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I replaced a 2 conductor NM wire running from a light fixture to the switch with a 3 conductor 12/3 wire, to bring a neutral line to the switch box for a smart switch.

Then I pulled out the old wire and realized that it actually was 14/2, not 12/2. I thought it was 12/2 so I picked up 12 gauge from the store, and this of course was after I already fished the line through the wall.

Is there potentially a problem switching out a wire for a higher gauge? Theoretically a 12 gauge wire can conduct more amps than a 14, so my assumption is that it’s not a problem because it can handle more current than the old wire, not less. But I thought I’d double check.

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  • 1
    That’s exactly what to do. No reason to own 2 sizes of /3 cable!
    – Harper - Reinstate Monica
    Commented Jul 27, 2020 at 1:23
  • 2
    Are you replacing the wire all the way back to the distribution board or just the last piece? There's a small-but-nonzero risk someone in the future might assume its all 12/3 based on looking at the end.
    – Criggie
    Commented Jul 27, 2020 at 11:00
  • @Harper-ReinstateMonica the reason is not to mix wiring color and keep everything easily identifiable.
    – werfu
    Commented Jul 27, 2020 at 14:25
  • @werfu you mean the yellow cable jacket? Wrap its first couple inches with white tape. Done :)
    – Harper - Reinstate Monica
    Commented Jul 27, 2020 at 16:19
  • @Harper-ReinstateMonica the trouble is not a the termination, but when opening a wall or a ceiling and trying to understand what wires goes where and do what.
    – werfu
    Commented Jul 27, 2020 at 17:07

3 Answers 3

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You're good to go. You could run #10, #8 or #6 if you wanted (of course you'd have a heck of a time terminating it in the box, but that's another story).

You're always free to run a larger gauge wire since, as you know, it can handle the higher amps.

Make sure you label the circuit (as others very properly noted) to remind yourself and future electricians that this is a 15a circuit with some #12 wiring and that it's not #12 all the way to the breaker.

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  • of course you'd have a heck of a time terminating it in the box Well, to be fair, box fill rules would probably limit you on the large end. Three #6 wires in the box consume 15 cubic inches of fill budget, another 15 if spliced, and that doesn't leave a lot for splices, other circuits, etc. Realistically there's a pretty quick limit on how big of wire you can use in branch circuit device boxes unless you intend to have ridiculousness like a 4-gang box with two blanks and two devices just to fit all the wire, etc.
    – J...
    Commented Jul 27, 2020 at 14:58
  • There was some sarcasm implied there, @J.... I guess it didn't come through all that well. :(
    – FreeMan
    Commented Jul 27, 2020 at 15:35
  • No, it wasn't missed - the human brain had a chuckle but the engineering brain left the comment. I guess I should have more clearly expressed my initial amusement first ;)
    – J...
    Commented Jul 27, 2020 at 15:39
  • that explains so much, @J... So much... ;)
    – FreeMan
    Commented Jul 27, 2020 at 15:52
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Using the larger gauge wire is OK as it can easily carry the current capacity of the lighter gauge wire that it was replaced with.

In an instance like this it can be a good idea to place a label in the electrical boxes where 12AWG wire is located to indicate the the circuit is only protected with a 15A breaker and that 14AWG wire is deployed in the rest of the circuit. This helps a future you or next owner of service person to understand that this is NOT a 20A circuit.

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The minimum wire size is based on the overcurrent protection, so assuming the breaker was properly sized to begin with then it should be a 15A breaker which will limit the current to about 15A. So wire that can carry 20A is fine since current will be limited by the breaker anyway. In fact often wire sizes are increased to provide lower wire resistance, which reduces voltage loss suffered due to length.

The charts based on NEC (310.16/310.16(B)(7)) are the maximum allowed current on the size wire shown.

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