Three Ph Serivce Breaker/Conductor Size

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FewerSparks

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I have a task where I have a Motor Control Center with several motors. Per NEC, I have used 1.25% of the largest motor and 100% of the remaining motor (Full Load Amps) and have come up with a load of 473 Amps. For sizing my main circuit breaker, would I use a 500 Amp breaker or do you have to apply the 125% rule to the 473 Amps (1.25 x 473 = 591 Amps) and go to a 600 Amp breaker.

For the conductors, are there any other code requirements for the size (like another 1.25) or do I just select the conductor with the ampacity for the circuit breaker you use?
 
Your 473 amps appears to be correct for conductor size per 430.24.
Other factors such as conduit fill, ambient , etc must be taken into account.
You need to look at 430.62 for your breaker sizing. It's a whole different set of multipliers.
 
Look at 430.62. You can install a breaker equal to the OC protection for the largest motor determined in table 430.52 + the sum of the FLA of the other motors.
 
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