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I have a dedicated 20A circuit going to my bathroom that I plan to use to hook up a fan/heater/light/night light combo unit. I would like to use the compact combo 2 or 3 decora rocker switches. A quick search online only shows 15A switches. Do I need a 20A rated switch (specifically for the heater part of this unit)? If not, why not?

This unit does recommend a dedicated 20A circuit be used. The heater is 1500W.

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Yes, if you have a 20A circuit for a 20A load, then you need a 20A switch. You can't put a 15A switch in there for the same reason you can't use 14 gauge wire - the conductor is to small, leading to a fire hazard.

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The switch needs to be rated for the load that the switch interrupts. (Turns on and off, but mostly we're concerned with the "off".) This applies to the load the switch is in series with, not the entire circuit. Your 1500W heater is 12.5 amps and that is what the switch must handle.

The ordinary, dainty little 75 cent switches are for small loads like lights and perhaps a low power fan. They make heavy duty switches in the $6 range for bigger loads like heaters and air conditioners.

The dainty switches often claim they are good for 15 amps. Baloney.

If you can't get a heavy-duty switch in Decora, there are two ways to go. #1 don't use Decora. #2 use a relay.

I recommend a third approach, which is to create an additional 1-gang space just for the heater, and put an Intermatic style rundown timer switch there. One with no "hold"/continuous position.

Some more food for thought.

Consider putting the bathroom lighting on a different circuit than the heater or outlets, e.g. A lighting circuit from other rooms. That way an overload or GFCI trip does not plunge the occupant into pitch black, now with a straight razor or red hot hair curler in their hand. Not good. That would require keeping respective hots and neutrals with each other and not mixed. Grounds can mix.

A 1500W heater and a hair dryer both running at once on the same circuit, will trip a 20A breaker, but only after a minute or two.

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