Acceptable E drop to sub panel for general loads

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howardrichman

Senior Member
I'm installing feeder conductors for a agricultural barn from a residence 355 ft. to panel in barn. I've calculated approx. 7.5-9KW loads for tub heaters, heat lamps, general lighting and outlets. I'd need 40 amps but will supply at least 50 amps to be safe for any future loads. Using aluminum; I'm calculated voltage drop by using a common formula by DR Watts; using the conductors resistance, distance, and amperage. According to calculations, I'd need either #1 or 2 XHHW AL, depending on what an acceptable E drop should be. To my knowledge, there are no compressor or motor loads. With #1 alum. I've calculated approx. 4.5V drop for 50 amps. Could I safely get by using #2 Alum. ? What's an acceptable E drop for a sub panel feeding general loads?

HR...
 

Nom Deplume

Senior Member
Location
USA
Why are you trying to cut it so close.
If you know you need at least 40 amps and the additional loads are unknown, just run a 100 amp feeder out there and be done with it.
 

suemarkp

Senior Member
Location
Kent, WA
Occupation
Retired Engineer
It is difficult to answer. One big consideration is are your loads 240V or well balanced 120V loads? Another is that resistive things don't care much about voltage drop. But if there is motor operated machinery in that barn it will care about the long feeder length, especially any that are 120V (because you'd want a 120V Vd calculation instead of a 240V one).

I'd up the wire 3 sizes if there will ever be 120V 12A or larger motors used, especially if most of the other loads are on. Would think that may be a common thing on a farm... Using #2 AL is probably fine if there could be 240V motors. I'd probably go with 1/0 AL on a 60 or 100 amp breaker.
 

howardrichman

Senior Member
Well; I'd figured I'd balance the 120V loads of those tub heaters and other similar loads across the panel. SA; 4- 1.8kW 120V loads on 20A breakers, on @ 100% should be calculated 30A 240V as part of the load calc, and lighting round up from there, after calculating light and outlet loads@ approx. 10A 240. so, service may not need be more than 50-60 amp/240V.

HR...
 
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