Circuit Breaker Sizing

If there is a local electrical code in force, it will prescribe the ratings of fuses or circuit breakers. In the U.S., the NEC (National Electrical Code) allows the secondary protection to be 1.25 times FLC, or the next higher standard size. The multiplier for the primary protection depends on the size of the transformer; I don't have a copy of the Code handy and don't remember the exact numbers.

thanks very much

For sizing a CB, the factor 1.25 or 1.5 for main (primary) or feeder (secondary) is not required for safety or magrin, for feeder there are not any fator for sizing if protection curve of CB is suitable for this feeder, for main CB a factor maybe required due to the biggest motor in feeder (if any) operation in acceptable overload condition (1.25 as per motor protection relay setting).

thank you very much.