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I've installed 20A switches on 15A circuits before, namely on the circuit supplying my kitchen lights. It was primarily to take advantage of the side-clamp wiring available on these beefier switches instead of the curl-and-screw flavor of side-wiring, or the push-in wiring, that you normally see on 15A S/P switches.

A friend of mine says that having a switch of higher capacity than the branch circuit breaker can cause the breaker to trip. I say he's full of it; a switch is a switch, and a 20A S/P will just be beefier than a 15A. As long as it's rated for at least the amperage draw of the circuit, you're fine, IMO.

Thoughts?

On a related note, does the same apply to outlets? He may be confusing using a 20A switch and using a 20A outlet; the outlet, unlike the switch, supports plugging in appliances that will draw >15A, and if you do, that WILL trip the breaker (though as 20A outlets support plugging in parallel-bladed 15A appliances, if you KNOW the branch is 15A and don't try to plug in a T-bladed plug, you're still OK).

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up vote 10 down vote accepted

You are correct on both counts:

  • There is no problem with using a 20A switch on a 15A circuit, as the breaker will trip and protect the switch from over-current.
    • Using a 15A switch on a 20A circuit is potentially dangerous though: the circuit may draw up to 20A, but the switch will be overloaded at that point which may cause it to overheat/melt/start a fire.
    • Really this is the same as wiring: you can use larger wires (with higher current ratings) but not smaller wires
  • A 20A outlet is not allowed on a 15A circuit. It's not really "dangerous" since the breaker will (should) trip at 15A anyway, but it is bad to knowingly install it this way.
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Re installing a 20A outlet on a 15A line: the .01% of people who can tell the difference will try to put more than 15A of load on the circuit and keep wondering why the breaker is tripping. :-) –  kdgregory Jan 28 '12 at 14:41
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@kdgregory The other 99.99% of people think if it fits it should work, so they may also put a 20A load on the 15A circuit. It could also be dangerous if the breaker malfunctions (which can/does happen), so it's best not to install 20A receptacles on 15A circuits. –  Tester101 Jan 28 '12 at 16:23
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Actually, even "15 amp" receptacles are rated at 20 amps total usage (much older ones not so), and can be placed on 20 amp circuits. This even applies in kitchens as the 20 amp requirement is for the circuit, not the receptacle itself. –  Skaperen Jan 30 '12 at 1:25
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