Yet another UL Question

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Mcamnl

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This one comes from my boss.
Going from our motor back, we have a contactor, manual motor starter, and now we are adding fusing to the chain. He wants to know if we can leave out the motor fusing if a fused disconnect is being used to power the machine? I told him no according to the examples on 6.1. His concern is with the need to go to the J class fuse on this unit. We are running a 10HP motor at 480V 3P. According to Table 31.1, with a Class CC fuse we can size at 300% FLA of the motor. That puts us over the 30amp limit of the CC class. I moved to the J Class to be able to go to 175% FLA of the motor. This takes up more room in the panel and adds more cost. That is his concern.
Am I right on my figuring here? We do need to keep the fusing the way it is.
 

jim dungar

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Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
What standard are you referencing?

Generally, but not always, the overcurrent protection can be located anywhere "upstream" from the device it protects.
 
jim dungar said:
What standard are you referencing?

Generally, but not always, the overcurrent protection can be located anywhere "upstream" from the device it protects.

The fuse provides short circuit protection for the motor AND the conductors feeding it. The overload protection is provided by the manual motor starter. Some type of manual motor straters do incorporate both overload and short circuit protection. I wonder why you have a contactor and a manual motor starter. Usually one would use a contactor as part of the combination motor starter or a manual motor starter. Seems like some redundancy occurs here.
 

davidr43229

Senior Member
Location
Columbus, Oh
We are running a 10HP motor at 480V 3P. According to Table 31.1, with a Class CC fuse we can size at 300% FLA of the motor. That puts us over the 30amp limit of the CC class. I moved to the J Class to be able to go to 175% FLA of the motor. This takes up more room in the panel and adds more cost. That is his concern.
Am I right on my figuring here? We do need to keep the fusing the way it is.
I would think NEC 430.52 is the article I would referrence here. You are correct with the max sizing of 175% of the FLA of the motor, providing your Service Factor is 1.15. If the SF is not 1.15 you can go up to 225% of the FLA motor. WHich means that 30amps is the max you can size a fuse to this motor. If your main or disco within the equipment is higher than that you need to install an additional fuse.
If size is a concern try this:
http://cooperbussmann.com/pdf/849fb67e-0855-45e2-b2f2-0d5d3e34c4c7.pdf
http://cooperbussmann.com/pdf/0c7f80df-8324-4a60-8f51-a452d37cecc7.pdf
Just my $.03, corrected for inflation
 
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