I have two mains lines coming into my detached garage. Is there any way to test if these are on opposite "phases" (I don't know what the correct word is) so that I can use them for 208/240 outlet?

Update: I think poll is the word I am looking for. Not phases.

  • Do you have a voltmeter/multimeter? – ThreePhaseEel Nov 12 '17 at 16:18
  • You may not be able to do what you're asking about anyway. Are these circuits arriving in the garage via separate cables? Each with its own Neutral and ground? Or was this wired with /3 cable where both circuits are sharing the same cable? Just having two circuits on opposite phases doesn't solve your problem. – Tyson Nov 12 '17 at 16:25
  • To share the neutral and make a 208/240 volt circuit out of these, the hot conductors would have to be (as in MUST) not only on different phases (if this is 3-phase power) or on opposite poles (and thus out of phase, if this is single-phase/split-phase power). The neutrals would have to be bonded both in the junction box in the wall, and in the service panel. The hots also must be on a two-pole (single phase) or 3 pole (3 phase) breaker, and some would recommend a 3 or 4 pole breaker so that you can also break the neutral. – Craig Nov 12 '17 at 17:12
  • It would be 208V if you have three-phase service, or 240V if you have single-phase (so-called split phase) service, which is typical of residential power in the U.S. and Canada. – Craig Nov 12 '17 at 17:15
  • @craig and they'd have to be in the same cable. – Harper - Reinstate Monica Nov 12 '17 at 17:17

Placing them on opposite poles is stupid easy but that doesn't help you.

You can't gang two cables to support the same load. It's not allowed and it probably won't work.

As an aside, it's a little weird to have two parallel cables to the same outbuilding. This, also, is not allowed unless the circuits are of different voltages or differently controlled (e.g. A switched circuit for a light). This prohibition may be your salvation: If both cables are /2, their only options are 120V or 240V, so there'd need to be one of each.

So that's the answer, rewire one of the circuits to be a 240V circuit.

If you need to switch a light, then you either use wireless smart switches of some kind, or fit motion sensors, or fit efficient LEDs and just leave them on 24x7.

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  • They sell devices to create 240V outlets by plugging into two 110/120V outlets but the outlets have to be on opposite polls. Also, I thought homes in North America only received two two 110/120V polls and bonded them to create 220V outlets for stove and dryer? – jetole Nov 13 '17 at 18:29
  • @jetole "They" sell them illegally, or to be more precise, their use is illegal. It is also illegal to use 2 cables for a single load. Paralleling neutral is also illegal. "Illegal" means your fire insurance won't pay out, and the DA may go after you for manslaughter. They will work electrically, so they "seem ok" but are dangerous. When they connect dryer/stove they use a single cable with an extra wire, called /3. It contains ground, neutral and both hots. – Harper - Reinstate Monica Nov 13 '17 at 19:40
  • Are you talking about Romex or NM 12/2, 12/3, 14/2, 14/3, etc? Now I don't want to build this from two outlets. I just saw that it can be done which made me thing. I want to do this from the breaker box using two bound breakers on opposite polls, which sounds to me like what you are saying that the dryer outlet already does where it has a hot on one breaker and a hot on another breaker and they are on separate (but bound) breakers and on opposite polls. @Harper – jetole Nov 13 '17 at 22:03
  • Just to clarify that the two cables I want to ground are the two hots from the house breaker box to the garage mains to the garage breaker box and I want to bind them in the breaker box using an NM 12/3 on two bound breakers. – jetole Nov 13 '17 at 22:06
  • Well, you're laying an XY problem on us, you are not discussing the actual problem you are trying to solve (X), but focusing on a particular method (Y) and asking questions about the method instead of the problem. That never gives good results. Related, trying to understand it by asking questions, but not knowing which questions to ask. I'm sure you can do what you want easily and safely, but you are trying to whittle a solution together from too little knowledge. What is the original problem? – Harper - Reinstate Monica Nov 14 '17 at 2:41

To chime with the above, it is important that the power supply to a separate building come through a single cable or a single conduit. Also this supply should originate from a single 2-pole breaker in the main building's main electrical panel. You will need an unbroken white conductor the right size run between each of the panels neutral buss bars. This white wire must be routed together with the power conductors. Also you need a ground wire properly sized included in the same wiring assembly run between the two panels. If I could correct one thing from Craig, once the neutral, the white wire leaves the main service panel it will never again be bonded to any of the grounding system, the bare of green wire. It will 'float' or stay separate from the ground wires.

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  • Well, it can go as 2+ cables, but only as I describe, separate control, separate voltage or a number of other weird exceptions. – Harper - Reinstate Monica Nov 12 '17 at 23:23
  • The circuits are on two different breakers in the main breaker box in the home that lead to the garage but I haven't removed the breaker box cover in the home and even if I did I wouldn't know how to check if they are on different polls. – jetole Nov 13 '17 at 18:30

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