From: Richard (metal@fullwave.com)
Date: Thu Apr 22 2004 - 20:32:18 PDT
Paul,
There are 4 things to consider:
1) Might you already have more than 15 amps
available? In other words, what is the service
rating coming INTO that garage box ?
Since there are -two- 15 amp circuits there, then
it's likely the service is -at least- 30 amps. In fact,
that would be a very minimal garage circuit. Most
are 60 amps or more.
You need to examine the main wires feeding that box.
What gauge are they? Where are they coming from?
And you need to tell us more info. Is it fuses, or breakers?
If breakers, is there a "main" breaker in the box? If so,
what's -its- rating ?
In any case, with a 30-amp or greater feed, you can put
a 20 amp breaker in that box and wire up a heavy-duty
20-amp outlet. Just don't use much else while you're using
it. (a few amps of lighting load is probably OK ).
2) Where is the feed coming -from- ?? Does it come
from a fuse/breaker in the main-panel in the house?
Or is it a separate feed from the meter ?
If you have/put a breaker-box in the shop that has a
main-disconnect breaker, then you can run a parallel
feed from the meter. I.e., it doesn't have to come from
the house's breaker-box.
If it's coming from a fuse/breaker in the house-panel, what
is -its- rating ?
3) How much can you spend ?
A breaker-box for 100-amp service is about 100 dollars.
Breakers for the box are $5-20 /ea.
100-amp service direct-burial rated aluminum 3-wire cable
is probably around a buck a foot...maybe more these days
(metals are way up over the past year). Copper is better,
but will be about twice as much. If you use the aluminum,
you MUST use oxidation-prevention compound on all
connections. This is important!
4) Do you currently have 220 in the shop? I.e., it's possible
that those two 15-amp circuits are on different sides of neutral.
You can check this with a little $10 digital multimeter. If you
don't have one of those, get one. Well worth having around.
If you have 3 wires plus ground coming into the box, it's likely
you have 220 there already. If so, and if you can rewire the
compressor motor for 220; you can run twice the HP on the
same 15 amps.
I STRONGLY recommend running compressors on 220, if
its motor can be rewired for that.
To summarize, first check and see what you DO already have.
It may be more capacity than you think. Many times the feed
wire is good for quite a bit, but people skimp on the breaker
box.
I've seen quite a few 150-amp services with only 60-amp boxes.
Second, determine what your future needs will be....because
there's a fair amount of labor in digging or properly hanging
the cable, mounting the box, etc.. If you're going to do the
work for a 60-amp box, it's not that much more work or cost
to put a 150-amp service/box in. If you're ever going to run
a welder out there, you want at least 100-amp 220v service.
hope this helps,
Richard
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