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When wiring an indoor switch, receptacle or fixture that has NM-B run into the box, is it legal to use a different type of wire (for example THHN) of the appropriate gauge to make pig-tails?

I can't imagine that this would not be legal, since one would connect NM-B to THHN in a box when transitioning from an interior run in cable to an exterior run in conduit, but just wanted to be sure.

If so, is it critical that the colors match, since, for example, it would be obvious by tracing the wire within the box that a short piece of white sheathed "neutral" wire was connected between a black "hot" wire exiting the box and the terminal on a standard toggle switch. Would putting some appropriate colored electrical tape on the wire be sufficient to remark the wire to be of the appropriate color?

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    But why? If I were the next person to work on the wiring, I'd be really pissed if you chose to use mis-colored pigtails. Spend a couple bucks for "scrap" wire and keep the colors matched. As you suggest, the cladding/bundle type is irrelevant inside a box. – Carl Witthoft Sep 14 '20 at 16:40
  • While the "why" wasn't explicitly stated, if one has run out of scraps of NM-B, then one may not have the correct color available in something else. However, the mismatched colors are confusing, hence the question about using tape. – FreeMan Sep 14 '20 at 16:43
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It's OK to mix types of wire.

There are no exceptions to the color-coding or re-identifying rules for pigtails. For example you could not use a green wire for pigtails with the hot or a black wire for a neutral pigtail. What's clear to you today when you do it may not be clear to the next person looking at the wires folded up in the box.

If you were to re-identify a pigtail by taping the insulation end to end, it's pretty hard to imagine anyone complaining about that, even if it's not strictly compliant.

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Bats has it.

The wire types are not actually different wires - the copper is the same. The difference is insulation.

The wire nut engages to parts of the wire which have no insulation. Hence the insulation type is irrelevant to the wire-nut.

Wire-nuts don't even care about solid vs stranded wire (though other connection types might).

Solid THHN wire is just stupid-cheap by the foot, so having a couple of feet lying around of 2 colors is a good idea. Remember THHN is almost always co-listed as THWN, which is allowed outside.

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    The home stores usually have scraps of THHN laying around in the wire bin that they'll just about give you for nothing... +1 – JACK Sep 14 '20 at 19:05

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