using fuses for motor overload protection

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petersonra

petersonra

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Northern illinois
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Semi-retired engineer
I have a customer who thinks its a good idea to get rid of overload relays and rely on fuses for motor overload protection.

I never thought much about this before.

looking at 430.32(A)(1) seems to imply fuses would be ok, but 430.32(C) specifically talks about overload relays, while 430.36 talks about fuses used as overload protection.

is the difference between using overload realys and fuses for this purpose that 430.32(c) allows a higher setting for relays?
 
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Given the latest generation of Class RK1 & RK5 fuses, yes it is possible to provide motor overload protection with fuses alone.
However - in my several decades of observations, I have never seen anyone actually do it.
What size motors?? Application?? Voltage, etc??
A couple of problems come to mind. Ongoing Maintenance - a year from now when a motor stops, will you have sufficient spare fuses of the correct size available? If the 7 amp fuse blows, maybe we should put in 10's? Poof, there goes your protection. Just seems like an unnecessary headache.

Also, you can't just push a button and reset fuses. Could get expensive.

So - yes it can be done.
Could you even find an Engineer willing to do it? Maybe - Not Me - but maybe.
Be careful out there
db
 
barclayd said:
Could you even find an Engineer willing to do it?
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Ironically, the guy that wants to do it is a PE.

I talked to him about it this morning. He says they have been doing it that way for years and "never had a problem". I told him he would probably have to substitute a 3A fuse for the 5A fuse he had on the drawings to meet the 125% rule, which he agreed to. My guess is it will end up with the 5A fuse that was on the drawings.

I also told him the motors needed a disconnecting means. This is now shown on the drawings as "by others".
 
ultramegabob said:
I wouldnt want to rely totally on fuses for a three phase motor.
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I agree. Not only would you have to spend money on replacing fuses, you might be spending money to replace motors because they were single phasing and burned up from a blown fuse.
 
chevyx92 said:
I don't think there is a good reason for doing it. Just the fact that the NEC allows it. It's all a design issue.
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I am guessing that someone 50 years ago did it this way and they do not wish to change.
 
chevyx92 said:
. . . you might be spending money to replace motors because they were single phasing and burned up from a blown fuse.
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If the fusing were done properly, single phasing would take out additional fuses and protect the motors. Granted, only qualified persons could maintain this type of protection and not just go to a larger fuse when they felt like it. A supply of fuses and trouble shooting knowhow would be require if trouble developed. :)
 
This might be a reason to use fuses instead of overload relays, it's the same

reason people use for every thing else, it's a lot less expensive!!! If done

properly it is no less of a job, it code compliant, and the bottom line is less

all the way around.

side note: it's just as easy to get a higher ampere overload.
 
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