Motor Circuit Breaker Selection

Motors and branch circuit conductors.

Motor circuit breaker selection. Motor calculations part 1. 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 motor protection circuit breaker or mpcb is a specialized electromechanical device that can be used with motor circuits of both 60 hz and 50 hz it has several functions that allow it to provide a safe electrical supply for motors. Protection against electrical faults such as short circuits line to ground faults and line to line faults.

For a 800 kva transformer in 1155 a. Eaton s motor circuit protectors are ul recognized instantaneous only breakers for motor short circuit protection. The cross section of the connecting leads to the motors are to be selected on the basis of the current setting of the thermal overload release of the circuit breaker. Motor protection circuit breakers our motor protection circuit breakers provide both short circuit and overload protection for individual motor loads and are available for a wide range of motor sizes and installation types.

Eventually the maximum permissible length of the connection is to be considered shock hazard due to touch potential in the case of a short circuit. I rated momentary current and ii rated symmetrical interrupting current. Icu minimum 38 ka from figure h46 the cbm indicated in the table is a compact ns1250n icu 50 ka circuit breaker selection for cbp duty. When used in tested motor starter combinations they provide complete ul listed motor control and protection and short circuit current ratings up to 100 000a.

There are two types of selection of circuit breaker ratings which require the computation of sc current are. Transformers in sizes s00 and s0 up to 22 a switching capacity 65 ka or 50 ka at 480 y 277 v. Selection of circuit breaker. Sirius 3rv27 and 3rv28 can be used according to ul 489 csa c22 2 no 5.

As circuit breakers cbs and overcurrent protection devices ocpd are designed for 100 rated current i e. For fuseless protection of systems motors and non motor loads e g. The mpcb can interrupt any electrical fault that is. 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.

Circuit breaker selection for cbm duty. Use as branch circuit protection device bcpd acc. Go back to cb selection tips. Breaking capacity icu required for these circuit breakers is given in the figure h46 as 56 ka.

The circuit breakers for use in ul markets. However this isn t usually the best choice for motors. They can be combined with our nema and iec contactors or solid state motor controllers. Difference between circuit breaker and isolator disconnector circuit breaker size calculation for continuous non contentious load.

Symmetrical sc current is obtained by using subtransient reactances for synchronous machines.