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In a question related to plugging in a device with a 6-20P Plug (240V Ground + 2 Hots) into a 10-30R (120/240V Neutral + 2 Hots) receptacle, I suggested replacing the outlet with a 6-20R and using the existing Neutral wire as a ground (moving it to the appropriate ground bus if necessary) by marking it as a ground on both ends.

I know it's ok to mark white wire with black tape or a marker when wiring switches, but I can't find any reference saying that it's ok to remark white or black (or any other color) as a grounding conductor.

Is it ever code compliant to mark small gauge (i.e. 10 to 14 AWG) insulated wire as a ground, or is it always required to be bare or green? I know there may be different rules for larger gauge wire, so this question is mainly about smaller gauge wire.

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There's also the possibility that the circuit was wired with a type SE cable, where the combined neutral and EGC is the bare wire... – ThreePhaseEel Jan 24 at 19:48
    
... then the least terrible hack is to use the bare conductor as a ground and not a neutral. Perhaps if they are going to another main panel (240/480 to 120-240 transformers can be had cheap) SE might be appropriate. – Harper Jan 25 at 1:19
    
How is using the bare conductor as a ground a "hack" when connected to the grounding terminal of a 6-20P and connected to the grounding busbar at the panel (or the shared neutral/ground bus bar)? Isn't that a 100% appropriate configuration? – Johnny Jan 25 at 1:34
    
I agree, it's 100% right if the cable is rated for that application. The SE caught me off guard, I've never heard of 10 gauge SE. – Harper Jan 25 at 1:49
    
@WolfHarper -- my understanding is that 10/anything or larger NM wasn't available back in the 40s/50s when the 250.140 exception was written, so SE was used instead. – ThreePhaseEel Jan 25 at 5:23
up vote 7 down vote accepted

To my understanding, no.

250.119 Identification of Equipment Grounding Conductors. Unless required elsewhere in this Code, equipment grounding conductors shall be permitted to be bare, covered, or insulated. Individually covered or insulated equipment grounding conductors shall have a continuous outer finish that is either green or green with one or more yellow stripes except as permitted in this section. Conductors with insulation or individual covering that is green, green with one or more yellow stripes, or otherwise identified as permitted by this section shall not be used for ungrounded or grounded circuit conductors.

Note that 250.119 (A) and (B) do allow marking the wire green at the terminations for wires larger than 6 AWG.

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At the end of the day it's about safety. So it pays to account for the user's force-of-will. If a code violation of one type or another is inevitable, it is a matter of least harm.

My first read of "Continuous outer covering" was "more than the usual band of black tape you find on switch loops", e.g. covering the wire with many spiraling loops of electrical tape, paint, or shrink tubing, to the full length it is visible in a box or legal splicing point. With the notion that the mark survive the kind of manhandling that often removes that black band. That may not be what code means, but now, back to our end-user who is going to solve his problem by some code violation or another.

The rationale of code allowing exception for very heavy wire seems to be that very heavy wire is expensive. Well, if General Motors can't afford to replace a 0000ga. wire simply because it is not green, maybe Joe Poor can't afford to replace a 12ga.

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AFAIK, >6 gauge wire only comes in black, that's why there's an exception. – Mazura Jan 25 at 1:55
    
Oh, they'll cheerfully make green for you if you pay them enough money! The point is, nobody wants to go to that expense. – Harper Jan 25 at 5:17

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