Fused Disconnect feeding breaker taps

[bjenks]

Say you have three motors individually protected off a tap by circuit breakers that was sized at 225% of NEC table 430.250 and the feeder is protected by a fused disconnect would you size the fuse at a) Largest motor breaker size + FLA of other motor taps b) same as previous, but do calculations based on the largest protection being sized at 300% of FLA? c) something else.

The code seems to support option (a) but I would think that might create an opportunity to blow the service fuse on motor start up.

 

[bjenks]

I forgot one other question: Suppose these three motors that are fed by the fused disconnect is fed by an ATS. The largest CB motor is shut down upon power failure to lower the generator load. Next question, do you size the generator feeder breaker based on what the service disconnect would have been sized at if that motor wasn't there or do you stick to the rule of sizing it based on the actual fuse?

 

[jraef]

I would think that you size the conductors for the feeder based upon those rules, i.e. 125% of aggregate motor FLA, then size the fuses to protect the feeder conductors. Use Time delay fuses to avoid problems with motor inrush current.

You don't size generators to fit fuses, you size generators to handle loads. Fuses are the final step and are used to protect the conductors feeding those loads.

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