circuit calc for 10hp 3ph motor

[bill@usps06492]

I have a 10hp motor @208v, 30aflc, 3phase. Do I rate the circuit breaker and the conductors at 250%of the flc? Iam using table 430.52 for inverse time breaker. That puts the overlaod protection at 75a and the conductors at #6. Do I need to divide the 75a/1.73=54.7a, or is the 75a what the circuit should be rated for?

 

[don_resqcapt19]

Re: circuit calc for 10hp 3ph motor

Bill,
The conductor and branch circuit overcurrent and short circuit protection are sized based on the value from Table 430.150. The table FLA for this motor is 30.8 amps. 430.22(A) requires that the branch circuit conductor have an ampacity of at least 125% of the table FLA. This requires a conductor with an ampacity of at least 38.5 amps. #8 copper has an ampacity of 40 amps and is suitable for this motor. The short circuit and ground fault protective device is sized per 450.52 and with an inverse time breaker is permitted to be 250% of the motor FLA. 250% of 30.8 is 77 amps. 77 amps is not a standard size breaker and Exception #1 to 430.52 permits rounding up to the next standard size which is 80 amps. The code permits an 80 amp breaker and #8 conductor for this motor.
Don