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My new home has ductless wall heaters (Cadet specifically). I want to add a new wall and install another heater in it. In checking my panel, I see that the seven original heaters are on four dedicated circuits; each being 240V, 30A. I want to tie the new heater on the line that only has one heater on it currently. However, in purchasing a heater and checking all of the current ones, it seems that the wiring that comes with the heaters is 18 AWG (for the fan) tied to 14 AWG (for the heating element), with 14 AWG exposed to connect to the main circuit. Additionally, every heater has a separate thermostat with 12 AWG wires. I cannot tell in the heater box what gauge the main wiring is, but it is definitely larger in diameter than the 14 AWG. As far as I understand, 30A breakers require 10 AWG wiring, so now I am confused as to how my house ever passed inspection and how I will pass for this remodel.

Am I missing something in the code that allows for appliances to have wiring connected to the 10 AWG of the circuit that is only 12 or 14 AWG? Is this safe?

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Code doesn't apply to the wiring within appliances

The NEC does not deal with wiring within appliances -- UL (or CSA, or Intertek, or MET, or TUV) is the one responsible for ensuring that's safe, and they work off a different set of wiring specs (Appliance Wiring Material or AWM for short) than the wiring used for building wire. This is codified in NEC 300.1(B):

(B) Integral Parts of Equipment. The provisions of this article are not intended to apply to the conductors that form an integral part of equipment, such as motors, controllers, motor control centers, or factory assembled control equipment or listed utilization equipment.

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    Because those runs are enclosed within typically metal apparatus (in this case whose purpose is making heat so who cares). Also because internal wiring can consider what the appliance will typically draw, wall wiring must be a size the breaker can protect. – Harper Oct 15 '17 at 12:03

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