Class CC or J fuses on small motors? (3-phase 480 under 30A)

Class CC or J fuses on small motors? (3-phase 480 under 30A)

  • Class CC

    Votes: 3 60.0%

  • Class J

    Votes: 2 40.0%
  • Total voters
    5
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emiller233

Senior Member
Location
pittsburgh, pa
Occupation
Controls-Automation engineer
Other than cost, no. Often a motor branch circuit is at the far end of a distribution line so the available fault current at that point is rarely going to be high enough to warrant a current limiting fuse. So for example an FNQ-20 fuse usually fits the same size holder as a CC, but is less expensive.

What you might find however is that control component mfrs have only listed their devices using current limiting fuses anyway, so getting an SCCR that you can use might require it anyway.
 
We are just starting to get requests from our customers for a high SCCR rating (65k). That's what we're starting to see is we have to use their 'combo of tested parts' to hit those numbers
 
emiller233 said:
We are just starting to get requests from our customers for a high SCCR rating (65k). That's what we're starting to see is we have to use their 'combo of tested parts' to hit those numbers
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Yep, people are finally getting wise to this issue and requiring it up front instead of having to try to deal with it after the fact when the panel shows up with a 5kA label on it and they have 35kA available.
 
I usually use Class CC fuses up to 30 amps and class J fuses up to a hundred. Above a hundred I find that a circuit breaker often becomes more cost-effective.
 
petersonra said:
I usually use Class CC fuses up to 30 amps and class J fuses up to a hundred. Above a hundred I find that a circuit breaker often becomes more cost-effective.
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I used to work at a panel design/build/program shop that did it the same way due to cost. Our biggest customer was always beating us up over price and nickel and diming us, so we had adopted that as a best practice. Caveat being that circuit breakers have poor SCCRs unless you spend big bucks, but most of our customers didn't care. But it was always an uphill battle when a high SCCR requirement came along, since they'd always balk at the price difference.

Last few years I have been in the habit of defaulting to J fuses, since that's what manufacturers usually specify to get a higher SCCR on their VFD/contactor/whatever. Still a struggle, but mostly due to other people. I've run in to people who think that putting a J fuse in the panel's main disconnect will give you a 300kA SCCR (despite every branch being protected by a cheap circuit breaker). Thankfully at my current place, our customers are usually light industrial, so they don't have high SCCR requirements (if they even know what SCCR is).
 
Can't dau that ive looked at it this closely yet. Do they specify class j (typically) or just 'current limiting'.
We're a distributor for the components too, so the price differences aren't quite as drastic at least. (Between class Cc vs J anyways)


olsondavidj said:
I used to work at a panel design/build/program shop that did it the same way due to cost. Our biggest customer was always beating us up over price and nickel and diming us, so we had adopted that as a best practice. Caveat being that circuit breakers have poor SCCRs unless you spend big bucks, but most of our customers didn't care. But it was always an uphill battle when a high SCCR requirement came along, since they'd always balk at the price difference.

Last few years I have been in the habit of defaulting to J fuses, since that's what manufacturers usually specify to get a higher SCCR on their VFD/contactor/whatever. Still a struggle, but mostly due to other people. I've run in to people who think that putting a J fuse in the panel's main disconnect will give you a 300kA SCCR (despite every branch being protected by a cheap circuit breaker). Thankfully at my current place, our customers are usually light industrial, so they don't have high SCCR requirements (if they even know what SCCR is).
Click to expand...
 
olsondavidj said:
I used to work at a panel design/build/program shop that did it the same way due to cost. Our biggest customer was always beating us up over price and nickel and diming us, so we had adopted that as a best practice. Caveat being that circuit breakers have poor SCCRs unless you spend big bucks, but most of our customers didn't care. But it was always an uphill battle when a high SCCR requirement came along, since they'd always balk at the price difference.

Last few years I have been in the habit of defaulting to J fuses, since that's what manufacturers usually specify to get a higher SCCR on their VFD/contactor/whatever. Still a struggle, but mostly due to other people. I've run in to people who think that putting a J fuse in the panel's main disconnect will give you a 300kA SCCR (despite every branch being protected by a cheap circuit breaker). Thankfully at my current place, our customers are usually light industrial, so they don't have high SCCR requirements (if they even know what SCCR is).
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As far as circuit breakers go you'd be surprised sometimes at how cost-effective they can be even with higher short circuit current ratings. The circuit breaker can also serve as the disconnecting means and often that means you could save a lot of space in your cabinet which cost money. You just have to run the numbers. It helps if you get a good multiplier on circuit breakers from your local supplier though.

One other thing, many times you can have much higher rating on your the current limiting fuse to meet the short circuit current rating requirements that is needed to deal with a motor so it is possible to perhaps put a 60 amp class J fuse and distributed from it to smaller Class CC fuses that feed each branch.
 
emiller233 said:
Can't dau that ive looked at it this closely yet. Do they specify class j (typically) or just 'current limiting'.
We're a distributor for the components too, so the price differences aren't quite as drastic at least. (Between class Cc vs J anyways)
Click to expand...

Most contactors, VFDs, overloads, etc. that I've seen will require a specific class of fuse, and they will also specify the maximum size of said fuse in order to achieve the high SCCR. This is completely manufacturer dependent because it has to do with how the manufacturer tested and listed the component.

Looks like AB (for example) says you can use either J or CC fuses for their small IEC contactors to get 100kA, but I've seen other manufacturers that will only list one or the other. And technically J and CC fuses have different current limiting characteristics, so as far as SCCR is concerned they aren't interchangeable without doing some calculations.

On that note, UL 508A supplement SB has an exception where if a "High SCCR" for a component is achieved by a specific class of fuse, you are allowed to swap it for another class of fuse AS LONG AS the peak let-through and I2t are NOT GREATER than the specified fuse. Note that to do this, you must use the umbrella fuse values for Ip and I2t that UL has; you aren't allowed to use the fuse manufacturer's data sheet for this.

If you need to do that, I highly suggest you read up on it to make sure it suits your application because there are some other details that you need to be aware of. In practice I don't find that this is very useful, but I haven't had a situation where I can't use what the manufacturer specifies. Usually it's more trouble than it's worth.
 
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