to answer the topic question itself directly: what actually happens?

If you'd like to give advice on whether you think this is a good or a bad idea, happy to hear it, but first would like to ask if you'd offer an answer to the topic question itself, so that I can understand the basis for your perspective, and learn the real answer. I ask for a direct answer to the topic question because most of the answers to related questions that I have seen online seem to be people saying something along the lines of 'don't ever do that!' or 'that'll work fine so long as you throw the 240V breakers', but not actually giving a clear answer to what will actually happen.

Thank you

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  • Has nothing to do with phase.
    – Ecnerwal
    Aug 21, 2023 at 18:11
  • 2
    What is the claimed BENEFIT of this nonstandard approach? Yes, modulo the increased load on neutral and losing the ability to use 240V appliances like stoves clothes dryers, it's possible with enough work... but, migawd, why?
    – keshlam
    Aug 22, 2023 at 5:18
  • 2
    Reading between the lines - you have a US style panel with normal 240V AC service and both 120 and 240V circuits. But when there's no utility power, you want to switch over to a local 120V generator and power only the 120V devices on both sides of the panel ? I'm trying to understand why....
    – Criggie
    Aug 22, 2023 at 5:19
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    "What is the claimed BENEFIT of this nonstandard approach?" The benefit is all 120 volt loads in the house has power. With 240 volt loads being things like air conditioning, clothes dryer, and kitchen oven this is a fair trade off since it means all the lights in the house work (which is much easier now that LED is more common than the old Edison bulb) as do things like cell phone chargers, televisions (again thanks to LED this is easier), refrigerator, and maybe some small kitchen appliances so a cold meal (or lukewarm one) can be prepared.
    – MacGuffin
    Aug 22, 2023 at 6:20
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    Ah. So trying to abuse a 120V backup power supply to run the entire house? I'd say get a more reasonable backup power supply. Especially since you are still going to need a safety transfer switch/linkage to avoid electrocuting line workers. And often, to minimize draw on the alternate supply, you don't want to power the whole house, just the most essential circuits; that's why the transfer is often done on a sub-panel which carries only those circuits.
    – keshlam
    Aug 22, 2023 at 15:50

4 Answers 4

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120V hot to neutral is single-phase. 240V hot to hot is also single-phase, indeed the same single-phase, without involving the neutral center-tap on the transformer.

The 120V loads see 120V

The 240V loads see 0V

240/120V loads (most dryers, many ovens) will see 120V but not 240V.

Any Multi Wire Branch Circuits (MWBCs) can overload their neutral wire. That's not good, and violates code. Normally they only send the difference between the separate 120V currents (from the 240V hots) down the neutral wire. In this setup they send the sum of those currents down the neutral wire.

So if you have any MWBCs, don't do this. If you don't know what they are, either don't do this, or learn and find out if you have any, then don't do it if you have any.

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  • Thank you for your reply. I've been reading about MWBC's. Can you tell me a simple, definitive way to know whether there are MWBCs or not? Can one look at the connections and the main panel to see a clear answer? Thank you again. Aug 21, 2023 at 18:20
  • Depends how your house is wired. If NM-type cables, you'll see a /3 cable with black/white/red/ground where black and red are both connected to a breaker. In the current era those should be either a two-pole breaker or adjacent and handle-tied, but earlier codes permitted less obvious and less safe breaker arrangements, so it's not guaranteed. If you use a voltmeter the hots on a MWBC will be 240V between them, not 0V, but there's no guarantee that a duplex receptacle on an MWBC will be split & connected to both sides. Common, but not guaranteed. If conduit they can be harder to spot.
    – Ecnerwal
    Aug 21, 2023 at 18:25
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    Please ask this as a separate question. It is important enough to stand on its own, since there are other reasons a homeowner might want to identify which circuits are MWBC. Aug 21, 2023 at 18:26
  • I will second that this will make a good question by itself.
    – crip659
    Aug 21, 2023 at 19:01
  • See related question: diy.stackexchange.com/questions/279770/… Aug 21, 2023 at 20:05
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Watch out for your neutral bars!

The main issue with feeding the two busses of a split phase panel from the same phase is that if you try to feed both hot busbars at the full rated panel ampacity, you'll overload the panel's neutral busses even if you have two full size neutral wires connected to them from the supply. This is because split-phase (and even three-phase) panels only need to have a 100% (same size as any one hot) neutral under normal circumstances, and thus are made with 100% neutrals as a result.

Of course, with your average generator feeding your average main panel, you're going to stall your generator well before you run out of neutral ampacity, but for more unusual scenarios (I once saw a paper that proposed using a split-phase panelboard fed with 2 120V windings in-phase from an isolation transformer for critical audio work), this can be a limiting constraint as 200% neutrals are only available as an option on commercial panelboards.

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  • 1
    Would buying a 200A panel and using a 100A or smaller main breaker be a code legal and cost effective alternative to buying the specialized panel with a 200% neutral? Aug 22, 2023 at 5:10
  • 1
    @DanIsFiddlingByFirelight -- certainly, as that'd achieve the same relationship between ampacity permitted on the hot busbars and ampacity available on the neutral busbars Aug 22, 2023 at 11:47
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Another gotcha is what happens when the utility power returns, and the incoming supply is 240V apart on each leg?

Your generator will be shorting the two sides of the panel together when supplying 120V to both, so you will absolutely need some kind of lockout/cutout that prevents your generator from shorting your incoming supply.

You absolutely need a transfer switch that will briefly kill the loads, while it disconnects the generator and then connects the utility supply.

When the switch-back to utility happens, every 120V device will see some level of discontinuity in the frequency. Rather than a continuous AC Sine wave, your worst cases are a reversal in the direction of the polarity, or a cut from +169V to -169V. In other words, it will be dirty power for a brief time.

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  • Transition switches exist for this very scenario. They can be either in Mains or Generator position, and usually have a neutral position inbetween. As long as the tie exist only on generator side, this is valid and safe solution.
    – Thomas
    Aug 22, 2023 at 14:12
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    @manassehkatz-Moving2Codidact Excellent point - OP never mentioned a transfer switch. They could have been doing anything, inclucng installing a breaker between the two sides for when running off generator. Or worst case, a big switch shorting the two sides. The potential for error is huge.
    – Criggie
    Aug 22, 2023 at 19:42
  • Read the 8th paragraph of the question slowly and carefully.
    – Ecnerwal
    Aug 23, 2023 at 1:08
  • @Ecnerwal yep - exactly. "a switch that does X" has the potential to be something other than a transfer switch. Imagine someone using a light switch for this purpose.... it could go very wrong.
    – Criggie
    Aug 23, 2023 at 3:03
  • 1
    Given that the transfer switch breaks the connection for a time (probably several cycles of AC) before making the new one, I think the only thing that might care about the phase swap is rotating equipment. And probably not most of that, but I imagine a permanent magnet synchronous motor would get a bit angry.
    – hobbs
    Aug 23, 2023 at 15:01
1

Nothing bad will happen in bonding both poles of your panel to the same 120 volt phase provided that it is limited to 15 amps.

I have two generators (and there's a long story behind that) where one has a maximum of 2000 watts and the other 6500 watts. The smaller generator outputs 120 volts and to attach that to the house for power I bond the two phases. The larger generator outputs 240 volts, two 120 poles out of phase like the utility power. Perhaps I'm pushing the limits a bit on the smaller generator as it could provide a bit more than the 15 amps that would otherwise trip an overload on a MWBC but if there's so much as 2 amps somewhere else on that 200 amp panel there's going to be a trip on the generator before there is damage to the wires.

Also, don't forget that it is not like a 15 amp breaker is fine with 14.9 amps and then trips instantly if that spikes to 15.1 amps. There's a variation on the trip levels among the breakers based on manufacturing, wear, and so on. This is in part why there is that 80%/125% rule for heating loads. The trip current on a 15 amp breaker is not likely to be exactly 15 amps, it is going to be some small margin above that to prevent nuisance trips but not so much higher that safety isn't maintained by letting any wires or contacts get too hot.

I expect that any generator that gets approval from UL, DOE, FCC, EPA, ATF, FAA, WTF, BBQ, and whichever other TLA that comes along will have a 120-0-120 output on a 4 prong outlet if it was sold in the USA and has an output greater than is safe from a NEMA 5-15R. Edit to add: I realized later that by being so emphatic I've crossed the line into hyperbole so I'll clarify this in my summary at the end.

Any 240 volt load on the panel will see zero volts. Given that there could be some small difference in resistance of the electrical path between where the panel has both poles bonded and the various loads branch off there could be some small voltage showing. This would hardly damage the large motors and resistance heaters seen using 240 volt supplies to operate, they'd likely see voltages like that from all kinds of weird induction and such.

A 240 volt load like an air conditioning unit might have 3 wires to it but the ground wire is not (or should not be if the thing is still intact) part of the circuit, any voltage difference between that and either pole (within reason) does not matter. On one pole is one end of a very sturdy electromagnet, heating element, or something, and on the other pole is the other end of this very sturdy piece of equipment. To get the high power flow through these devices there's a low resistance, and if there's someone trying to turn these things on while the two 120 volt poles are in phase the voltage will only get lower. The lower the voltage the less current, the less current the less power. That voltage I refer to is, again, the voltage between the two poles and not the 120 volt input and so will be very very low, as in fractions of a volt. Some current might flow but it would be very small, not enough to make any motor move and not enough to produce any meaningful heating.

If the total current input is limited to less than 15 amps by a circuit breaker at the generator then even MWBC circuits will operate safely. Given some margins for error, "phantom loads" around the house sucking up a few amps, and so on I'd think even a 20 amp or 25 amp breaker is safe. But then what I consider safe for my house might not fly for you and your insurance company so it's on you if taking my advice breaks something. I do want to spell out why bonding the poles is considered a safe practice and done routinely. Generator power is a temporary situation, done in an emergent situation, so there's going to be some safety rules that get set aside. Do know when you are breaking the rules, why one rule overrides the others, and what is at risk if there's another emergent situation that comes up to cause a reevaluation of the risks.

With a generator producing 120 volts at a maximum of 15 amps there's no issues with 240 volt loads or MWBC, the circuit breaker (or GFCI if it has one) on the generator should trip before anything in the load panel trips.

Edit to add: For those that believe I was hyperbolic before I'll emphasize again my important caveat on what would make this safe, there's a 15 amp breaker at the generator. Also, there's a at least a half dozen reasons to argue this is safe if the breaker at the generator is 20, 25, or maybe even 30 amps. The important detail is knowing why bonding the 120 volt poles could cause a safety issue, by knowing the cause there's ways people can mitigate against any hazard. If you don't know when and why bonding the two 120 volt poles on the breaker panel is safe then it may be wise to not do so. If you know why it is not safe then that will inform you on how to make it safe.

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  • 4
    "Also, don't forget that a 15 amp breaker is fine with 14.9 amps and then trips instantly if that spikes to 15.1 amps" - That's not how breakers work in the real world. Typical Square D breakers need 20x rated current to trip within 1 sec, 5x rated current to trip in 10 sec, 2x to trip in 1 min, 1.2x takes about 5 min. See tripping curve (pdf) here: download.schneider-electric.com/…
    – MTA
    Aug 22, 2023 at 12:11
  • @MTA That's not how I intended to write that, I meant quite the opposite. I must have had a brain fart in the middle of typing that. Answer edited for clarity and correctness. Thanks.
    – MacGuffin
    Aug 22, 2023 at 14:44
  • I expect that any generator that gets approval from UL, DOE, FCC, EPA, ATF, FAA, WTF, BBQ, and whichever other TLA that comes along will have a 120-0-120 output on a 4 prong outlet if it was sold in the USA and has an output greater than is safe from a NEMA 5-15R. Actually, no that is not the case. In a quick search I have found plenty, such as this Westinghouse from Home Depot. Basically, these are based on you either Aug 22, 2023 at 14:53
  • 1
    hooking up a bunch of standard 120V stuff or connecting to a 30A 120V RV connection ("travel trailer"). < 2400W and you're good with a 20A 120V/120V/don't worry about the neutral because you can't overload. >5000W (and in many cases down to 4000W) and you almost always end up with 240V (e.g., a 30A 240V connection is good for up to 7200W). The problem is in-between. Aug 22, 2023 at 14:56
  • 1
    @manassehkatz-Moving2Codidact That's a fair point that bonding the poles on a 30 amp 120V generator to a two pole 240 volt panel with MWBC could be considered a safety problem. I'd have to think about how to reword that in the answer to be clearer on how and when such safety issues could arise. It would be somewhere in that 2000 to 4000 watt range that generators could reasonably not have a 4 pole outlet and be able to produce power that could be unsafe to bond the poles.
    – MacGuffin
    Aug 22, 2023 at 15:18

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