Load a circuit to 80 percent

Why this rule exists

A breaker's rating is its trip threshold, not a comfortable continuous target. For loads running three hours or more, continuous loads, the NEC limits sustained draw to 80 percent of the breaker and conductor rating, a margin so the wire and terminals do not sit at their thermal limit for hours. Thermal breakers also trend toward nuisance tripping when held near 100 percent. The 80 percent rule keeps connections cool, insulation intact, and the breaker reliable. Run a 20-amp circuit at a steady 20 amps and you are asking for heat and trouble.

In practice

For continuous loads, size the circuit so the load is at most 80 percent of the rating: a 20-amp circuit carries 16 amps continuous, a 15-amp carries 12. Equivalently, size the breaker and wire to at least 125 percent of the continuous load. Add up what actually runs together for long stretches, space heaters, EV chargers, servers, and do not stack them on one circuit. For dedicated high-draw appliances, give them their own correctly sized circuit rather than sharing.

Example

Continuous load: 16 A
Breaker >= 16 / 0.8 = 20 A
Or: breaker >= 16 x 1.25 = 20 A
Use 12 AWG copper (20 A rated)
Max continuous on a 20 A circuit: 16 A
Never design a 20 A circuit to draw 20 A

When it doesn't apply

The 80 percent limit applies to continuous loads; genuinely intermittent loads may use more of the rating, and some assemblies are listed for 100 percent continuous duty and marked as such. The margin is a floor for safety, not a target to design up to; leaving more headroom is always fine, and often wise for future loads.