What happens if you connect the same phase AC (from a generator) to both sides of an electrical panel?

Question

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

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.