What is the end of a circuit according to the NEC?

Question

A comment on another question stated the NEC does not allow smaller gauge wires on a circuit than allowed by the breaker, even if the wires themselves are rated higher(90C).

Many devices/appliances are tested(UL) to have smaller wires to connect to the household/home circuits. Either by plugs or junction boxes/hard wired.

I have assumed that a circuit ends at the device/appliance itself. Maybe NEC ends the circuit at the receptacle/junction box.

Where does the NEC and/or CSA(Canada) ends the circuit for the gauge of wire allowed? At the wall/junction box or at the device/appliance?

I am talking about wires/cables supplied by the manufacturer, not what we can add to the circuit ourselves.

Answer 1

The NEC (and the Canadian Electrical Code), insofar as it prescribes rules for home wiring, is concerned only with permanently installed wiring, as in what's inside the walls/floors/ceilings/cabinets/etc. In effect, the circuit ends at the receptacle or at the junction box in the case of permanent fixtures. The cables that plug into those receptacles and the wires inside a fixture, appliance, or other device are instead the purview of testing and certification bodies such as UL.

These bodies can assess the wiring requirements of devices on a device-by-device basis, and as such may agree with the designers that larger wires are not necessary given the likely current draw of a device. Installed wiring, on the other hand, needs to be able to safely handle whatever might be plugged into it, and carry as much current as the breaker will allow, without burning the place down. Therefore, the requirements for installed wiring are rigid and predefined, whereas devices can be certified individually and vary wildly in their wiring needs.