Rule 22 of 40 · Chapter IV — Conductors and Loads
Size the wire to the load
Why this rule exists
A conductor turns current into heat, and too small a wire for the current it carries runs hot enough to cook its own insulation and start a fire. Ampacity, how much current a conductor carries continuously without exceeding its temperature rating, depends on wire size, insulation type, and installation conditions. Undersizing to save money, or because it is a short run, ignores that the wire heats along its whole length. The load defines the current; the current defines the minimum wire. Get that order right and the breaker and everything downstream can do their jobs.
In practice
Start from the load's continuous current, apply the 80 percent rule and any derating, then pick a conductor whose ampacity meets or exceeds the result. Common copper baselines: 14 AWG for 15 amps, 12 AWG for 20, 10 AWG for 30, before derating for heat, bundling, or long runs. Use the correct insulation temperature rating and do not exceed the lowest-rated component in the circuit, usually the terminals. Match the breaker to the wire, not the other way around. When in doubt, go one size larger.
When it doesn't apply
Terminal temperature ratings often cap you below the conductor's own higher-temperature ampacity, so you must use the lower value, typically the 60 or 75 degree column. Voltage drop on long runs can force a larger wire than ampacity alone requires. When the load is not a simple resistive branch circuit, check the specific requirements.