14-2 outdoor direct bury rated wire to tap into a 12-2 wire 20am circuit I thought it was 15a anyway at the time. I wired a 20amp GFCI switch that will see intermittant use through the wall about 1 foot away. Is this really that bad? I realized afterward I 'shouldnt' have done that techincally. I know that particular wire might get slightly hot if in use by something that is pulling 20a. But it is outdoor grade. It is going literally a foot away.... I know what code says but what are the real risks?


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    Hello, and welcome to Stack Exchange. The "intermittent use" argument doesn't really hold water; who knows who and what this outlet will get used for in the future, and they won't know that you've promised not to use it for extend periods. – Daniel Griscom Sep 15 '16 at 15:22
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    @DanielGriscom Right. You can't build things into buildings that only you know how to use safely. – Harper - Reinstate Monica Sep 16 '16 at 17:09
  • So I took it apart and changed it out to 12-2. Before I did that I ran some power tools off it. Didn't get ridiculously warm. It probably would have been fine forever but found some 12-2 anyway. – mps123 Sep 16 '16 at 19:58
  • a) follow the law (the N.E.C.) and b) follow the principle of least astonishment. :) – Craig Sep 16 '16 at 20:30

The question isn't whether it's "bad," but whether it is legal. Running a 20A branch circuit over #14 wire is not legal (there are very uncommon configurations that it is pointless to explore here).

If you have any questions at all, bring in an electrician.

If it isn't legal, there is a reason it isn't legal, and the reason generally boils down to fire safety. You could (illegally) push 50 Amps over a #14 wire and power a load, but the wire is going to get hot. The more continuous the load, the hotter the wire is going to get. Voltage drop due to resistance is probably going to damage the load (especially if it's a motor or electronics), and chances are good that you'll cook wire connectors and start a fire, which may burn your house down and kill everyone in their sleep.

The code errs on the side of safety. I've actually seen horribly incompetent cases where, for example, a 240V water heater on a 30A circuit (two pole 30A breaker) was being served with #14 wire and it had been working for years. But maybe the next family in that house taxes the water heater harder and the wire heats up enough to start a fire.

Did you know that the vinyl sheath on NM cable will burn and carry a flame right through the walls of your house (stopping that flame from propagating down the wire is one of the main purposes of NM cable staples).

The bottom line is that it is not legal to put a 20 Amp breaker on a #14 wire. You need to replace the wire, or replace the breaker with a 15A breaker.

The breaker protects the wire, and protecting the wire protects lives.

  • That's a very interesting assertion about the staples stopping fire transmission. I sought more information, but came up short. Do you have any references? – isherwood Sep 15 '16 at 19:55
  • Sure. Nail a length of NM cable to a board with NM staples, light one end of the cable sheath and observe what happens when the flame reaches the staple. :-) – Craig Sep 15 '16 at 21:13
  • @isherwood And... the insistence of multiple electrical contractor uncles and their employees over the years. – Craig Sep 15 '16 at 21:21
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    Forgive me for being skeptical, but that's precisely how at least a few of our major world religions were formed as well. :P I was hoping for something along the lines of technical specs or code references. – isherwood Sep 15 '16 at 21:25
  • Wait... so you can use NM cable as a burning fuze? The kind you light to delay a firecracker, not the electrical kind. – Harper - Reinstate Monica Sep 16 '16 at 17:14

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