Breaker Larger than Fused Disconnect Rating

E

elecshop

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Location
FL, USA
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Electrical Engineer
This is probably a straightforward question but for some reason I keep getting confused.

I have a 2-pump motor (lift station) that has 60A breakers integrated in its front panel (1 pump is the primary, 1 is a standby). FLA=20A for 1 pump.
The OCPD rating should be 250% x FLA1 + FLA2 = 250%x20 + 20 = 70A.

Can I use an existing 100A (3-phase) circuit without replacing the 100A breaker with a 70A breaker? Since the OCPD is already installed inside the equipment.
 
Is there a code section in NEC Art 430 that states that the breaker is allowed to be rated higher than the integral motor OCPD? I know the table that sets the maximum overcurrent protection rating as a percentage of FLA, so shouldn't this also be applicable to the branch circuit breaker, not only to the equipment's integral OCPD?
 
elecshop said:
Is there a code section in NEC Art 430 that states that the breaker is allowed to be rated higher than the integral motor OCPD? I know the table that sets the maximum overcurrent protection rating as a percentage of FLA, so shouldn't this also be applicable to the branch circuit breaker, not only to the equipment's integral OCPD?
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The 100 amp circuit is a feeder. Think of it like the 60 amp breakers were in a 400 amp panelboard. In your application the 100 amp wire replaces the panelboard bus.
 
petersonra said:
I think there is a provision to allow the feeder conductor ampacity to be smaller than feeder ocpd rating for multi motor feeders.
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But only where the feeder OCPD is not greater than largest feeder breaker permitted for the circuit. The largest feeder breaker for this circuit is 70 amps. If the feeder OCPD was 70 amps the feeder conductors could be 8 AWG assuming everything is rated at 75°.
It is my opinion once the feeder OCPD is larger than what is permitted 430.62(A) you treat it like any other circuit...that is the conductor ampacity must equal or exceed the rating of the OCPD.
 
augie47 said:
As I read 430.62, 70 amp would be the largest allowed.
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We don't need to use 430.62. Any size feeder is permitted to supply the two 60 amp breakers. We only need to use 430.62 of the feeder conductors have an ampacity less than the rating of the feeder OCPD.
 
augie47 said:
As I read 430.62, 70 amp would be the largest allowed.
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If the conductors are sized per 430.24, agreed, as per 430.62(A). But if you want to use a larger OCPD, then 430.62(B) says that's fine, you just need to match the ampacity of the conductors to the larger OCPD used, you no longer get to size them per 430.24.

(Or what Don said).

Cheers, Wayne
 
don_resqcapt19 said:
The 100 amp circuit is a feeder. Think of it like the 60 amp breakers were in a 400 amp panelboard. In your application the 100 amp wire replaces the panelboard bus.
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based on 430.62: a feeder supplying a specific motor load(s) shall be provided with a protective device having a rating or setting not greater than the largest rating or setting of the branch circuit short circuit and ground fault protective device for any motor supplied by the feeder (based on 430.52) plus FLA of the other motors.

applying this: assume squirrel cage 3 phase, inverse time breaker: --> 250% FLA + FLA = 70A

assuming the conductors are #3, shouldn't the breaker protecting this 100A feeder conform to the limits above and not exceed 70A?
 
wwhitney said:
But if you want to use a larger OCPD, then 430.62(B) says that's fine, you just need to match the ampacity of the conductors to the larger OCPD used, you no longer get to size them per 430.24.
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thanks. this is the code section I am searching for.
 
don_resqcapt19 said:
We don't need to use 430.62. Any size feeder is permitted to supply the two 60 amp breakers. We only need to use 430.62 of the feeder conductors have an ampacity less than the rating of the feeder OCPD.
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agree... I read into the OP the conductors were smaller than allowed on the 100 amp breaker... my bad....
 
augie47 said:
As I read 430.62, 70 amp would be the largest allowed.
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If the conductors have ampacity of less than 70 amps.

Which in OP's case if they are 20 amp motors the conductor ampacity can be 20 x 1.25 + 20 =45 amps minimum and be on said 70 amp breaker.

If you wanted to use a 100 amp breaker here then you would need to have 100 amp conductors.
 
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