Adding 220V to my garage


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? What amperage breaker would I use?
Thanks in advance for any help.
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I would upgrade the main to 200 amps, you might then use your old main as your new gaarage sub panel,
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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.

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)
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Okay, I appreciate the advice. Once I've figured out the total amperage of all items which could be switched on simultaneously, then I can add a breaker. How do I then know the guage of the wire I'll need (ie, is there a chart somewhere that I can access?)
The furnace manufacturer recommends the furnace be on its own 15 amp breaker, and aside from that, I normally would only be using one power tool at a time. I've been using the garage as a shop during the summer months, and I've yet to trip the 15 Amp breaker, if that helps.
I would like to add the 220V wiring while I'm at it, though at present, my only 220V tool is a pressure washer, which would, whenever used, be used on its own (just to wash the truck, etc.)
I'd rather not upgrade my service to 200 AMP unless absolutely necessary. If that were the only way to get 220V in the garage, I'd pass for now, and simply run a new 15 Amp run to the furnace, and stop there.
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Under those conditions, you're probably fine with an 8/3g on a 40 amp main

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