Conductors, Cables, and Wiring Devices
Ampacity and temperature
Table 310.16 gives ampacities in 60, 75, and 90 degree columns. Which column you may use depends on the weakest link: conductor insulation, terminals, and equipment. Under 110.14(C), circuits of 100 A or less use the 60 degree column unless the terminals are listed otherwise, which is why 14 AWG copper is a 15 A conductor in practice. NM-B cable is limited to the 60 degree column. Ampacity is then adjusted for more than three current-carrying conductors in a raceway or cable and corrected for ambient temperature before the overcurrent device is selected.
Cable and device basics
Know the common cable types: NM-B for dry interior, MC and AC for armored installations, SE for service entrance, USE for underground. The grounded conductor is white or gray, the equipment grounding conductor green or bare, and ungrounded conductors any other color. Receptacles must match the circuit: a 15 A receptacle may be on a 20 A circuit only where the circuit serves more than one receptacle (210.21(B)). Tamper-resistant receptacles are required in dwellings, and GFCI protection is a device-level requirement in many locations.
Traps
Using the 90 degree column because the wire is THHN; forgetting the 240.4(D) limits on 14, 12, and 10 AWG; swapping the grounded and grounding conductor colors; assuming a 20 A receptacle is needed on every 20 A circuit.
Worked example
Six 10 AWG THHN conductors in one raceway: 40 A at 90 degrees, adjusted to 80 percent equals 32 A, then capped at 30 A by the 60 degree termination limit.
Rule of thumb
Size for the lowest of: insulation ampacity, termination ampacity, adjusted ampacity, and the overcurrent device.