4160 feeder to 750 KVA transformer

[steve66]

Designing a new building feeder with a 750 KVA xformer. (The transformer and feeder are outside). According to my calculations, the primary Full load amps = 104. According to table 450, I can fuse the primary at 300% of the primary FLA, or at 300A.

The problem: If I fuse the primary at 250 amps to keep the transformer inrush from blowing the fuse, then I have to size the feeder for 250 amps. Correct?? It seems silly to use a 250A feeder for a 100A load.

Is my only other option to reduce the primary fuse size?? Would anyone reccomend using a 150A slow or super slow blow fuse?

Steve

 

[charlie b]

Re: 4160 feeder to 750 KVA transformer

Originally posted by steve66: If I fuse the primary at 250 amps to keep the transformer inrush from blowing the fuse, then I have to size the feeder for 250 amps. Correct??

I have often wondered the same thing. I can't find a trail through the code (notably including 240 and 450) that says otherwise. I don't even know if you can get away with primary conductors rated for 125% of the 104 amps, by taking credit for secondary protection at that level. So I'll be watching for the collective wisdom of the membership to clear up this mystery.

 

[bob]

Re: 4160 feeder to 750 KVA transformer

Steve
I think you would need a 175 or 200 amp fuse but check here for additional info.
"http://www.hi-techfuses.com/downloads/3phHighAmpere-Cooper(Ver1.3).PDF"
Copper Power and Kearney.
Does the customer have their own substation?
I ask this because if you are on a standard distribution sub with other customers the utility will have a max fuse size that will coordinate with the sub breaker.

[ December 08, 2005, 07:49 PM: Message edited by: bob ]

 

[steve66]

Re: 4160 feeder to 750 KVA transformer

Thanks Bob, 200A does sound better. I could use #1 on the primary.

STeve

 

[bob]

Re: 4160 feeder to 750 KVA transformer

Steve
Check with the MFGs I suggested and ask about the time delay fuses. It may be possible to reduce the ampacity even lower.