Wiring Infrared Patio Heating Systems

Question

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°?

Answer 1

You're stuck with the two-circuit approach, although I'd use a single 12/2/2 cable for the homerun for that

As it turns out, the NEC doesn't allow taps off 40 or 50A branch circuits to feed space-heating appliances in a dwelling unit (this is a combination of 210.19(A)(4) and 424.3(A)). So, you're required to use the "5 wire" approach, running two 20A, 240VAC branch circuits to the switch box and thus onward to the heating appliance. While you could use the approach of pairing 12/2s to make the homerun, I would use a single 12/2/2 cable for the NM leg of the circuit to avoid silly issues with circuits getting "split" across cables and thus violating Code.