Thoughts....

A

Alwayslearningelec

Senior Member
Location
NJ
Occupation
Estimator
This is temporary power. Think this is most efficient way to run power to the 100A panels? Was thinking maybe one 400A feeder then redciue feeder size as you go up as less power would be required.



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Alwayslearningelec said:
Was thinking maybe one 400A feeder then redciue feeder size as you go up as less power would be required.
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Then you could eliminate the 100 amp disconnects and apply the tap rule to each panel. The taps would have to be larger than the #2 Aluminum risers that they show.
 
infinity said:
Then you could eliminate the 100 amp disconnects and apply the tap rule to each panel. The taps would have to be larger than the #2 Aluminum risers that they show.
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So might not be miore effecient then what is shown. Anyway we could use the 90 degree column for sizing the AL MC cable?
 
If you apply the appropriate tap rules could you not eliminate the 400 amp disconnect ?

(#2 AL is not sufficient for a 100 amp breaker)
 
Yes.
(and if I understand that rule correctly for >10 fr to 25 ft his conductor would need to be rated around 140 amps.)
The 10 ft tap is more lenient but thats true on a tap from his 400 amp disconnect also,
 
augie47 said:
Yes.
(and if I understand that rule correctly for >10 fr to 25 ft his conductor would need to be rated around 140 amps.)
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Correct. From his drawing it looks like the 400 amp disconnect is providing the secondary OCPD within 25' of the transformer so I'm wondering how that OCPD could be eliminated.
 
I looked at "detail A" and that looked like the disconnects were real close to the transformer. If the 10ft tap rule won't work my idea is moot
 
augie47 said:
I looked at "detail A" and that looked like the disconnects were real close to the transformer. If the 10ft tap rule won't work my idea is moot
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Ah I think that I've got it. You would get rid of the 400 amp disconnect and come out of the transformer 4 times, once to each of the 100 amp disconnects.
 
infinity said:
You need an OCPD but I think that Augie was saying if you came out of the transformer 4X directly to each 100 amp disconnect then you wouldn't need the 400 amp disconnect.
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Well that would make more sense for use if allowed by code. We are coming out of the disco 4x's so why not just come out of xfrmr? How do I determine fo that is allowed( not using 400A disco)?

Would we need the (4) separate 100A disconnects(detail A) to feed each of the panels?

I though a disconnect switch on the secondary side of xfrmr is usually always needed when feeding panels located away from xfrm in my pic.
 
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infinity said:
You need an OCPD but I think that Augie was saying if you came out of the transformer 4X directly to each 100 amp disconnect then you wouldn't need the 400 amp disconnect.
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Sorry....so your saying you would come out of xfrmr 4x's to each 100A disconnect and they run to each 100A panel from each disconnect?

How the heck would you land 4 sets of conductors in the xfrmr? Can land multiple wires under each lug?
 
I suggested coming out of secondary side 4x's to each 100A disconnect and I was told it can't happen because line and load need to be fused. Not sure I understand that.
 
Alwayslearningelec said:
I suggested coming out of secondary side 4x's to each 100A disconnect and I was told it can't happen because line and load need to be fused. Not sure I understand that.
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Don't worry I don't understand that either. What does line and load have to do with the secondary conductors of a transformer? You can come out of the 112.5 KVA once to the 400 amp switch which will provide the overcurrent protection. OR as Augie ss saying you can come out 4 individual times, once to each 100 amp disconnect and eliminate the 400 amp altogether.
 
In any event, you need to follow the 240.21(C) tap rules for your transformer secondary.
Simple if you come to a 400 amp disconnect > conductors rated at at least 400 amps.
If you go to 4 separate 100 amp disconnects, I don't see a problem with conductors rated 100 amps or greater.
If you have to use the 25 ft tap rule the 100 amp conductors are questionable (ratios, 1/3 ampacity.etc...I've not tried the math)
I'm sure Rob understands that rule better than I and he can advise on the conductor required.
4 runs to a 112.5 kva should not be a problem connection-wise if you go that route.
 
infinity said:
Don't worry I don't understand that either. What does line and load have to do with the secondary conductors of a transformer? You can come out of the 112.5 KVA once to the 400 amp switch which will provide the overcurrent protection. OR as Augie ss saying you can come out 4 individual times, once to each 100 amp disconnect and eliminate the 400 amp altogether.
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Think they mean disconnects must be fused???
 
augie47 said:
Simple if you come to a 400 amp disconnect > conductors rated at at least 400 amps.
If you go to 4 separate 100 amp disconnects, I don't see a problem with conductors rated 100 amps or greater.
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In post #28 Augie has given you the other option.

The way it's depicted in the Detail A you have the XFMR>400 amp Disconnect>Wireway>Tap Conductors to line side of 100 amp circuit breakers. After that you have 4 feeders to each individual temp panel. Those feeder are currently designated as #2 Aluminum. Since #2 Al is only rated for 90 amps and 90 amps is a standard size they cannot be used for 100 amps.

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