When sizing motor over-current protection, CSA 22.1 28-200 3a states that a circuit breaker can be sized up to 250% of the rated full load amperage (FLA) of the motor.
E.g. 100hp, 575VAC motor draws 99A (CSA 22.1 Table 44): 99A * 250% = 247.5A
Usually I would've rounded up and selected a 250A circuit breaker. I just noticed today that in the over-current example on page 662 that they always size the the next lowest standard size and specified a 225A circuit breaker. This makes sense since a 250A circuit breaker is past 250% of FLA. However, Rule 28-200 4c goes on to state that if the circuit breaker above isn't large enough for the motor to start that you can go up to 300% of the motor FLA (297A). Therefore, 250A isn't in violation.
- As a rule of thumb, when sizing motor branch over-current protection, should you round up or down to the next standard breaker size? If rounding up, do you need to prove that the motor couldn't start with the smaller sized breaker?
- Does the answer to #1 change when considering feeder over-current protection?