Size and Drop

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kennyp

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Charleston, SC
I have a problem with a feeder size to a 277/480 to 120/208 step down xfmr. The xfmr is a 75kva left in the existing space by the old tennant and the new one wants to use it to feed a 100 amp 3 phase panel. The new service is 350 feet from where I want to install the xfmr and panel. All the voltage drop calculators I go to calculate a #1 thhn be used for not more than a 3% voltage drop. The contractor has an engineer that says I can feed it with a #4. I don't feel comfy with that. :confused:
I am using a 60 amp cont. and 40 non cont. load for the calc.
 
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kennyp said:
I have a problem with a feeder size to a 277/480 to 120/208 step down xfmr. The xfmr is a 75kva left in the existing space by the old tennant and the new one wants to use it to feed a 100 amp 3 phase panel. The new service is 350 feet from where I want to install the xfmr and panel. All the voltage drop calculators I go to calculate a #1 thhn be used for not more than a 3% voltage drop. The contractor has an engineer that says I can feed it with a #4. I don't feel comfy with that. :confused:
Click to expand...
Tell him to sign off on it to the local inspector then you can install it twice.
 
Using a 100 amp load at 350 ft my calculations are
#1 cu 9 Volts L-L 5.2 volts L-N 2.8%
#4 cu 19 Volts L-L 11 volts L-N 4%
at the transformer. You will have additional VD thru the transformer and the
208/120 volts wiring.
 
A 3?,4w,Wye panel at 100A shows each #1 cu, 350 foot feeder operating at 60c (rather warm); and at this temperature rise the proper voltage drop is 4.4%

That temp rise goes above 60c for any continuous loading near 100A, and blows most residential equipment listings (lugs, breakers, boxes, & panels).

At an 80A peak, same feeder & unity Pwr factor, using #4, the operating temp is 73c, corresponding to a 7.2% VD. Continuous loading at 80A rises #4 to 97c, and 7.75% VD.
 
ramsy said:
A 3?,4w,Wye panel at 100A shows each #1 cu, 350 foot feeder operating at 60c (rather warm); and at this temperature rise the proper voltage drop is 4.4%

That temp rise goes above 60c for any continuous loading near 100A, and blows most residential equipment listings (lugs, breakers, boxes, & panels).

At an 80A peak, same feeder & unity Pwr factor, using #4, the operating temp is 73c, corresponding to a 7.2% VD. Continuous loading at 80A rises #4 to 97c, and 7.75% VD.
Click to expand...
I was using 60 amps continuous and 40 non
 
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