ECN Forum Forums General Topics of the Trade General Discussion Area Transformer Overcurrent Protection

My question is concerning transformers over 600V. The sizes of fuse or circuit breaker protection is based off of Table 450-3(a) (1999 Code).

Why is the size of a circuit breaker permitted to be larger than a fuse?

I thought that fuses would offer better performance and therefore be allowed to be rated higher.

I thank you for any guidance you can give me on this.

You will find a similar situation for motor short cirucit protection from Table 430.52. Fuses get set tighter to the load than breakers to avoid false trips.

Thank you very much. That makes perfect sense.