3-Conductor wire and backup circuits

[craig72]

Hello everyone!
My name is Craig. New to the board today :)
I have a question. I am looking to install a 5240 and have a question about picking which circuits to backup. In one of the Generac manuals, it stated that if a circuit is wired with 3 conductor wire and shares a return with another circuit, you should backup both circuits or neither, to protect against possible overload of the return.
A lot of my panel was wired like this from the builder (GRRRR!).

My question is: Why would both be required? If I only take one circuit to the transfer switch, how could I possibly overload the return line? The other circuit is not seeing any power, correct? I understand that the circuits are in opposite phase and they cancel each other out, but the return line (being 14awg) is rated to handle 15A, correct?

I'm sure I'll have more questions.
Thanks!

[Kelly Myers]

Without getting into this question to deep, I'll try to explain.

Your panel has 240 volts in it. Two phases of 120v that come together to make 240 volts.

Phase A will be every odd numbered breaker.
Phase B will be every even number breaker.

For instance if you're looking at the first breaker on the top left, it would be phase A, the one below it is phase B, then phase A, and so on. This is why two pole breakers will always yield 240v, because they're deriving power from two different phases.

Now we have to talk about neutrals. For ease of explanation, we will think of the neutral wire as a return wire for the hot wire.

20amp circuit powering a sump pump with its own neutral:

PhaseA, 20amp circuit with a sump pump drawing 15amps.

Phase A hot, and the neutral would both have 15 amps of draw on it.


Now if you have Phase A hot, and a Phase B hot, sharing a neutral (three wire romex) they actually equal out load on the neutral.

Three wire romex setup:
Phase A hot, 15 amp load. Phase B hot, 15 amp load, the neutral load would be 0.

Now for the scary part, if you have Phase A hot, 15 amp load, and accidentally use another Phase A hot with a 15 amp load, you end up with 30 amps of load on the neutral wire, which will burn it up.

Not sure if this makes sense to you, but if you have any questions, ask, and I'll try to best explain it. Guardian is making sure that you keep proper phasing when you hook up your transfer switch.

[craig72]

Now if you have Phase A hot, and a Phase B hot, sharing a neutral (three wire romex) they actually equal out load on the neutral.

Three wire romex setup:
Phase A hot, 15 amp load. Phase B hot, 15 amp load, the neutral load would be 0.

Now for the scary part, if you have Phase A hot, 15 amp load, and accidentally use another Phase A hot with a 15 amp load, you end up with 30 amps of load on the neutral wire, which will burn it up.

Not sure if this makes sense to you, but if you have any questions, ask, and I'll try to best explain it. Guardian is making sure that you keep proper phasing when you hook up your transfer switch.

Hi Kelly.
Thanks for the response. I understand that if I wanted to backup both circuits on a 3 conductor set-up, I would need to keep them on opposite legs in the transfer switch box. What I dont understand is why I have to take both if I'm only interested in backing up one of the circuits.
Example: On Phase A (breaker 5, black wire) is my freezer. On Phase B (breaker 6, red wire) is a circuit of outlets that I dont care to backup. According to Guardian, If I want to pull breaker 5 over, I have to take breaker 6 as well. Why is that? I dont understand how I could run the risk of overloading the return if only breaker 5 is powered.
Hope I explained that okay..:o

[Kelly Myers]

Hi Kelly.
Thanks for the response. I understand that if I wanted to backup both circuits on a 3 conductor set-up, I would need to keep them on opposite legs in the transfer switch box. What I dont understand is why I have to take both if I'm only interested in backing up one of the circuits.
Example: On Phase A (breaker 5, black wire) is my freezer. On Phase B (breaker 6, red wire) is a circuit of outlets that I dont care to backup. According to Guardian, If I want to pull breaker 5 over, I have to take breaker 6 as well. Why is that? I dont understand how I could run the risk of overloading the return if only breaker 5 is powered.
Hope I explained that okay..:o

You will be deriving a neutral from one panel and "hot" wires from two different panels which is against NEC.

NEC states you must derive the neutral from the same panel as the branch circuit.

Will it matter for safely reasons?? Probably not.. The neutrals are tied together anyways. But you should follow code.

[banshee]

If the utility restores and your still on generator power the phase could be off between the genset and the circuit still tied to the utility power. If the phase is close or the same as the generator phase you can have the problem Kelly stated with 30amps on the neutral.

[craig72]

Got it, I understand now. I will definitely follow that. Thanks for the feedback!

Next question.

I added a 100A sub panel next to my main panel to have additional circuit capability. Some of the circuits I want to back up are in this sub-panel.

If the 30A breaker for the transfer switch is in my main panel, is it okay to backup circuits from both the main and sub panel?

It seems that the transfer switch is essentially just another sub-panel with backup capability.
Thanks again
Craig