%Voltage Drop and NEC

[winnie]

Transformer impedance is a measure of the voltage drop 'built in' to the transformer.

You expect voltage drop in a wire run; when you have no load on the wire, the voltage at the end of the wire is the same as the voltage on the supply side. As you increase the load and draw more current, more and more voltage gets 'used up' in the wire, and the voltage at the load side goes down.

A similar effect is seen in the transformer itself. The voltage on the output terminals goes down as the load increases. Different transformers have different impedance ratings. The impedance rating on a transformer is simply the % voltage drop at full load versus no load.

In your installation, you are using 75kVA transformers at about 60kVA. If these transformers had 3% impedance rating each, then the string of 3 transformers would result in a 7.2% voltage drop versus no load, substantially in addition to any voltage drop in the conductors themselves. This number should give you an idea of the issue that you are dealing with, but is not a hard number by any means.

This answer is going to be approximate for several reasons: I am ignoring things like the power factor of the load and the long line, and I am just guessing at the transformer impedance ratings. You can shop for different impedance ratings; just don't ignore this rating and buy 75KVA transformers on price. If you oversize the transformer the 'voltage drop' at any given load will be reduced.

I am not qualified to answer your question about the difference in drop between single conductors and cable. I would expect a difference in cable inductance and thus drop associated with power factor, but I don't know the details.

-Jon

 

[ramsy]

If the FAA won't let the local POCO run their UG primary all the way to the building transformer, then consider running your own 13.8 KV UG from where the POCO is now going to set their pad mt. xfmr

Would the FAA go for this, if the POCO puts a remote disconnect at their pad mt, then comes back to energize it when your ready. That could save the cost and inefficiency of running two extra xfrms.

 

[ramsy]

FAA won't allow (13.8kv utility run primary directly to the pad) for whatever reason.

If the utility can provide a 480vac 3?, that 4400 ft V-Drop @ 480vac with 58 kva is only 15 volts dropped (L-L) or 3% with 1-per ? 500 kcmil in PVC @ PF=1. Using EDR, Electrical Designer's Reference. EDR smokes IEEE methods, but also suffers the 600vac max limit. 4400 ft of #500 would cost you, but still only require one single phase step down xfmr. near the main distribution.

Your best design, and lowest cost would be running the service under the airport near the building.

Using a reverse engineered spreadsheet matching EDR's above figures, which also assumes a balanced system, but accepts 13800vac 1?, that 4400 ft V-Drop @ 58 kva or 4.2A fits 2/0 cu with Ze=0.440 and 2 volts dropped. Using #6 cu in PVC @ PF=1 this equation results in Ze=2.16 and 9 volts dropped over 4400 ft. I'm not comfortable with Z's near 0.50, but then never worked with MV either.

 

[ramsy]

I think reactance, proximity, and skin effects are the main problems with V-Drop calcs below 600vac. MV may also require conductor ampacity calcs. like NEC 310-15c that needs Engineering Supervision. You should probably ask the POCO for the cable, raceway, and calcs they would use to run 13.8kva 4400 ft underground.