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I'm rewiring a a small part of a wall and I'm confused about a combination of the NEC and the products that I'm able to find.

I'm working on a 20 amp circuit (with 12g wire of course). I'm connecting 4 receptacles each in their own box, 15 amp receptacles for normalcy. But the terminals don't seem to accept 12g wire, even on the crews. My first reaction was to make pigtails 14g pigtails, so that the 14g tap goes directly to the receptacle and is not used to feed the rest of the circuit. 12g in the box, 12g out of the box, and 14g inside the box only.

Is that type of splice allowed, or should I look harder for a 15 amp outlet that can more easily accept 12g wire?

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I'll have to look through the code for the exact section, but I'm fairly sure there are situations were #14 copper can be used on a 20 ampere circuit. Though these situations are likely very specific, and as a general rule #12 should be used. –  Tester101 Feb 1 at 14:29
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4 Answers

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I would say 14 gauge wire anywhere on a 20 amp circuit is not OK. The purpose of the breaker is to cut off power before the wiring overheats. If you plug in several devices on an outlet that total 20 amps, you will exceed the safe working capacity of the 14 gauge wire without tripping the breaker. (You may be under 15 amp for each individual outlet but between the two outlets on a duplex receptacle you can exceed the 15 amp rating of 14 gauge wire).

12 gauge wire is a little tougher to work with but I've never had much of a problem getting it attached to outlets. (Working with the extra stiffness & bulk in the junction box is the issue I've usually noticed.) However, maybe you can pick up some 20 amp receptacles, even if you don't anticipate using any 20A appliances? Obviously a 20A receptacle needs to support 12GA wiring...

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This is the best explanation. I didn't consider that a device could malfunction, or a power strip could overload and draw more than 15 a across the 14g wire. –  andyortlieb Feb 1 at 13:26
    
Henry - You are right and I don't see why this person should use 14 gauge. However when working with Aluminum 12 gauge it is not so easy. I see Tester commented some exact code but inspectors in our area in most cases have had no issue with using 14 to pigtail on a 20 AMP circuit - and if they do we simply change out the breaker to 15 AMP. Fact is 12 gauge copper will snap 40 year old Aluminum to pieces. Not sure if this is a regional thing or not. –  DMoore Feb 4 at 4:44
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National Electrical Code 2008

Article 210 Branch Circuits

II. Branch-Circuit Ratings

210.19 Conductors — Minimum Ampacity and Size.

(A) Branch Circuits Not More Than 600 Volts.

(2) Multioutlet Branch Circuits. Conductors of branch circuits supplying more than one receptacle for cord-and-plug connected portable loads shall have an ampacity of not less than the rating of the branch circuit.

Which means in your case, you'll have to use 12 AWG conductors for attaching the receptacle.
note: A duplex receptacle actually counts as two receptacles according to NEC

However, there are situation were 14 AWG conductors can be used on a 20 ampere circuit.

(4) Other Loads. Branch-circuit conductors that supply loads other than those specified in 210.2 and other than cooking appliances as covered in 210.19(A)(3) shall have an ampacity sufficient for the loads served and shall not be smaller than 14 AWG.

Exception No. 1: Tap conductors shall have an ampacity sufficient for the load served. In addition, they shall have an ampacity of not less than 15 for circuits rated less than 40 amperes and not less than 20 for circuits rated at 40 or 50 amperes and only where these tap conductors supply any of the following loads:
(a) Individual lampholders or luminaires with taps extending not longer than 450 mm (18 in.) beyond any portion of the lampholder or luminaire.
(b) A luminaire having tap conductors as provided in 410.117.
(c) Individual outlets, other than receptacle outlets, with taps not over 450 mm (18 in.) long.
(d) Infrared lamp industrial heating appliances.
(e) Nonheating leads of deicing and snow-melting cables and mats.

Check the Device

Back of a 15 ampere rated tamper resistant duplex receptacle
Click for larger view

Push in #14 CU solid wire for 15A branch circuit only.

Installation Screw Terminal > #14 - #12 AWG CU Wire Only.

Notice on the back of this 125V 15A duplex receptacle, it states that you can use 14 AWG or 12 AWG copper wire when terminating at the screw terminals, but that 14 AWG copper must be used when terminating at the push in terminals. It also says that if you're using the push in terminal, and 14 AWG copper, it can only be used on a 15 ampere circuit. If you're installing this on a 20 ampere circuit, with 12 AWG wire, you'll have to use the screw terminals.

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NEC 240.4(D)(3) states that 14 AWG must be protected at 15A. You can not use 14 AWG anywhere on a circuit that has a 20A breaker.

If you are putting 15 amp receptacles on a 20 amp circuit with 12 gauge wire, then you MUST use the screw terminals, not the back stab terminals. I wouldn't use those even with 14 gauge wire as I've seen the wires bend and break right at the point where the shielding was stripped and the plug goes dead or the breaker trips. Just use the side terminals. Make sure that the wires are wrapped clockwise around the screw so that tightening the screws pulls the wire tighter and not so it's pushing the wire out.

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I echo the sentiment: don't use the backstab. But if you dislike screw terminals, for a bit more money you can get backwire outlets, which have a really nice easy connection method that works with 12 gauge wire. –  Bryce Feb 1 at 6:11
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I think you've got the wrong code section. NEC 2008,2011,2014 240.4(D)(2) is for 16 AWG copper. 240.4(D)(3) is what you're looking for ("14 AWG Copper. 15 amperes"). –  Tester101 Feb 3 at 12:20
    
Good catch, Tester101. Fixed to NEC 240.4(D)(3) for 14 AWG. –  dslake Feb 3 at 19:36
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There are VERY few times when you can have #14 on a 20A breaker, and this is NOT one of them.

Also, as stated, NEVER use the quick-wire holes in the back of devices. They are extremely failure prone and are simply NOT worth the 2 seconds in time they save.

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