I'm installing 240 volt in-floor heating mats. The amperage draw is 21 amps. I intended to connect them to a 2-pole, 240 volt, 20 amp breaker. My thought was that you have two legs of 20 amp power (40 amps total). Is this correct of how the 240 volt breakers work?

Its 20 Amps total, you will need #10's and a 2 pole, 30 amp breaker.

I was afraid of that. I think I will run two circuits since I already have a lot of 12/3 wire. The mats are in different rooms anyway. Thanks

For reference, since it's space heating, you multiple the current of the heaters by 1.2 and size up to next available breaker size, hence 30 A.

I was afraid of that. I think I will run two circuits since I already have a lot of 12/3 wire. The mats are in different rooms anyway. ThanksHow much draw and what volts for 1 heater?

The amperage draw depends on the size of the mat. The largest one will pull 15 amps. The remaining ones are smaller and pull 6 amps combined. I'll split them into two 20 amp services which should work fine. Thanks for your input...