40 amp breaker feeding a 30 amp rated disconnect

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Steve McKinney

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Having differences of opinions on the following. Can I feed a 30 amp rated disconnect with a 40 amp breaker? Feeding an overhead crane in a factory. More fuses in the crane control panel but from breaker to disconnect....aren't they feeders?

Thnx in advance guys.....
 
Steve McKinney said:
Having differences of opinions on the following. Can I feed a 30 amp rated disconnect with a 40 amp breaker? Feeding an overhead crane in a factory. More fuses in the crane control panel but from breaker to disconnect....aren't they feeders?

Thnx in advance guys.....
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A little more detail but if it is a fused disconnect generally yes. With motors and non fused disconnects, you might have higher breaker setting than disconnect rating in some instances - check HP rating of the disconnect.
 
kwired said:
A little more detail but if it is a fused disconnect generally yes. With motors and non fused disconnects, you might have higher breaker setting than disconnect rating in some instances - check HP rating of the disconnect.
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Yes Sir. It is fused and we are fusing it at 30 amps. My buddy and myself were under the impression that it would have to be rated at least to the rating of its' feeder breaker. I can not find anything in the Code to justify it tho......
 
Steve McKinney said:
Yes Sir. It is fused and we are fusing it at 30 amps. My buddy and myself were under the impression that it would have to be rated at least to the rating of its' feeder breaker. I can not find anything in the Code to justify it tho......
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Common practice is to make taps to larger feeders to make subfeeders or branch circuits. That is one reason to use a fused disconnect, to reduce circuit rating on downstream side. If supply side is already protected to level you need, fuses become sort of redundant, though you may still need a disconnecting means for various reasons.
 
Steve McKinney said:
Having differences of opinions on the following. Can I feed a 30 amp rated disconnect with a 40 amp breaker? Feeding an overhead crane in a factory. More fuses in the crane control panel but from breaker to disconnect....aren't they feeders?

Thnx in advance guys.....
Click to expand...

The way I look at it depends on if is considered a motor load...or not. If it is not a motor load then the wiring between the breaker and the fused disconnect must be rated 40 amps, downstream of the disconnect could be 30 amp wire. If it is a motor load then in most cases fuses are selected at 175% of the motor load and breakers can be sized at 225%. (other rules also apply) This could account for the two different overcurrent protection ratings. as kwired pointed out upstream of the feeder is usually protected at a higher rating......that's good practice, but it doesn't have to be. Motor feeders and branch circuits are usually sized at 125% of the motor amp draw (from the NEC motor amp chart or the equipment name plate if multi motor
 
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