In article . com, "maurice" wrote:
hi. I've got an attached garage with a couple of plug-ins, an overhead
door, and a couple of lights. I want to use the garage as a shop.

I've purchased a forced-air furnace for the garage, and want to run
some new electrical for it. The house was built in the 1960s, and I
have an existing 100 Amp service. It appears that the garage shares
some circuits with part of the basement.

I also want to have 220V available while I'm at it.

My thought is to run 8/3 wire from my existing panel to the garage
(approx. 40'), and then run a sub-panel from that. does that sound
reasonable?

No. You don't have enough information yet to size the wire. When you do,
you'll probably discover that 8/3 isn't anywhere near big enough.

What amperage breaker would I use?

You're putting the cart before the horse. The first question is, what's the
amperage rating of the furnace? Add to that the total amperage of whatever
shop tools you may have running simultaneously (for instance, a table saw and
a dust collector). Then add the amperage of your lighting plus 25%. The total
of those is the minimum size of breaker you should have.

Then you size the wire to the breaker.

--
Regards,
Doug Miller (alphageek at milmac dot com)

It's time to throw all their damned tea in the harbor again.