Motor Protection Circuit Breaker Sizing

Maximum loading of any circuit breaker is 80 of rating for non motor loads with exceptions as noted below.

Motor protection circuit breaker sizing.

Sizing a circuit breaker circuit breakers are designed to carry 100 of their rated current while the nec dictates an 80 application. The best method for providing overcurrent protection for most circuits is to use a circuit breaker that combines overcurrent protection with short circuit and ground fault protection. A 30a circuit breaker can safely carry the exact 30a current but nec suggests 80 as a safe current limit as compared to the rated current. Breaker cannot be larger than ampacity of wire except for motors and a few other loads.

Lps rk sp lpn rk sp frs r and frn r fuses are sized for motor running back up protection and provide superb short circuit protection at the same time. Circuit breaker size 2. Locate where your motor falls in table 430 52 based on the full load current and motor size. Nec for 2002 a new rule for the conductor sizing was added for devices that are listed and marked suitable for use as tap conductor protection.

This table is used to size the short circuit protection check the motor data sheets for the motor s full load current. Motor protection circuit breaker and motor circuit protector specifications bulletin 140m summary of changes. With rare exceptions the best method for providing overcurrent protection in these cases is to separate the overload protection devices from the short circuit. Breaker size must be rated for a minimum of 125 of the full load current of the motor it is protecting rounded up to the next standard breaker size.

As circuit breakers cbs and overcurrent protection devices ocpd are designed for 100 rated current i e. Lpj sp tcf and lp cc fuses are sized a little larger but are even more current limiting providing an even greater degree of short circuit protection for the motor circuit. However this isn t usually the best choice for motors. The table will give you the size of the short circuit protection as a percentage of the full load current.

For example the full load current of a 5 hp motor at 460v is 7 6 amps.