Recently during an electric review at our training center, where we were using Mike Holts ELectrical Exam Preparation I saw an illustration 11-4 showing an AC unit with a minimum circuit ampacity of 23 amps and a maximum fuse size of 40 amps with the motor rated at 18 amps. Ultimately they show it wired with #12 THHN and fused at 40 amps (225%).
Maybe because I have a limited understanding of fuses and all their characteristics, but somewhere down the line there is certainly a panel where this circuit is feeding from and it will be a 20 amp circuit breaker protecting this #12 THHN. So I do not understand why fuse at 40 amps? When the circuit breaker will surely do it's job a long time before the 40 amp fuse will come into play?
Maybe because I have a limited understanding of fuses and all their characteristics, but somewhere down the line there is certainly a panel where this circuit is feeding from and it will be a 20 amp circuit breaker protecting this #12 THHN. So I do not understand why fuse at 40 amps? When the circuit breaker will surely do it's job a long time before the 40 amp fuse will come into play?
