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I am extending a lighting and outlet circuit in a basement. The current setup has the following characteristics

  • 15 amp. circuit breaker (200 amp service)
  • 4 existing lighting fixtures using CFL bulbs (~ 20 W each)
  • 1 existing double duplex outlet box (no regular appliance in use)
  • 14/2 and 14/3 NM cabling (lighting switched, outlets not) throughout
  • less than 100 ft of cabling so far

I plan on adding three lighting fixtures (for LED spots, less than 20W each) and one duplex outlet. I will tap off one of the existing boxes to get both a switched and a constant hot line. Additional cabling will be less than 30 ft.

Is there any reason I cannot use 12/2 and 12/3 NM cabling for this project (I happen to have some on hand)?

Is there any practical benefit (e.g., less voltage drop in this small section of line)?
Is there any disadvantage in doing so, other than higher cost of cabling and the slightly harder handling of thicker wires?

If the cabling were concealed, would the presence of a 12 gauge wire in an outlet box or fixture convey something to an electrician or future DIYer that is misleading based on the existence of unseen 14 gauge wiring upstream?

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  • 14 gauge is not rated for 20A in the first place. With 14ga you should only have 15A breakers.
    – The Evil Greebo
    Feb 15 '13 at 14:51
  • @TheEvilGreebo You are right. Some of the breakers are 20A, but the ones with 14 gauge (including the one I am extending) are 15A. I edited the Q.
    – bib
    Feb 15 '13 at 15:03
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    Possible duplicate: Is it ok to wire light switches and lights with 12/2 cable?
    – BMitch ♦
    Feb 15 '13 at 15:09
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    No problem at all (other than wasting money). You can always up-size wire, you just can't down-size.
    – Tester101
    Feb 15 '13 at 16:01
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    The downside of mixing 12/2 wiring on a 15 amp is that it confuses electricians, and someone could try to upgrade the breaker without realizing that there is also 14/2 wiring on the circuit. But in this situation, that seems to be less of a concern since the breaker itself is wired with 14/2.
    – BMitch ♦
    Feb 15 '13 at 17:01
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The OP has a great reason for using 12 ga. wire. He already has some on hand, so using it is cheaper than going to get new 14 ga. wire. It's perfectly safe to use it. Don't worry about confusing future workers. If they want to add something to the circuit, they will have to go turn off the circuit breaker first, and then they will see they are working with a 15 amp circuit. Then, if they have any brains / experience at all, they will see that there is 14 gauge wire hooked up to that 15 amp circuit breaker, so they will know not to upgrade it to a 20 amp breaker. Your plan is perfectly fine.

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  • "then they will see they are working with a 15-amp circuit" It's not always as simple. In some circumstances, it is advisable to use an oversized gauge of wire, for example to minimize resistive loss on longer runs, so you can't just say "I'll upgrade it to 20-amp since it's #12".
    – sleblanc
    Feb 14 '18 at 5:57
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It is perfectly acceptable to use 12AWG, though I would, personally, choose not to.

I wouldn't use 12 gauge wire for the project for a few reasons:

  1. 12AWG isn't required for the size of the circuit

  2. going from 14AWG to 12AWG confuses the use of the circuit by changing wire size -- future workers will see 12AWG and may make an assumption about the size of the circuit

  3. 12AWG is more expensive than 14AWG

  4. 12AWG is more difficult to work with than 14AWG

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  • @downvoter, any suggestions on how I can improve my answer?
    – Matthew
    Mar 19 '13 at 17:44
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    There is no reason he can't use 12AWG. Some electricians I know only use 12 for 15 and 20 amp. If anything he is making it easier to upgrade to 20 amp in the future. Money isn't an issue either since what he currently has is free compared to buying something he doesn't have. And saying 12AWG is more difficult to work with... yes maybe it takes an extra 1 minute per outlet. The answer to the question is yes and I think this confuses those with lesser electrical knowledge - most DIYers.
    – DMoore
    May 4 '13 at 18:16
  • @DMoore I edited my answer to be less ambiguous.
    – Matthew
    May 6 '13 at 15:19
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    I don't plan to ever use anything smaller than 12AWG for anything ever again. From voltage drop issues (minor, maybe, and more $$ but electricity flows with less resistance, voltage drop is lower and appliances run slightly more efficiently with bigger wire) to future upgrades to 20A circuits. In fact, for longer 20A runs I may just proactively use 10AWG wire. Maybe. In the building I work in, there are all kinds of screwy issues with voltage on 100+ foot 20A runs on #12 where we've put extra laser printers in close proximity to workers. No such problems on the shorter 20A runs.
    – Craig Tullis
    Jun 8 '15 at 4:24
  • @DMoore I think it's important to maintain consistent wiring conventions. If everything everywhere in the house on 15 and 20 amp circuits uses 12 gauge, then use 12 gauge. If everything on a 15 amp circuit uses 14 gauge, I think it's best to be consistent and use 14 gauge.
    – CorayThan
    Nov 12 '16 at 7:21
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It is perfectly acceptable to use 12 gauge wire on a 15 amp circuit.

If the cabling were concealed, would the presence of a 12 gauge wire in an outlet box or fixture convey something to an electrician or future DIYer that is misleading based on the existence of unseen 14 gauge wiring upstream?

It shouldn't, many older homes were wired with 12 gauge throughout. Apart from that, it is actually explicitly allowed by code.

From the NEC:

Table 210.24 Summary of Branch-Circuit Requirements

enter image description here

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  • I don't see where it's "explicitly allowed by code"? Looks like 15A circuit wires and taps should be #14 to me.
    – Tester101
    Feb 15 '13 at 17:33
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    @Tester101 Ha I guess you are right, it is more implicit really. Conductors (min. size) implies that 14 gauge is the minimum so bigger is acceptable or it would be called out.
    – maple_shaft
    Feb 15 '13 at 17:47
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    It does say "min." but it doesn't say whether the entire circuit should be the same gauge or not. I guess since it doesn't say not to do it, that you would be allowed. IMO best practice would be to use the same gauge for the entire circuit.
    – ryanwinchester
    Feb 15 '13 at 17:55
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    Side question: why is it permitted to put a 50 A receptacle on a 40 A circuit?
    – Hank
    Feb 18 '13 at 18:49
  • @HenryJackson Thats a very good question actually... you should ask it on the site :)
    – maple_shaft
    Feb 19 '13 at 13:04
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"explicitly allowed by code"

I think the term TAP is confusing.

In Part M of Article 410, Special Provisions for Flush and Recessed Fixtures, appears Section 410-67(c), which reads as follows:

(c) Tap Conductors. Tap conductors of a type suitable for the temperature encountered shall be permitted to run from the fixture terminal connection to an outlet box placed at least 1 foot (305 mm) from the fixture. Such tap conductors shall be in suitable raceway or Type AC or MC cable of at least 18 inches (450 mm) but not more than 6 feet (1.83 m) in length.

Note that the 18-inches and 6-foot lengths are for the raceway (or cable sheath), not the conductor. (The tap lengths in Sec. 240-21 are for the conductors, not the raceways or cables.)

The other controversial matter in this Section is the use of the term “tap.” Some believe that the high-temperature wire from the junction box to the fixture must have a lower ampacity than the branch circuit overcurrent protection rating, due to the definition of “tap” in Section 240-3(c), new in the 1999 NEC:

As used in this article, a tap conductor is defined as a conductor, other than a service conductor that has overcurrent protection ahead of its point of supply, that exceeds the value permitted for similar conductors that are protected as described elsewhere in this section.

The “tap” referred to in Sec. 410-67(c) is not the “tap” as defined in Sec. 240-3(c), because that definition starts out with “as used in this article.” Therefore the definition applies only in Article 240, and the tap in Art. 410-67(c) need not have overcurrent protection greater than its ampacity. As a matter of fact, where higher temperature insulation is required, these conductors probably have an ampacity greater than the rating of the branch circuit overcurrent protection.

There, now it's not confusing.

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I don't know why everyone is saying this is okay, and trivializing the matter of "confusing future electricians". This is a big deal. You should not mix gauges of wires in the same circuit. Regardless of what code says, I guarantee this will not pass inspection in some jurisdictions and depending on how bad a day the inspector is having. Not recommended.

14-gauge wire is cheap compared to 12-ga. You should save the 12-ga wire for when you need it. Or sell what you have and buy new 14-ga.... profit!

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