How do you size a circuit breaker for a branch circuit?

Circuit breaker sizing is based on conductor ampacity and continuous load requirements. The breaker rating must not exceed the conductor's ampacity. For continuous loads (operating 3+ hours), the load cannot exceed 80% of breaker rating, so a 20A breaker can only continuously carry 16A. Breaker rating should coordinate with upstream and downstream devices for selective coordination. Motor circuits have special sizing rules based on full-load current.