Voltage Drop

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jemsvcs

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A property owner (who thought he was doing himself a favor) installed over 200 feet of 1 1/4 SCH 40 from his residence to a detached garage that as built a few months ago. The voltage drop, however, if we use 2 AWG for a 100 A subpanel, would be close to 6%.

Sizing wire to keep Vd to 3% would require 2/0 - which isn't happening in 1 1/4 SCH 40 :cool:

I had another electrical contractor tell me it should be ok to run he 2 AWG in spite of the voltage drop.

Any input would be much appreciated.

Thanks, Jemsvcs
 
Re: Voltage Drop

What's wrong with running four 2/0 wires in 1-1/4 Sch. 40? Just get yourself a big bottle of wire lube and jam it in! :D

Back to reality...

Actually, I think the 2 AWG wire might be OK. When you calculate voltage drop, you want to look at the actual load, not just the size of the panel. Is the homeowner really going to be pulling 100A in his garage? Sure, to be completely covered, you can calculate based on the 100A, but realistically, I doubt he's going to be pulling that much current in his garage.

I have a friend who put in a 100A panel in his new detached woodshop, and he did it with 4 AWG wire. He hasn't had any issues with voltage drop, despite using heavy-amp tools, mainly because he never pulls more than about 30A at any one time.
 
Re: Voltage Drop

I agree with Jeff this 2 awg should be fine.

2 AWG Copper

240 Volt

80 Amp load

3% voltage drop

Using those numbers I come up with 236' of distance one way. (472' of conductor)

189' one way if you figure a 100 amp load, but I would not.
 
Re: Voltage Drop

Thanks for the quick reply guys. I realize that the total draw in the garage will probably never get close to 100A, but I always try to think about future add-ons and liability issues that might pop up.

Jeff...I don't see a thing wrong with cramming 4 2/0's in an 1 1/4 SCH 40...as long as i'm not the one doing the pulling :D By the way, I usually run three wire to a detached garage and bond at the subpanel.

Bob...your calculations are on for copper...but I usually use Al to feed a subpanel in a detached garage. Same result however.

Thanks
 
Re: Voltage Drop

You might want to check out the subpanels thread in this forum. One of the moderators talked about how subpanels should have their neutrals and grounds separated and why. This is one of the reasons I run four wires rather than three to subpanels.

Then again, the last time I talked to an inspector on this subject, he indicated that either way was OK as far as he was concerned.
 
Re: Voltage Drop

Jeff...I'll have to check that out...thanks. As far as Code is concerned however, as long as you don't have parallel paths for your return you are OK.

Jemsvcs
 
Re: Voltage Drop

I have but are there Main Breaker panels available less than 100A...all the 70A panels I see are Main Lug only. Unless there are some back-feds out there somewhere?

Thanks,

Jemsvcs
 
Re: Voltage Drop

Backfeeding a panel using a main retainer is OK. BTW I did a calc using #2cu (.194) =3.23%VD. @ 200' to the garage. I think I would reduce the main breaker size but thats just me.

[ August 11, 2004, 06:55 PM: Message edited by: buck33k ]
 
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