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My understanding of color coding allowed by NEC is:

  • Ground: Bare, Green or Yellow/Green
  • Neutral: White or grey
  • Hot: All other colors

If I need to run 3 distinct circuits in conduit, I might choose:

  • Ground: Green
    • Of course, if I'm using metal conduit & boxen, I can skip the ground wire entirely and use the conduit for the ground.
  • Hot: Black, Red, Blue
  • Neutral: White, Grey, and ???

I can pair the White/Black and the Grey/Red, but what do I pair with the Blue to provide a neutral for it?

In thinking about this, I believe that a MWBC could be a way around this, but I'm not certain if that technically only applies to cable (i.e NM-B/"Romex"), or if MWBCs can be used in conduit-based wiring as well. Additionally, from what I've been reading here, there's a strong trend away from them, especially for GFCI/AFCI protected circuits since handle-tied *FCI breakers are somewhat unobtanium.

Note: This is purely a theoretical question. I do not have an actual application for this at this time. I'm simply trying to learn ahead of a home addition/remodel that's coming up.

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  • can u get your hands on dark and light greys? Yes it won't be as obvious as green/blue but still differentiable
    – Hobbamok
    Jan 12 at 12:29
  • 1
    @Hobbamok as noted, this is theoretical, so I don't really know. I'd guess probably not, or some of the very experienced electricians who have provided answers already would have mentioned it.
    – FreeMan
    Jan 12 at 12:31
  • Or gray for one and grey for another. Jan 12 at 14:28
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    @Hobbamok -- "slate" THHN is not unheardof, no idea if it's legal for a neutral though Jan 14 at 4:10
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You can mark the white or gray neutral wires with colored tape. Unlike marking a switch loop wire in cable (where it changes white from neutral to hot), this does not change the function but is simply an identifier. Real simple:

  • Black + White or Black + White-with-black-tape
  • Red + White-with-red-tape
  • Yellow + White-with-yellow-tape
  • Blue + White-with-blue-tape

Of course, the colors don't have to match like that, but that would make it super easy to keep track of things.

With only two different circuits, yes white & gray as two different neutrals makes sense. Once you get to taping things, it is arguably easier to use white for all neutrals instead of having two different colors, but that depends on how much wiring you do.

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  • There seems to be consensus that marking an grounded conductor (which has to be white or grey) in conduit is acceptable for identification purposes, and does not change the function of the wire (i.e. does not make the conductor and ungrounded ("hot") conductor), but I can't find this in the Code. Can you provide a citation? Thanks for the help! Mar 22 at 16:07
  • Yes, there is that consensus. But I'm not the code expert - I just follow what Harper, ThreePhaseEel and EdBeal tell me to do. One of them should be able to cite code... Mar 22 at 16:41
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You're required to clearly mark pairs

So the "next guy" can figure out what you did.

It's all well and good to use gray as a color. But that alone is not enough unless the usage is obvious.

Take a very typical situation you might find in my work: the colors are orange, purple, white and gray. Which hot goes with which neutral? --- ???? --- you can't tell. That's not good enough.

You might get away with black-purple-white-gray. Since black and white are an obvious pairing.

However, realistically, you simply need to either wrap the pairs at both ends, or use phase tape to mark them, or ink, or paint. As manassehkatz points out, you have more liberty to do markings in THHN wire than you do in cable.

Mark them the same at both ends, as always.

Be consistent

However, there is a Code requirement that whatever you do, you be consistent throughout your facility. For instance you'll never find any work of mine where purple/gray is not either GFCI protected downline, or 0-10V dimming. (I picked the GFCI protected pair before I knew it was a standard for 0-10V dimming, but I trust they won't be confused).

I quite agree with Ed Beal that there's no reason to own more than black and white... (and green if you like non-metal conduit)... you can "git-r-dun" just with phase tape, of which I own all 10 colors. (black + a cheap 5-pack of red-blue-yellow-green-white + individual brown, orange, purple and gray).

An example

Mind you I also own 10 colors of wire, but that's me being fastidious. But a little color goes a long way. Consider a piccadillo I have with 2 conduits to a machine shop:

  • Conduit 1: #10 Red-Black... #10 Red-Black... #10 Red-Black... and #10 Red-Black.

  • Conduit 2: #12 Black-White... #12 Black-White... #12 Blue-White... and #12 Black-White.

They used numbered cloth phase tape to indicate circuits, and it's all fallen to the bottom of the box. Ugh. So I need to rearrange these so that it's easier to tell which circuit is which. My plan is:

  • #10 Black-Black..... #10 Red-Red..... #12 Black-White..... #12 Red-Gray
  • #10 Black-Black..... #10 Red-Red..... #12 Black-White..... #12 Blue-Gray

See, being 240V circuits, there's no need to distinguish L1 hot from L2 hot. So they can simply be the same color. Bolds are the wires that are either changed or simply marked with phase tape. Now, the presence or absence of blue will tell you which conduit you are in so conduit 1 black-black can be distinguished from conduit 2 black-black.

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  • 1
    I understand that colours are obvious - is there anything against using modern heatshrink printed labels? They're more permanent than sticky tape.
    – Criggie
    Jan 12 at 7:44
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I run multiple circuits with all black, white and ground all the time. I tape the black conductors a specific color at each end. I also tape the white at both ends, yes you can use multiple colors but tape is cheaper for small jobs than multiple spools of different colors. I usually mark them close to the end and a foot or more back on both ends, it is rare for the tape to get pulled off, another method is paint pens put 1 ,2 or 3 stripes around the wire and that matches with circuit 1,2 or 3 each branch circuit is unique. I caution not to tape the 3 wires together except at the start because the tape holds the wires together and now it wants to stay straight so it is harder to pull and each corner hangs on the tape so it makes pulling much more difficult. Most electricians reserve white for 120v work and gray for 277v this way the voltage of the circuit can be identified, but it is ok to use both gray and white on either voltage neutral.

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  • I started at a young age using those pulling baskets and always used solid but never used backstabs. Started using stranded when doing instrumentation panels... never went back to stranded, lol +1
    – JACK
    Jan 12 at 13:07
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Taping or labeling works

Using tape to redesignate a white wire as a hot only is usable in cable -- when working in conduit, a taped white wire still means "neutral", so you can use matching colored tape to "phase tape" the neutrals to match their corresponding hot wires. If tape and colors aren't sufficient or suitable for some reason, you can also use self-adhesive wiring labels (you can get books of these anywhere electrical supplies are sold) to mark which wires go with which circuit, as long as you're consistent about what you're doing! You can also use heat-shrinkable labels or heat-shrink tubing instead of tape, if you're up for heat-shrinking things and want something more permanent than a sticky label.

If you're hooked up, striped THHN is also a thing

If you have a really good supply house and are willing to spend a bit more, though, another option is to get striped THHN wire. This is typically available as a white wire with a colored stripe on it, usually black, red, or blue, although other stripe colors and even color combinations may be available by special order. This has the advantage that it's a lot harder to screw up than taping, especially if you're pulling a lot of wire for your job. Note also that this is how multi-circuit cables (NM, MC) are often color-coded, at least in sizes typically found in light-duty wiring. (Fat "homerun" or "control" type MC cables with >3 circuits worth of wire in them use a different scheme, similar to what's done in industrial tray cables.)

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I've run into that a few times and just used white stranded for the third one. A penny a foot extra but had plenty of every type of wire and didn't want to mess with taping.

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  • I use stranded for everything in conduit easier to pull and it doesn't break at the strip point like solid wire will. But yes a penny per foot more might be a game changer but when pulling multiples I know few electricians that pull solid wire and those are usually using backstabs so my opinion of them is low to start with.
    – Ed Beal
    Jan 11 at 17:43

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