Garage electrical question, sub panel install
 

Hi,
I need more power in the garage! I've got a detached garage, 20x25", that needs 220V. I've bought a 100A sub panel, and am getting ready to trench for the wiring. The total wire distance between the main panel and sub panel is +/-100ft. Electricity demands are as follows: light circuit, 20A; 110V electrical outlet circuit, 20A; 220V, 20A circuit for 5HP compressor motor; 220V outlet for welder, 30A; 220V circuit for mill/lathe, 30A.

Okay, I don't have the welder, mill, or lathe yet, but want to be ready with the electrical when they come. I also realize that the total amperage for all circuits exceeds 100A, but the mill, lathe, and welder will likely never be run at the same time. The compressor will be hard wired to a switch, so, if I'm working in the garage, it could come on at any time. There is also the possibility that I will need a 220V circuit for a combo heat pump/ A/C unit I have laying around.

Question: what gage wire should I run from the main panel? My boss (who's wired all of the 3 phase stuff in our machine shop), says I'll be fine with 8ga. I was thinking 6ga. The guy at Lowes just told me that, to be on the safe side, I should run 4ga. The wire will be buried in conduit, but, remember, it is about a 100ft run.

Any thoughts? I'd like to trench this weekend.

Thanks,
Jim

I think I would use #6

But thats just me.

This may help you
http://www.electrician.com/vd_calculator.html

Jim I would also look at just putting in a whole new service at your garage. That is what I did and have a 200 amp panel. I know you are anxious to get started, but a year from now you sure want to be happy with what you have to live with.

Here's my garage's sub panel (wired it myself with some help from fellow Pelicans) - I don't remember the guage of wire, but someone should be able to tell from looking at the picture.

http://www.rennlight.com/shop/image/2100302.JPG

I'd go with at least a #6, #4 recommended. If you're welding you'll be running 20-30 A continuous. Then you could have an air compressor or heat pump turn on at the same time, drawing another 50-60A or so. And this doesn't include the peak motor start-up currents which are higher yet. You want to size the wire so you don't get a significant voltage drop to the panel, and on out to the tool or equipment. Use this wire size calculator to help you size it out., based on distance and expected max. peak amps.

I should mention - my compressor does not run off of this panel.

I would strongly recommend the above. I'd stay with 20 amp circuits for almost everything (which you're already doing), for the 220v compressor I'd go with 20 amps for each leg, 40 total; and if you think you'll need more air later, then 30 amps for each leg. Always build in substantial headroom on the current capacity to allow for start up power draw.

You really need a 200 amp box, straight from a separate service.

Years ago I worked for a mobile home company, for a 100A service we would install a 2/2/4 aluminum. Aluminum is higher resistance than copper, so you can go slightly smaller with copper. I would think no smaller than 4 gauge though, better too big than too small. This 220V 100A service was enough to run an entire 4 bedroom 2 bath home, A/C condenser and blower, dryer, etc.