Fuse Choice, NEC

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FionaZuppa

FionaZuppa

Senior Member
Location
AZ
Occupation
Part Time Electrician (semi retired, old) - EE retired.
Here's the setup & issue.
208v3ph60Hz service, fused disco is the main, then dumps into a breaker panel.
Service (disco) is fused 100A. (currently assumed ~35kva / 100A service)
Shop installed 208v3ph 77A rated equip (motors).
The breaker panel has 100A 3ph to the equip.
The std "fast" fuses in disco pop (usually one) during equip start.

Can the disco fuses be replaced with slow-delay type if the breaker panel has equip and other branch ckt's on it?
I would recommend an upgrade to 200A service, but shop guy does not want to go there just yet, hence the ask about changing fuse type.
 
Unless the "standard" fuses were sized for motor protection (doubtful), I would go with the delay-fuses,
Very common on motor loads.
 
The main question is, can delay fuses be used if the disco is feeding a breaker panel, which is not just for the equip?
 
petersonra said:
Have you ever seen a series rating between a fuse and a CB that required the use of fast blow fuses?
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Yes. I was surprised recently to discover there is not a series rating from a 200A RK at 65k to a standard 10k MCCB. Would need class T. If you look through the series rating chart, you will see many more options with quicker fuses.
 
petersonra said:
Have you ever seen a series rating between a fuse and a CB that required the use of fast blow fuses?
Click to expand...
Yes.
The general class of fuse, like RK, does not tell you it's time characteristics so most series ratings charts aren't based on the actual fuse speed, however it doesn't hurt to double check.

Also, for arc flash mitigation the actual fuse speed is often an important criteria and should be considered even when simply changing between brands.
 
jim dungar said:
Yes.
The general class of fuse, like RK, does not tell you it's time characteristics so most series ratings charts aren't based on the actual fuse speed, however it doesn't hurt to double check.

Also, for arc flash mitigation the actual fuse speed is often an important criteria and should be considered even when simply changing between brands.
Click to expand...
Interesting. I can't generally use series ratings because most of my loads are motors, although these days many times they are all VFDs.

I don't recall the series tables with fuses ever required a fast blow fuse. Most often they required a class T, J, L, or RK fuse. I don't recall it ever requiring a fast blow fuse, although I think maybe all class T fuses are fast blow, but I don't recall ever seeing a table that only had class T fuses listed for a series rating.
 
electrofelon said:
What's in there now? Maybe change to an RK5?
Click to expand...
Fast acting is in there now. 208v3ph was there, then they brought in some equip, popped in a tied 100A3ph breaker.
Counting breaker amps, it's way oversubscribed. 1st gonna check inrush and running amps of the equip. It really needs a service upgrade.
 
petersonra said:
Interesting. I can't generally use series ratings because most of my loads are motors, although these days many times they are all VFDs.

I don't recall the series tables with fuses ever required a fast blow fuse. Most often they required a class T, J, L, or RK fuse. I don't recall it ever requiring a fast blow fuse, although I think maybe all class T fuses are fast blow, but I don't recall ever seeing a table that only had class T fuses listed for a series rating.
Click to expand...
General series ratings are done with an umbrella fuse built specifically to UL specifications. By using this special fuse for the series rating tests, you can be sure that any commercially available listed fuse will perform correctly as long as the fuse Class matches.
As you leave the world of general series ratings it is possible, although unlikely, for a manufacturer to test very specific combinations of fuses and breakers. Substitution would not be allowed for these very specific series ratings.

My point was really to warn the OP to check if series ratings had been applied and consider how any system changes (including adding motor loads) might affect them.
 
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