Motor Circuit Breaker Sizing

This article will address the history use and recent developments in available high resistance grounding technology used with ungrounded voltage systems.

Motor circuit breaker sizing.

Enter the motor horsepower and voltage. Breaker cannot be larger than ampacity of wire except for motors and a few other loads. The fuse sizing in column 4 for lps rk sp lpn rk sp frs r and frn r fuses provides a degree of motor and circuit overload protection to back up the normal motor overload protective device. Maximum loading of any circuit breaker is 80 of rating for non motor loads with exceptions as noted below.

22a x 2 5 55a next size up 60a. Locate where your motor falls in table 430 52 based on the full load current and motor size. Table 2 load maximum circuit breaker size of current. 80 of breaker max single phase breaker size wire size based on copper thhn wire increase one size when using aluminum conductors at 115 volts at 230 volts 15 amp 14 1 380 2 760 20 amp 12 1 840 3 680 30 amp 10 seldom used on 115 voltage 5 520 40 amp 8 seldom used on 115 voltage 7 360 50 amp 6 seldom used on 115 voltage 9 200 60.

Minimum circuit size is 12 with 20 amp cb. Therefore it is good practice to size the breaker at 15 amps. Difference between circuit breaker and isolator disconnector circuit breaker size calculation for continuous non contentious load. 1 1 2 3 5 7 10 15 20 25 30 40 50 60 75 100 125 150.

For example the full load current of a 5 hp motor at 460v is 7 6 amps. 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. Size the branch circuit short circuit and ground fault protection device per 240 6 a and 430 52 c 1 ex. Unrestrained fault current can cause severe equipment damage product destruction and personnel injury.

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. This level of protection requires a well designed true dual element fuse. For an inverse time breaker. Circuit breaker size 2.

This calculator will also give you motor amps and recommended breaker size starter size heater size and conduit size. High resistance grounding is a means of limiting ground fault current in electrical systems. This table is used to size the short circuit protection check the motor data sheets for the motor s full load current. Let s try sizing a motor feeder conductor.