ECN Forum Forums General Topics of the Trade General Discussion Area how to find voltage drop wire size

Before you get too far in this, you might check around for availabity of the feeder size Aluminum. Factories are quoting up to 26 weeks for delivery.

Hi Steve,

Actually, a PF of 1.00 using 2KIL is the most liberal, not consevative, of the PF values to use. The lower the PF factor, the more the voltage drop will rise and this could easily mandate a larger conductor size.

Using V = IR, R incumbases all factors contributing to the circuit resistance. One of the major factors is PF along with ambient temperature.

Check out IEEE Std 141 exact voltage drop formula or just download Volts from ECN's store to see how PF and ambient temperature affect voltage drop.

I guess that depends on whether or not you start with kW or kVA when you assume PF=1!

Ambient temperature will certainly have an impact on what the conductor temperature is, but in the end, it's only the conductor temperature that matters.


[This message has been edited by SteveFehr (edited 10-13-2006).]

Hi Steve,

"I guess that depends on whether or not you start with kW or kVA when you assume PF=1!"

Power factor is a independant measure of the cosine of the phase angle offset. If you look at the power triangle, the bottom leg is real, true or working power(P) in units of watts, kW or HP. The hypotenuse (top line) is apparent power(Pa) in units of VA or kVA, the verticle leg is Imaginary or Reactive power in units of VAR or kVAR. Using a little trig will derive the angle of reactive power or theta. The cosine of this angle is the circuit's power factor.


"Ambient temperature will certainly have an impact on what the conductor temperature is, but in the end, it's only the conductor temperature that matters."

The conductor's temperature is the accumulation of conductor heat from it's resistance and ambient temperature.

As a brief note, I am the author of Volts. Since Volts' download is free for 10-days you can try this problem as well as other scenarios to view the results.

Looking over the above ramblings and studying the original post, some of us have missed the subtleties..."I am putting out to bid a service..." apparently lite bulb ain't bidding on it, he's looking for bids and help in writing the specs. Bob hit the nail on the head, I think. And the wisdom of hauling all those amperes 300 feet seems lacking when at this point the whole installation (& how do I know?) is still on the drawing board. If lite bulb is designing the service, put the transformer closer to the switchboard, buddy. Let the POCO save you the voltage drop. And a building that requires that level of 208 might be better served by spec'ing the prime movers at 277/480 and setting a much smaller dry type xfmr for the small amount of convenience receptacles and computer equipment. But I have no way of knowing the occupancy and what kind of equipment is to be in the building, so this is all shooting in the dark. But there are more efficient ways of getting power to the building. Some people and POCO's are inflexible. Recently I overheard a HO in a supply house asking about "what size wire do I need to run secondary 800 feet from the transformer to feed a 200 amp service..." I guarantee that a single pipe with primary cable will cost less than 1/8 of a mile of 250/250/4-0+ G Al...

Electrical design is not a dart board, it is Physics + Economics+ NEC. Wal-Mart has evolved to where their services are multiples of 600 & 800 amp 277/480 V services, panelboards that don't require ground fault protection that you would need if the stores used a bigger single switchboard. Typically, they use a 800 A. PB for HVAC,another one for single pole lighting circuits, a third one for balers and other machine loads and this third one also feeds several dry type 120/208 xfmrs feeding PB's for snack bars and office receptacles, etc. I am sure that there have ben a few changes to the above with the advent of the Supercenters, but the basic scheme is to have the highest voltage to reduce voltage drop and break it up into less than 1,000 amp packages to keep from having expensive gfi switchboards...And btw, we used .9 pf in all our load calc's.

[This message has been edited by Almost Fried (edited 10-14-2006).]

[This message has been edited by Almost Fried (edited 10-14-2006).]

[This message has been edited by Almost Fried (edited 10-15-2006).]