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Can I use 14 AWG pigtails on a 20 amp 12 AWG circuit with 15 amp receptacles?

This is follow up to this question asked by another user. I didn't (fully understand) the answers and wanted to clarify and get further input (but can't post comments yet).

I find that 12 awg wire in boxes quickly gets crowded and 12 awg tw wire can be difficult to bend, creating a potential hazard. Pigtails are often necessary for connecting multiple devices (ex. a switch and two outlets on same circuit) 12 gauge TW (NM-B) rated for 25 amps. 14 gauge THHN rated for 25amps (NEC 310-16) and retains better value in higher ambient ranges.

I like the idea of using 14 gauge thhn solid for pigtails within the box. It will make room and is more easily managed. I don't see any NEC conflicts. Solid, not stranded, because many devices use screws instead of clamps. Stranded for clamps and wing nuts is even better.

Question: Am I missing something or is the use of THHN 14 awg pigtails allowable on a 20 amp, tw fed branch circuit within a box? If so, what are the down-sides?

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  • Interesting, the 310-16 tables specify ampacities, but explicitly reference the overcurrent tables in 240 that specify lower ampacities that take precedence. I'm not an electrician, I wonder what the 310-16 tables are used for if wires must always be protected at much lower ratings?
    – jay613
    Dec 9, 2023 at 19:15
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    @jay613 310.16 is used for derating (fill and ambient temp), and as specified in 240.4(D) the table (G) are exempt from (D). For instance a 13A min circ ampacity Air Condittioner or motor needs 125%, so 16.25A rating. If using NM you are llimited to 60°C by 334.80 to #12, but THWN you could still use #14 since it's 75°C rated for 20A. Dec 9, 2023 at 21:27
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    "You didn't care for the answers" Well, code doesn't care what you care for, or not. The answer to the other question is correct.
    – Ecnerwal
    Dec 9, 2023 at 22:46
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    I appreciated NoSparksPlease's answer. It showed depth of understanding of both the code and the theory and gave me the direction I needed to do further research and understand, which is what I was asking for. You were right to call out my flippancy to the other question. I should have been more respectful.
    – RG Hughes
    Dec 9, 2023 at 23:09

3 Answers 3

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It is not allowed.

(D) Small Conductors Unless specifically permitted in 240.4(E) or (G), the overcurrent protection shall not exceed that required by 240.4(D)(1) through (D)(8) after any correction factors for ambient temperature and number of conductors have been applied.

(4) 14 AWG Copper 15 amperes

I've read that UL/CSA/ETL Listing specs that 15A devices with screw terminals are 20A feed through rated, but I don't have access to those specs. Also 15A stab-backs that have holes sized to reject #12 are only Listed for 15A circuits.

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  • Agreed: NEC does not allow 14 gauge wire in a 20 amp circuit. I dug into the derating and found a few articles that further clarify, including one that states in a comment that NM-B wires utilize thhn insulation and are de-rated based on the jacket. diy.stackexchange.com/questions/76859/nm-b-vs-thhn-amp-capacity. Thanks again for providing enough information to help understand the 'why' rather than just 'thou shall not'. It makes understanding the NEC rules a bit easier.
    – RG Hughes
    Dec 11, 2023 at 13:54
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and 12 awg tw wire can be difficult to bend, creating a potential hazard.

No, it doesn't create a hazard at all. There's a difference between an inconvenience and a hazard. If you want more working space, use larger boxes. If you don't want to wrangle #12, install 15A circuits or do what I do, use stranded wire.

12 gauge TW (NM-B) rated for 25 amps. 14 gauge THHN rated for 25amps (NEC 310-16) and retains better value in higher ambient ranges.

That is completely false. See NEC 240.4(D).

I like the idea of using 14 gauge thhn solid for pigtails within the box.

I like the idea of not paying taxes.

It will make room and is more easily managed. I don't see any NEC conflicts.

Because you are "selectively interpreting" the NEC, only considering the clauses which agree with you. This is the "alternative facts" sort of method that certain political parties use to advance That Which Is Not So. This is simply flat wrong.

What I use is #12 THHN stranded. Lovely stuff and much more flexible than #14 solid. But you must be selective about the type of receptacle terminations you place it on.

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As already stated in other posts and in another answer here: NOT ALLOWED.

But you have actually hit on another very common problem: box sizes.

There are very specific rules for calculating the amount of space required for wires/cables and devices (switches, receptacles, etc.). While theoretically every installation always follows all the rules (ha!), it is certainly quite plausible that a box started out large enough but is no longer large enough. For example, a box that originally had one cable + duplex receptacle and now has additional cables to power other devices in other boxes from the same circuit.

There are also situations where a box was large enough with the original device but is not large enough with replacements. I have found that in my house when replacing very old ungrounded receptacles with modern good quality grounded receptacles - the new receptacles simply won't fit in the old boxes. I replaced the boxes to solve the problem. This is a particularly common problem with GFCI/receptacles. So if you have box with a regular receptacle and just enough space and replace it with a GFCI/receptacle and an extra cable to connect to another receptacle you can easily find yourself out of room. Yes, using 14 AWG wire for pigtails will help, but if it is really that close, just replace the box.

What? Replace the box? That will cost a lot of money and time! No, it won't. A good deep steel box might cost $2 - $4 at Home Depot/Lowes/etc., a little more at a small hardware store, a little less in bulk or at an electrical supply house (if they give you the contractor pricing). As long as your old boxes are mounted on studs, this is really easy to do.

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    Very good point, and I have done this as well. At the risk of rebuke for my use of plastic, the Southwire boxes with the screws on the inside make it simple to get a deeper footprint (18.5 cu in) and solid install, or for junctions where access is limited.
    – RG Hughes
    Dec 10, 2023 at 2:01

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