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I have 20 Amp power run to part of my house. The conduit that carries it runs down one of the walls of my garage. I've cut the conduit, inserted a metal junction box, and intend on putting in a single 20 amp outlet in my garage for power tools, etc.

The conduit carried four wires: blue, yellow, gray, gray. As it turns out, I'd thought about this as I turned off two breaker switches, but yes, the conduit is actually carrying two 20 amp circuits. I cut the wires too quick without thinking about it though, and now find myself wondering if it matters which power is matched with which neutral.

I tested by installing the outlet with yellow, and one of the grays, and turned the breakers on (and off) one at a time. I thought that if a lamp plugged in to the outlet turned on, I would have my match. Problem is, I know which breaker is for yellow and blue respectively now, but the lamp works with either neutral.

Given that this is two distinct circuits, AND one of the circuits powers a room with expensive electronics, I'm paranoid.

So, does it matter which neutral is connected in pair with which power? If so, how do I test and figure out which neutral is which?

  • Do you have a clamp-on meter or the ability to borrow/rent one? – ThreePhaseEel Aug 2 '17 at 1:49
  • What is the cost difference between wiring a house in steel conduit vs. NM? Some years back I walked through a mansion under construction in Highland Park, a posh inburb of Dallas, that has always required all houses to be wired in steel conduit, and thought wow and nice. – Jim Stewart Aug 2 '17 at 10:39
  • @JimStewart -- that's a totally different question (and probably beyond the scope of this site -- Chicago code, most notably, requires conduit as well) – ThreePhaseEel Aug 2 '17 at 11:43
  • The OP wrote "The conduit carried four wires: blue, yellow, gray, gray." Is the use of these different colored wires normal for wiring in conduit? Why wouldn't the neutrals also be wired in different colors? – Jim Stewart Aug 2 '17 at 12:45
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Yes, it matters. It is vital to pair the neutrals with their partner hots. You should have marked the wires at the time you installed them, either by tagging the wires with tape, or by bundling the pairs with tape, or any other method that makes sense.

Now, your best bet is to document carefully which outlets are on which hots. Then, pull one of the neutrals off the neutral rail and insulate it. Make sure that all the outlets on one circuit do work, and all the outlets on the other circuit do not work. Now you know the still-connected neutral is associated with that hot. Mark it!

Do not allow a situation where some outlets on hot A return on neutral A, and other outlets on hot A return on neutral B. That is a wiring defect that should be corrected ASAP because it will result in the neutral being overloaded. Neutrals do not have circuit breakers, so the system absolutely depends on neutrals not serving two hots.

  • is it stnadard practice when wiring in conduit to have labels in the panel to indicate which neutral is paired with which hot? – Jim Stewart Aug 2 '17 at 2:35
  • @JimStewart you would still need to tape, paint or otherwise place a durable mark on the wires themselves, or use tape to bundle the pairs. There is no way to do this without marking the wires, because you can't distinguish two random gray wires coming out of a conduit. – Harper Aug 2 '17 at 2:38
  • That is exactly what I was asking about. I have only worked with NM. When pulling wires through conduit is it standard practice for the electrician to apply identifying bands (colored or numbered) to the ends of the wires before they are combined in a bundle and then "hidden" inside conduit? – Jim Stewart Aug 2 '17 at 10:14
  • Yes, you must do that before installation, or ring it out after. You either mark the wire, or bundle the pairs with tape, or both. And you will want to use markers that endure, so the next guy doesn't find your withered markers in the bottom of the panel. It's difficult to imagine not marking wires, as it would make things so much harder later. Sometimes you make the wires too long, and have to "move" a mark by remarking before you cut. – Harper Aug 2 '17 at 14:06
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What really matters is that two hot conductors, fed through two different circuit breakers, do not share a neutral if the two hot conductors are on the same pole (or the same "phase leg" as some folks call it). If that happens, there is a potential for the neutral to carry up to twice as much current as it is rated for, because the amperage from the hot conductors is additive, which could result in equipment problems or even a fire. If the hots are on different legs (different poles), they can share a neutral because the power on each leg is 180 degrees out of phase with the power on the other, and they cancel each other out like any other example of wave interference, like sound waves. In fact electricity moving along a wire is an electromagnetic wave essentially the same as a light wave moving through the air. Anyway, the neutral will carry the imbalance between the two hots--always at or below its capacity. You also must use a 2-pole breaker (or an adjacent pair of handle-tied breakers or another mechanism allowed by the electrical code in your area) and connect both hots to it in this case, so that they are both de-energized if the breaker trips.

Since you have 2 hots and 2 neutrals, just assume they are two entirely separate circuits which should NOT share a neutral.

Having said all of the foregoing, the neutrals in the conduit are interchangeable, presuming that a) they are the same size and b) they both go back to the same panel. They both connect to the same common bus in the panel. It doesn't matter which one you pair with which hot conductor, as long as it is paired with one and only one hot conductor.

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Presumably before cutting the wires you powered down all the expensive equipment and shut off the breakers, right? Now plug an extension cord into a receptacle that powers the equipment and bring the other end of the extension cord to the place you have cut the wire. Use a VOM in resistance mode to check the continuity of the hot slot in the cord with each of the two (distal) hot ends to identify which hot goes to the electronics.

Similiarly use continuity of the neutral slot in the cord to identify which of the two (distal) neutrals goes to the electronics. Of course, now label these.

Now consider the two proximal hots and the two proximal neutrals. Presumably the breakers are labeled or you remember which was used for the electronics so that takes care of identifying the hots.

If there are receptacles between the cut and the panel, you can use that with the extension cord to identify the neutral end that goes to that receptacle. If there is no receptacle and if all the four current carrying wires of these two circuits stay in the same conduit to the panel, it does not matter which neutral you use for the electronics room.

Are the neutrals labelled in the panel? If they are, you can of course use that.

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