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My father-in-law is a builder. He has put in 20 amp and 15 amp breakers, and ran 12-2 as the home runs to the switches and the first plug to make them hot. He then ran 14-2 to feed the rest of the plugs, lights, and ceiling fans.

Is it okay that he used different gauge wires on the same circuit?

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  • I use 12AWG on 15A breakers when I am dealing with an obsolete panel (looking at you Pushmatic) and there are 15A breakers in the panel. Rather than source expensive breakers I just find circuits where 15A will work and wire it all #12 in preparation for the happy day when I change panels. I don't own any #14 wire and don't see buying it unless I have a very long outdoor run somewhere and I was doing my transformer thing, then it would be UF or THHN obviously.
    – Harper - Reinstate Monica
    Commented Dec 14, 2017 at 0:16

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It is fine for the 15 amp circuits, but not for the 20 amp circuits.

The National Electrical Code requires circuits protected by a 20 amp circuit breaker to use #12 AWG copper wire throughout the circuit.

The National Electrical Code allows circuits protected by a 15 amp circuit breaker to use #12 AWG copper wire but #14 AWG wire cannot be protected by anything larger than a 15 amp breaker for power and lighting.

Regardless of where it is in the circuit, even if it is after the first receptacle, #14 wire cannot be used on a power or lighting circuit protected by a 20 amp breaker.

The following are pertinent excerpts from articles of the National Electrical Code:

210.20(B) Conductor Protection. Conductors shall be protected in accordance with 240.4.

240.4(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.

...

(3) 14 AWG Copper. 15 amperes

...

(5) 12 AWG Copper. 20 amperes

The #14 wire that is installed on 20 amp circuits would need to be replaced with #12 or the breaker on that circuit needs to be replaced by a 15 amp breaker.

Usually, 20 amp circuits are used for receptacle circuits and 15 amp circuits are used for lighting circuits.

Good luck!

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    Good answer but "Regardless of where it is in the circuit, even if it is after the first receptacle" might imply to some that this could actually matter in some way. Everything should be wired in parallel so it would seem completely irrelevant where in the circuit it is. I guess my point is, whether you think the electrical code is too strict or not, this is just plain wrong.
    – JimmyJames
    Commented Dec 12, 2017 at 16:08
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    Another "good answer but": The two lines about the gauge and breaker combinations right at the beginning could be clearer. It seems to say if you want to use #12 wire, then you have to put in a 20 amp breaker, but it's really if you want to build a circuit protected by a 20 amp breaker, you must use #12 wire. You also might clarify if the exact gauge is required or if a larger than required gauge is acceptable. E.g., can you use #12 wire for a 15 amp circuit? (I grant that the answer is implied in your first sentence, but it might help to be explicit)
    – Todd Wilcox
    Commented Dec 12, 2017 at 17:01
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    @ToddWilcox You can use 12 gauge on a microamp circuit if you really want to, but a 12 guage wire MUST be protected by a 20 amp fuse/circuit breaker OR LESS. Circuit Breakers and fuses protect wire, not equipment. The reason is heat. A heavier gauge wire can carry more electricity and stay cooler. On my sailboat's AC circuits, I ran 10 Gauge on my 20 amp circuits and 12 gauge on my 15 amp circuits. Part of the reason I do this is because I am bundling wires along the same path. The more wires you bundle together, the greater the heat so I run larger wires to keep things cool.
    – Escoce
    Commented Dec 12, 2017 at 18:46
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It sounds like your father-in-law "reasoned things out" and "used his own judgement" rather than "blindly following" the "common practice" (in this case the electrical code). He was possibly thinking about the way water pipes (supply and drain) are sized. Unfortunately this does not work for home electrical systems. It actually works well under most normal operation, but fails if there is a combination of loads which allows close to 20 A to flow through the #14 conductors. A #14 copper conductor carrying 20 A would get very hot.

The immediate thing to do is to replace the 20-A breakers with 15-A breakers on any circuits which have any sections of 14 AWG copper wires. Then analyze the wiring and you would see that some circuits could be rewired to disconnect the 14 AWG part from the #12 and these all #12 sections could be upgraded to 20-A breakers. New #14 cables from 15-A breakers could then be installed to power the lights which are on #14.

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    Question for those knowledgeable about real world performance of power wiring: What would happen to the #14 sections of circuits with mixed #12 and #14 protected by a 20-A breaker in the event of a dead short in one of the #14 sections? Would the 20-A breaker trip so fast that no significant heating of the #14 would occur?
    – Jim Stewart
    Commented Dec 12, 2017 at 11:51
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    In the first case, the breaker should trip within a single cycle. In the second situation, the wire probably won't get hot enough to start a fire. The problem comes with the trip characterization of the circuit breaker, when responding to an overcurrent situation. A 20 ampere breaker can allow 30 amperes of current to flow, for up to 6 or 7 minutes before opening. 40 amperes can flow for 2 or so minutes, before the overcurrent protection kicks in. So the question becomes, how hot will the #14 get in 2 minutes with 40 amperes flowing through it?
    – Tester101
    Commented Dec 12, 2017 at 13:55
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    Sorry from a 15a breaker to the first plug. He put 12-2 wire for home run. And from there to the other plugs he ran 14-2. The 12-2 wire is the hot to feed to the plugs. And the lights are on a 20a breaker. He done the same thing. Ran a 12-2 to the switch's box for a hot to feed them. And ran 14-2 out of the switch box to the lights. And fans
    – Mikey
    Commented Dec 12, 2017 at 17:38
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    Wow, great that the drywall is not up! Any circuits that you want to be on a 20-A breaker MUST be entirely #12. (But it is OK to use #12 on a 15-a breaker and might even make sense if there is a long home run.) I think that most lighting circuits would only have to be 15-A in this day of the LED. I don't know if in modern houses only some receptacles need to be on 20-A circuits. In my 1970 tract house all the 120-V receptacles were in 15-A circuits, except for two in kitchen and dedicated ones for clothes washer and dish washer.
    – Jim Stewart
    Commented Dec 12, 2017 at 17:53
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    This comment above is clearer and easier to follow than your answer.
    – Harper - Reinstate Monica
    Commented Dec 12, 2017 at 19:08
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You can always use a larger gauge than is required.

So if you have a 15A circuit, you are required to use at least 14 AWG wire. However, if you want to use #12, #8, 4/0... whatever, you can, as long as your wiring methods are proper.

There's one hitch, but it's a "practical, implementation" issue more so than a rule. That's attaching the wire to the device, receptacle, switch, whatever, or making the larger wire fit somewhere.

For instance, I just looked at a 15A receptacle, and it is listed for #12 or #14 copper wire -- and only #14 solid if using the backstab (which is a bad idea). You have to comply with all that. So if you upsized to #10, you can't connect it to that receptacle directly, you must pigtail the receptacle with a #12 (or possibly #14 if on a 15A circuit).

Or say you have 10 of these conductors to put down a 1/2" EMT conduit. That's legal with #12, but if you upsize to #10, that exceeds conduit fill rules.

And of course, you still have to follow all the other circuit rules. So if you have a 20A socket, it still has to be on a 20A breaker even if the wire is #8.

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    I'm now picturing the face of someone who opens a wall to find massive 0000 gauge wiring to each outlet. (aka 4/0) That's 1.2 cm in diameter, almost half an inch thick.
    – Criggie
    Commented Dec 13, 2017 at 9:25
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If a 15 amp breaker is installed on a 12 gauge wire, any electrician should think the wire is a long run and not that it was incorrectly installed on a 15 amp breaker.

Before changing the 15 amp breaker to a 20 amp breaker, it is the responsibility of the electrician to make sure the entire circuit is run with a minimum wire size of 12 gauge.

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Yes you can use on a 15 amp circuit breaker a 12 gauge wire from your panel to the first receptacle then continue with 14 gauge wire. It would in fact need to be done that way if your first receptacle requières a wire that was more than 50 feet long. Then, assuming the other receptacles would be in the vicinity of the first receptacle, a 14 gauge wire could be used to connect the rest of the receptacles

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