Xfmr upgrade question

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martsco11

Member
Location
Anaheim, CA US
Gentlemen, I have an existing 45kva xfmr (480 3ph to 120/208v 3ph) that I need to upgrade to a 75kva. The existing conduit run is 400' of 1" emt. I need to upgrade the wire to at least #2 thhn to handle the 90-100amp primary. According to the wire fill table I can fit 3 - #2 wires into the existing conduit. Here's the question...
Do I need to derate the wires (use larger wires), or can I re-tap the xfmr to accommodate the voltage loss (3%)? I'm trying not to replace the conduit... Most xfmrs have taps to allow for lesser or greater voltages.
 
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xformer

Senior Member
Location
Dallas, Tx
Occupation
Master Electrician
martsco11 said:
Gentlemen, I have an existing 45kva xfmr (480 3ph to 120/208v 3ph) that I need to upgrade to a 75kva. The existing conduit run is 400' of 1" emt. I need to upgrade the wire to at least #2 thhn to handle the 90-100amp primary. According to the wire fill table I can fit 3 - #2 wires into the existing conduit. Here's the question...
Do I need to derate the wires (use larger wires), or can I re-tap the xfmr to accommodate the voltage loss (3%)? I'm trying not to replace the conduit... Most xfmrs have taps to allow for lesser or greater voltages.
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Why would deration be necessary? You Have no more than 3 CCC's in the conduit. What about the EGC? Still have room for that along with the 3 - #2's?? :)
 
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petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
martsco11 said:
I'm going to use the emt as the grounding conductor.
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good choice.

#2 is good for 115A at 75 deg C for thhn. I'd be inclined to see if I could get a 115A CB. I doubt such a beast exists so maybe 110A. I don't see why it needs any derating unless there is a temperature issue and that can likely be resolved by using 90 deg C wire.
 
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martsco11

Member
Location
Anaheim, CA US
Well, other than driving a ground rod or finding building Steel, or locating a cold water ground nearby it's the least of my options. I cannot fit a number 8 ground with the three number twos in a 1 inch conduit according to the NEC. Believe me, I'd feel better pulling in the ground. But I will test it to make sure it's up to par.
 
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xformer

Senior Member
Location
Dallas, Tx
Occupation
Master Electrician
petersonra said:
good choice.

#2 is good for 115A at 75 deg C for thhn. I'd be inclined to see if I could get a 115A CB. I doubt such a beast exists so maybe 110A. I don't see why it needs any derating unless there is a temperature issue and that can likely be resolved by using 90 deg C wire.
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I concur unless AHJ trumps it. :)
 
electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
martsco11 said:
Well, other than driving a ground rod or finding building Steel, or locating a cold water ground nearby it's the least of my options. I cannot fit a number 8 ground with the three number twos in a 1 inch conduit according to the NEC. Believe me, I'd feel better pulling in the ground. But I will test it to make sure it's up to par.
Click to expand...

You still need a GEC in addition to the EGC run with the primary conductors.
 
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