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What breaker size do I use for an S811+ soft starter?

Breakers are not sized based on the size of the soft starter, but rather the size and type of the load (motor)

Breaker sizing is based on NEC 430.52. NEC allows for maximum settings base on motor type and provides a maximum % for breaker or fuse sizing. Code will allow for exceptions where settings based on 430.52 are insufficient for the starting current of the motors. Code does not address duty cycles for breaker sizing. Selection of the BCPD is based on FLA values found on table 430.250 for 3 phase AC motors.

There is no special sizing criterion for breakers or fuses. When it comes to “Severe Duty” you must leverage NEC sizing rules for BCPD and with soft starters you will most likely never in run in to the exceptions due to the already reduced starting effects of the soft starters.  Breaker selection then will be the same for standard and severe duty. You must select as noted above per the NEC defined motor types and the provided tables. Decisions on where to apply the allowable % is up to the engineer/marketing preferences. NEMA selection guide charts are a good guides as to what is generally will recommended for sizing.

The industry in general is running into breaker trip issues in the field is with high premium efficiency motors where transient inrush’s (20-28 times) can occur which can trip the breaker out.  In some cases one is forced to have the breakers replaced with a larger size per the exceptions allow in code so that a breaker can be put in place that is able to hold at 20-28 times. The effects of initial transients currents up to 28x of premium efficiency motors rated FLA has presented problems in the industry overall with circuit breakers tripping. With these motors, the high transients are typically 8-10 times but can be much higher depending on where the voltage wave is when energized, when at zero it could be very high and couple with their lower power factor under locked rotor conditions, make for a high transient LRC.