Conductor size/ over current protection

[Chris egbert]

Got a compressor with a local 3p disconnect fused at 30amps mounted adjacent to it. The feeders serving this are #6awg cu thhn, and are protected by a 3p 100a breaker. Are the #6 conductors undersized? Is the 3p 100a breaker rating to large? My thought is do change breaker to a 3p 60a, but am not sure if this is necessary because the compressor is protected by fuses.

 

[chris kennedy]

Got a compressor with a local 3p disconnect fused at 30amps mounted adjacent to it. The feeders serving this are #6awg cu thhn, and are protected by a 3p 100a breaker. Are the #6 conductors undersized? Is the 3p 100a breaker rating to large? My thought is do change breaker to a 3p 60a, but am not sure if this is necessary because the compressor is protected by fuses.

Could 240.21 apply?

 

[rlundsrud]

While I would want to know the horsepower rating of the compressor, it is probably correct in that the OCP is sized for 175% of the listed current whereas the conductors only need to be rated at 125%. I wouldn't change anything without further investigation.

Bob

I apologize, I didn't read your post carefully. I don't know why there is a 100 amp disconnect feeding the 30 amp fuses. I do think the fuses effectively protect the wiring even though they are downstream, but I can't site a relevant code of the top of my head. I am certain one of my more learned colleagues will be able to provide that citation.

 

[augie47]

I'm with chris, the only way I can think of it being acceptable would be if the #6s met one of the 240.21 tap rules.

 

[starbolin]

Got a compressor with a local 3p disconnect fused at 30amps mounted adjacent to it. The feeders serving this are #6awg cu thhn, and are protected by a 3p 100a breaker. Are the #6 conductors undersized? Is the 3p 100a breaker rating to large? My thought is do change breaker to a 3p 60a, but am not sure if this is necessary because the compressor is protected by fuses.

Table 430.52 allows a higher maximum rating for the circuit protection breakers than for the fuses. Rating of the breakers is then often limited by the ampacity of the circuit conductors. Referring to table-310.15(B)-formerly-table-310.16, and for the sake of discussion, ignoring other derating factors #6 copper is good for 75A and picking the next size breaker gives you an OCPD of 100A.

 

[suemarkp]

Wouldn't the motor rules apply if there is nothing else on this feeder but motor loads? Did there used to be a larger compressor or more motors?

If the 30A works for that compressor, you probably can't go up to 100A using any of the motor over current allowances though.

 

[augie47]

Table 430.52 allows a higher maximum rating for the circuit protection breakers than for the fuses. Rating of the breakers is then often limited by the ampacity of the circuit conductors. Referring to table-310.15(B)-formerly-table-310.16, and for the sake of discussion, ignoring other derating factors #6 copper is good for 75A and picking the next size breaker gives you an OCPD of 100A.

I don't think you could ignore the termination limitations of 110.14, but assuming for a second that you could, the next standard size breaker over 75 amps is an 80.

 

[Chris egbert]

Wouldn't the motor rules apply if there is nothing else on this feeder but motor loads? Did there used to be a larger compressor or more motors?

If the 30A works for that compressor, you probably can't go up to 100A using any of the motor over current allowances though.

To the best of my knowledge this is an original installation from 1993 and I can see no evidence of additional electrical equipment/ motors etc.