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400 vs. 350 Amps Circuit Breaker
2

400 vs. 350 Amps Circuit Breaker

400 vs. 350 Amps Circuit Breaker

(OP)
We do have a 200Hp/460V/3ph/60Hz motor. The NEC tells me that the size of the circuit breaker (CB) should be 350 Amps. If we use a 400 Amps CB, is it violation of the NEC?
FLA of this motor is 240 Amps.

Thank you indeed in advance!

RE: 400 vs. 350 Amps Circuit Breaker

Which part of NEC tells you that? Is this a VFD?
NEC would allow up to 250% of the FLA rated CB. Look up Table 430.52 in NEC 430.

RE: 400 vs. 350 Amps Circuit Breaker

(OP)
Thank you Rbulsara!

I see it now at that 2005 NEC table 430.52 last column.
The motor is not on the VFD but there is an auto transformer starter. I was not sure what it is the maximum rating for these CB’s,... Thanks again!

RE: 400 vs. 350 Amps Circuit Breaker

The Project I am working on; we are using a 400A Thermal-Magnetic (Inverse time) circuit breaker as stated in the Table 430.52 far column.  According to the code we could use 2.5 *240FLA = 600A but we have chosen a 400A with a trip setting of 7.5X or 3000A which is 12.5 X FLA.  The Motor Short Circuit Protector NEC Reference 430.52 (7) can have a trip setting of no more than 1700% FLA for high efficiency motors which we chose 1250% for the energy efficient motor loads.  The Motor Overloads will protect the motor and cable in the event of a overload condition.  We are using a 3/C 350 kcmil cable which is good for 310A at 75 deg C according to Table 310.16.  This exceeds the 1.25 * (240)FLA = 300A that is required per the continuos Duty requirement 430.22 (a).

RE: 400 vs. 350 Amps Circuit Breaker

(OP)
Thank you JBUDA54!
It is helpful information.

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