I need a subpanel in detached garage/shop 250′ away. Nothing too big a draw, so I figure 60 amps 4-6 circuits should be enough.
FHB magazine seems to say #6 guage is ok, but doesn’t refer to distance. Am I in for too much voltage drop here. Just how much?
Replies
Nannygee,
Looks like you'll need #4 copper or #3 Al to get to or below 5 percent drop in the feeder. The NEC recommendation is for 5% or less total drop (feeder + branch). If you want to get to 3% drop in the feeder, you'll need #3 copper or #1/0 Al. That's based on a 60 amp non-continuous load, which is a lot--unless you're running a Unisaw, a big dust collector, and electric heat (or AC) all at the same time.
If you can set up the stationary power tools to run on 240V, they'll draw half the current. So you can run a smaller feeder.
For a maximum load of 30 amps (say 5 amps for lights and 25 amps of motor load), #8 copper or #6 Al will get you about 5%, and #6 copper or #3 Al will get you just about 3%.
You'll have to figure out what the maximum load will be. Keep in mind that motor startup current draw will be 3-5 times the running current. If you don't size the feeder for motor startup peaks, you're going to see a big voltage drop out when you start the table saw (or the AC compressor kicks in). The lights will flicker big time.
Good point on the article--it was left implicit that the gages were for standard run lengths (i.e., less than ~100 feet). But then, I never intended the article to provide all the information needed to install a subpanel. There's just too many variables to deal with, to cover it all in six pages.
One of the best books I've seen on residential wiring is Ray C. Mullin's "Electrical Wiring Residential" by the Del Mar Press. "Wiring a House" by Cauldwell and the "Code Check Electrical" by Kardon (both by Taunton Press) are very good. And if you want to play with voltage drop calcs yourself, try Mike Holt's website. I used a program on my Handspring PDA (I've got a conduit sizing calculator on it too--both real timesavers).
Good luck with the project--
Cliff Popejoy
Sacramento, California
Edited 5/9/2003 12:43:21 AM ET by CAP
Thank You! I may try 30 amps. I'll have to mull it over for a while...