dickricker
Member
- Location
- Eugene, Oregon, USA
Hi,
Following NEC 2011, I have a question concerning motor feeder circuit breaker size.
Motor 1: 100 HP, 460 VAC, soft-starter controlled, NEC FLC = 124 A (Table 430.250)
Motor 2: 500 HP, 460 VAC, soft-starter controlled, NEC FLC = 590 A (Table 430.250)
Motor branch-circuit conductor ampacity not less than 125% NEC FLC (430.22)
Motor 1: 124 A x 1.25 = 155 A minimum ampacity
Motor 2: 590 A x 1.25 = 738 A minimum ampacity
Motor branch-circuit IT circuit breaker maximum 250% NEC FLC (Table 430.52)
Next standard size up if not exact
Motor 1: 124 A x 2.50 = 310 A, use 350 A
Motor 2: 590 A x 2.50 = 1475 A, use 1600 A
Motor feeder-circuit conductor ampacity not less than 125% NEC FLC of largest motor + 100% NEC FLC of other motors (430.24)
(590 A x 1.25) + 124 A = 862 minimum ampacity
Motor feeder-circuit IT circuit breaker not greater than largest motor breaker plus 100% NEC FLC of other motors (430.62)
Next standard size down if not exact
1600 A + 124 A = 1724 A, use 1600 A
So, now we have a 1600 A breaker ahead of a set of 1600 A + 350 A breakers.
This doesn't seem right.
Am I misreading something?
Is this a normal setup with a large motor and a small motor on the same feeder circuit?
Thank You!
Following NEC 2011, I have a question concerning motor feeder circuit breaker size.
Motor 1: 100 HP, 460 VAC, soft-starter controlled, NEC FLC = 124 A (Table 430.250)
Motor 2: 500 HP, 460 VAC, soft-starter controlled, NEC FLC = 590 A (Table 430.250)
Motor branch-circuit conductor ampacity not less than 125% NEC FLC (430.22)
Motor 1: 124 A x 1.25 = 155 A minimum ampacity
Motor 2: 590 A x 1.25 = 738 A minimum ampacity
Motor branch-circuit IT circuit breaker maximum 250% NEC FLC (Table 430.52)
Next standard size up if not exact
Motor 1: 124 A x 2.50 = 310 A, use 350 A
Motor 2: 590 A x 2.50 = 1475 A, use 1600 A
Motor feeder-circuit conductor ampacity not less than 125% NEC FLC of largest motor + 100% NEC FLC of other motors (430.24)
(590 A x 1.25) + 124 A = 862 minimum ampacity
Motor feeder-circuit IT circuit breaker not greater than largest motor breaker plus 100% NEC FLC of other motors (430.62)
Next standard size down if not exact
1600 A + 124 A = 1724 A, use 1600 A
So, now we have a 1600 A breaker ahead of a set of 1600 A + 350 A breakers.
This doesn't seem right.
Am I misreading something?
Is this a normal setup with a large motor and a small motor on the same feeder circuit?
Thank You!