We're thinking about installing an infrared patio heater – according to a local dealer a CD-6024 (2 - 3000 W / 240 V elements for a total of 6000 watts) would do the job. For the controls they recommend the 14-4320 dual-stack switch.
The heater can draw a total of 25A (or 12.5A per element), since the heater could be left on for more than 3 hours, it's a continuous load and the conductors need to be sized at 125% of the load (31.25 A total / ~16 A per element). The indoor portion of the circuit will be NM-B cable, the outside portion will be THWN in PVC conduit.
I have some questions about the wiring design:
The control appears to use a Leviton 5627 (20 A duplex) switch. The switch terminals will accommodate 12 AWG wire. The wiring diagram Infratech provides shows one side of the 240 V circuit going to the switch and the other side going to the heating element. So, question #1 if a common feed is brought to the switch, it would need to be sized for 30+ A, so 8 AWG for the NM-B portion of the circuit. The switch terminals won't accommodate 8 AWG wire, is it safe / to code to feed the switch and the rest of the run to the heater using 12 AWG THWN?
I'm currently planning to use a single two-pole 40 A breaker and 8 AWG NM-B cable for the run to a junction box on the outside of the house, and then from there run underground in PVC conduit with 8 AWG THWN to the controls. Would it be acceptable to treat each element as a separate device, using two two-pole 20 A breakers and feeding each circuit with 12 AWG NM-B?
When transitioning from the NM-B to the THWN how do the temperature ratings work? The individual conductors in the NM-B cable are rated at 90° C, but the cable assembly is rated at 60° C. The run in conduit is THHN/THWN which is rated at 75° C for wet locations. Could I use a 35 A breaker and size the THWN at 10 AWG (35 A @ 75° C) with 75° wire nuts? Or does the whole run have to assume 60°?