What breaker and wire gauge can I use for my heated floor cable?

Question

I will be installing a SunTouch heated floor for 200 square feet of space. I will wire this at 240V using a double pole breaker. The specs on the website indicate the amperage draw is 10 amps for the 783 feet of heating cable. This will be a dedicated line used for only the under floor heating system. Also, the line running from the heating system the panel will be around 25 feet.

1.) Should I use a 15, 20 or 30 amp double pole breaker? The specs page indicates that the maximum circuit load is 20 amps and 15 amps with a a SunStat control (which I have.)

2.) What is the smallest wire gauge I can use? Based on some other research, it seems like 8 gauge is the way to go.

Answer 1

If the load of the amount of cable you will be installing is 10 Amps at 240V, then you need 12.5A @240V for code to be happy with "adequate supply for a heating load"

Nobody makes a 12.5A breaker, so you need a 15A breaker (240V double pole.) If the wording of the "specs page" is as I suspect, the MAXIMUM size breaker you can use is a 20A. There's no particularly good reason to use a 20A breaker, though.

If you use a 15A breaker, you can use 14AWG wiring. If you want to waste the 3rd wire in the 14/3 you have lying around, you can use that cable. You only need two hots and ground for a 240V heating load.

If you use a 20A breaker, you must use 12AWG or larger (diameter, smaller gage number) wiring.

There's no sensible reason to use 8 AWG (40A) wiring here, which I suspect might mean you found a 12VDC ampacity chart to mislead you, without noting that 3 volts drop on 12VDC is 25% loss, while 3 volts drop on 240VAC is barely over 1% loss.

Answer 2

8 gauge for 10 or 20 amps would have to about 500 feet away from the panel.

For a 20 amp circuit you need 12 gauge minimum, and for a 10 amp draw that should be plenty.

You might have been looking at an automotive or boating DC 12 or 24 volt type table.

You want a 20 amp double pole breaker.