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I'm rewiring a bathroom, and will install a GFCI receptacle on a 20 amp circuit breaker (dedicated to the bathroom only). I am also going to have a fan and vanity light on the same circuit, downstream of the GFCI outlet (the fan is above the tub, thus requiring GFCI protection). Question: downstream of the GFCI outlet, am I required to continue to use 12 AWG cable for the rest of the circuit, or can I switch to 14 AWG? A code citation would be most helpful along with any rationale for why this allowed/not allowed, whether this is a good idea/ terrible idea, etc.

  • My rationale for switching to 14 AWG downstream is that the combination of the fan, fan light, and vanity light won't come anywhere close to 15 amps, so 14 AWG would be sufficient for this portion of the total load on the circuit. The outlet could see more than 15 amps, so it makes sense that it would require the 12 AWG to it. – the_meter413 Mar 5 '15 at 22:32
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    Why would you want to? You're only looking at a few dollars difference in wire costs for an extra 10 foot run of 12 gauge wire. In any case, I don't believe this is allowed under the NEC (I'm sure someone will answer with a definitive answer), if you have a 20amp breaker, the whole circuit needs to be wired 12 gauge or larger. Even if your fan is only rated for 2 amps max, if has a fault and it shorts out, it's going to be pulling the full load of the 20amp breaker until it trips. – Johnny Mar 5 '15 at 22:57
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    One more reason this is a bad idea, because you probably won't be the final owner of this house. Maybe the next owner will come in and think that it's too cold in the bathroom and since he sees that 20A breaker and "knows" that he has a 20A circuit to the fan, he replaces it with a 2000W resistance heater/fan and ends up overloading the 14AWG wire. – Johnny Mar 5 '15 at 23:08
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    You're thinking about this backwards: you pick the circuit & wire size first based on usage, and then you size the breaker to protect the wiring. A 20 amp breaker can't protect 14 GA wire. – Hank Mar 6 '15 at 2:17
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    To expand on @HenryJackson's comment - if the fan motor were to flake out and start drawing 18 amps, the breaker wouldn't trip, but the 14 GA wire would become a hazard. You can't think in terms of 'normal load' here, you've got to think in terms of failure conditions. – Michael Kohne Mar 6 '15 at 2:33
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Yes, all conductors protected by a 20 ampere circuit breaker must be at least 12 AWG copper. There are a few exceptions and loop holes, but none of them apply to your situation.

National Electrical Code 2014

Chapter 2 Wiring and Protection

Article 240 Overcurrent Protection

240.4 Protection of Conductors. Conductors, other than flexible cords, flexible cables, and fixture wires, shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G).

(D) Small Conductors. Unless specifically permitted in 240.4(E) or (G), the overcurrent protection shall not exceed that required by (D)(1) through (D)(7) after any correction factors for ambient temperature and number of conductors have been applied.

(5) 12 AWG Copper. 20 amperes

The rationale here, is that 12 AWG copper conductors are rated for 20 amperes, whereas 14 AWG copper conductors are only rated for 15 amperes. So if you put 20 amperes through a 14 AWG conductor, the conductor could heat to dangerous levels.

That would make it a terrible idea to use 14 AWG copper conductors, on a circuit protected by a 20 ampere circuit breaker.

  • Thanks, @Tester101. That answers my tangential question, too: why don't the luminaires and the fan have to use 12 AWG, too? My understanding is that they are exempt due to the "fixture wires" clause in the main body of 240.4. – the_meter413 Mar 5 '15 at 23:29
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    @the_meter413 Not exactly... The fixture itself will be UL approved, meaning it has passed UL testing (or a sample has). Since the device is actually tested, NEC doesn't apply. Technically if you had an engineer approve the design, you might be able to use 14 AWG conductors. Since it's probably cheaper to just use 12 AWG, it's not worth it to hire an engineer. – Tester101 Mar 5 '15 at 23:38
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    Further on @Tester101 's explanation re luminaires: the amperage they can draw is limited by the nature of the fixture and the bulb max limitation. You would need about 1500 watts before you approached the limitations of a 15 amp circuit. If there are that many bulb sockets, the lamp wiring would likely be heavier by design (and would need to be for UL approval). – bib Mar 6 '15 at 15:15
  • @Tester101 - is it true that an engineer sign off will let you legally deviate from the NEC? It seems like that could result in a lot of non-standard wiring since a home builder might find it worthwhile to hire an engineer to let him save a $100 on a few hundred identical houses by deviating from the NEC in cases exactly like this one. – Johnny Mar 6 '15 at 17:59
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    @Johnny Yes, if an engineer signs off on it and provides the proper documentation, it can override the code. Code is a broad outline of minimum safety standards, there's always a possibility that specific situations can call for a deviation from code. However, the engineer is staking his career, livelihood, reputation, and possibly freedom on these decisions. They're going to be sure any deviation from code is safe. It's not likely you'll find an engineer who'll write up a plan, just to save a contractor a few dollars. – Tester101 Mar 6 '15 at 18:15

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