Fused disconnect wire sizing

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krussell@scs-usa.com

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So I have a fused disconnect on a piece of equipment. Lets say it is fused for 60 amps.

If it is a .... 150 amp disconnect. Do you size for the fuses, or for the disconnect ?

I think you size for the fuses, but I had someone say they were sizing for the full disconnect potential (the total 150) even though the fuses prevent the load.

(these are hypothetical fuse sizes and disconnect sizes, but you get the point)
 
You can put 30 amp fuses in a 200 amp disconnect if the circuit only requires 30 amps. The over-sizing of the disconnect is irrelevant.
 
I had an inspector tell me the same thing several years ago. I had a 400 amp service, but the pipe in the ground wasn't big enough for a 400 amp aluminum wire, so I changed the main breaker to a 300 amp. Inspector tried to tell me I had to change to Copper conductors so that I could put a 400 amp wire in that pipe because the sticker inside said it was a 400 amp bus

He had an article number he was citing, I can't recall which one it was now. It did say that but it was a misapplication.

I asked about it on this forum. I'll go see if I can search and find it.
 
The opposite example is an 8 AWG conductor on a 20 ampere circuit breaker, the wire oversized for voltage drop or its what was on hand. Whats to keep someone from installing a larger CB or larger fuse?
 
krussell@scs-usa.com said:
So I have a fused disconnect on a piece of equipment. Lets say it is fused for 60 amps.

If it is a .... 150 amp disconnect. Do you size for the fuses, or for the disconnect ?

I think you size for the fuses, but I had someone say they were sizing for the full disconnect potential (the total 150) even though the fuses prevent the load.

(these are hypothetical fuse sizes and disconnect sizes, but you get the point)
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The wire is sized for the load.

The fuses protect the wire.

The fuses then are based on the load, not the disconnect. If you put 400A fuses in there, the conductors would have to be rated for at least 400A (unless feeding a motor starter).

The size of the disconnect is only relevant in that it has to be 115% LARGER than the load. How much larger is a function of your budget and willingness to deal with whether or not the fuses fit.

James L said:
I had an inspector tell me the same thing several years ago. I had a 400 amp service, but the pipe in the ground wasn't big enough for a 400 amp aluminum wire, so I changed the main breaker to a 300 amp. Inspector tried to tell me I had to change to Copper conductors so that I could put a 400 amp wire in that pipe because the sticker inside said it was a 400 amp bus

He had an article number he was citing, I can't recall which one it was now. It did say that but it was a misapplication.

I asked about it on this forum. I'll go see if I can search and find it.
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The stickers in the SOURCE panel said it was 400A? Why would that make any sort of difference? Even if the destination was a panel with 400A bus, that's ridiculous. If you put in a 300A feeder and conductors rated 300A, the destination bus bar size has to be at least 300A, but even if it were 600A, you still can feed it with a 300A circuit.
 
Jraef said:
The stickers in the SOURCE panel said it was 400A? Why would that make any sort of difference? Even if the destination was a panel with 400A bus, that's ridiculous. If you put in a 300A feeder and conductors rated 300A, the destination bus bar size has to be at least 300A, but even if it were 600A, you still can feed it with a 300A circuit.
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it was a single panel service.
Power came from Transformer, through CT cabinet, to 400 amp main breaker panel inside the building

He kept insisting that the 300 amp trip unit in the main breaker didn't matter. Told me it needed 400 amp conductors

That fiasco was his last. It turned out he had dozens of services held up, particularly 125 amp, and 225 amp panels fed with 100 amp and 200 amp cables.

He got fired, and two days later I had power
 
petersonra said:
As far as I know fuse reducers are not listed. Both fuses and disconnects are required to be listed.
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That's news to me. We've been using them for the 35 years that I've been in this business. :unsure:
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petersonra said:
I looked those up. I don't see much in the way of class R fuses these days. Bussman makes them for class J fuses but those are not listed.
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I think that you're correct that there are probably some fuse versions that are not listed.
 
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