Voltage Drop Calculation

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TacomaMike

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I have a 120/240v 100 amp 1-phase panel with light load (about 15 amps). The pad mount trasformer feeding this panel is 1000' away. When calculating the wire size for the voltage drop do I use 100 amps or do I use 15 amps in the formula? (I realize this is fine assuming the load will never change).
Using the formula Vd = 2k x L x I / Cm
 
Running 240v 1000' is brutal for voltage drop calcs.

I just did a 1300'+ run, but I had the power company set a 480v 3 phase service then used a stepdown transformer at the load.
 
You use 100A for sizing the panel feed under the assumption that the load might increase to that someday and you do not want to have to repull wire.
You use 15A for sizing the panel feed under the assumption that you will never have a higher load.

And if the only protection at the panel end of the feed is a 100A main in the breaker, and/or you have a 100A feeder breaker or transformer secondary breaker at the transformer also, then the size of wire you need for it to be protected by those 100A breakers will quite likely be quite large enough for voltage drop control with a load of only 15A, even at 1000'. Work it out to confirm this. :)
(15/100 is close to 1/6. A 1000' conductor sized for 100A will have roughly the same resistance as a 150' conductor sized for 15A, so you might want to go a bit larger even for the 15A calculation.)
Be sure to use the two way distance for a 240V load and the one way distance for 120V balanced loads.
 
Thanks for the input... NEC has some 'recommendations' regarding voltage drop (FPN 210.19 (A)(1) Note 4 and 215.(A)(4) Note 2), relating to the 3% and 5% value. Can I assume these are values are based on actual load draw and not a 100A load panel?
 
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