Motor Protection Circuit Breaker Sizing

Circuit breaker size 2.

Motor protection circuit breaker sizing. Breaker cannot be larger than ampacity of wire except for motors and a few other loads. Difference between circuit breaker and isolator disconnector circuit breaker size calculation for continuous non contentious load. As circuit breakers cbs and overcurrent protection devices ocpd are designed for 100 rated current i e. For example the full load current of a 5 hp motor at 460v is 7 6 amps.

With rare exceptions the best method for providing overcurrent protection in these cases is to separate the overload protection devices from the short circuit. However this isn t usually the best choice for motors. 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. Maximum loading of any circuit breaker is 80 of rating for non motor loads with exceptions as noted below.

This table is used to size the short circuit protection check the motor data sheets for the motor s full load current. Therefore they are selected for the type of installations described above however some boundary. The table will give you the size of the short circuit protection as a percentage of the full load current. Circuit breakers with motor protection characteristics mpcb manual motor controllers according to ul 508 are multifunctional devices providing motor overload protection disconnector isolator properties and short circuit protection.

Sizing a circuit breaker circuit breakers are designed to carry 100 of their rated current while the nec dictates an 80 application. 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. Motor protection circuit breaker and motor circuit protector specifications bulletin 140m summary of changes. 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.