Simple VD Question

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bkludecke

Senior Member
Location
Big Bear Lake, CA
Occupation
Retired Electrical Contractor
I know how to calculate voltage drop but this has been bugging me since I started thinking about it. I'm running a feeder - 120/240 volt 1 phase 3 wire 450 feet to a building. The feeder is protected at 150 amps and the assumed connected load is the same.

I'll be pulling in aluminum conductors so I calculated 400 kcmil. Then I got to thinking; that was based on 240 volts. Should I have used 120 volts for the calculation and hence figure on 800 kcmil? This should be an easy one but I've been working too many hours this week.
 
If you really want to be 'worst case scenario' about voltage drop, then you would need to consider the case of a all the load being on a single 120V leg, and worse, that the load has starting current that exceeds the 150A feeder rating for a short time.

Imagine, if you will, that the load consists of two 120V single phase motors, each with a starting current of 700A. Now size your feeder to maintain less than a 3% voltage drop :)

Seriously, if the load is extremely 'pathological', then you might need to use a 120V base rather than 240V to figure voltage drop, but the reality is that you can almost certainly balance the loads so that 240V is fine.

-Jon
 
Yeah that's the way I think also. In fact, come to think of it, I have resolved VD issues by simply balancing the load on a feeder. Thanks Winnie.
 
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