I am planning to run power out to my small shed/shop in the backyard.
I would like to run a subpanel out there with two 20 amp circuits on
it. The wire from the main breaker box will be approximatly 100 feet
long between main and sub pnal boxes, all inside conduit.
What size wire should I use? would 8 guage work ok? or do i need to go
to 6 or better? I ask because I have a roll of 8/3 sitting here in
the house and I dont want to have to go spend $100 on new wire if this
The set-up out in the shed will have my compressor running off one
dedicated 20 amp circuit and a couple small lights along with outlets
running on the other circuit for small powertools (circular saw, miter
saw etc..). The compressor is my most power hungry piee of equipment.
What size breaker in the main panel should I run it from? 40? I can
see the compressor kicking in just as I fire up the circular saw and
popping a 30 amp breaker so i want to have at least a 40 amp supply
out to the shed.. Can someone help me work out the details?
Use a transformer 240 - 120 V at the required amperage. It may be pretty
big but that's the only way to get a 120 V motor to run off 240 V. Why do
you want to do this anyway ? Just size the wire for the right current of
the compressor and run it in a conduit under ground or get cable rated for
Excellent question Jim. Halves the number of amps right there.
And also help during the 'starting up' surge of current as the
compressor motor kicks in!
As someone mentioned the run is a bit long; that's concerning a
1% voltage drop requirement; IF some 40 amps was to be drawn. But
for practical purposes and provided the wiring is safely
installed and of the right type should be OK?
Since you have 8-3, you can run a 240V feeder. This can provide up to two
40A loads at 120V at the same time without popping the breaker. Since this
cable run is rather long, I'd breaker it at 30A (double pole) and put in a 4
slot (minimum) subpanel. If all you want is two separate 20A circuits, then
you have 10 more amps available that you'll never use (but you must put each
20A breaker on different supply "legs" which usually means different rows of
the panel). This will let you grow some if needed, or accomodate a 240V item
(like a different compressor or power tool).
Make sure that 8-3 you have is rated for wet locations (e.g. UF cable or
THWN individual wires, NM cable is NOT suitable for wet locations), as an
undergorund conduit is considered a wet location. If the cable you have is
not suitable, consider going to 6-3. You never know how your requirements
On 16 Nov 2003 16:13:46 -0800, firstname.lastname@example.org (Matt) wrote:
Based on distance you'd want to upsize this anyway to 6 gauge for a 40
amp breaker, plus if you have 8/3 and it's not UF you likely can't use
it anyway. Locally, a run like that would be required to be buried in
conduit, so Romex would be out and you be doing THHN anyway.
As long as you're doing this, you might go to a 50 Amp breaker just in
case you get the urge to add another branch in the shed for landscape
lighting, a sprinkler system or any of a million other uses that will
bite you later.
As always, check with your local building department for their
Sorry, nothing personal, but I am getting a little tired of
this "just go out and spend a fortune and plan for all kinds of
future stuff that you may never even dream of" mentality.
Original poster: Your current wire that you own: is it UF? if
so, as he just said, you can bury it without using conduit, if
it's UF or NM (aka "Romex") you're actually NOT permitted to run
it in conduit (again, as he just said), but the following still
applies if you end up buying wire to use with conduit.
Is your existing stuff 8-3 w/ground or just 8-3 or maybe it's
8-2 w/gnd and you're just calling it 8-3? If it truly is 8-3
w/ground (and it's UF, you're burying it) then use a double-pole
30A breaker to feed it from your load center. You'll be
under-rating the wire, but you will have some voltage drop over
the 100+ feet, so it will work out well. Think of it as running
a 30A circuit but using heavier wire than needed (10-3 meets
spec for 30A).
Most important: you DON'T need a double-pole 40A in the main
panel!...Don't worry about interaction between your compressor
and power tools/lighting. Your 2 * 20A circuits (as long as
they are used as you described) will not combine and overload
your breaker, they'll be on opposite sides of the line.
You could add a couple of 15A breakers to your subpanel (one
on each side of the line) and run *three* convenience/lighting
circuits along with your dedicated 20A for the compressor.
Just remember you could theoretically not overload any of
them individually but still total out to 30+ on either side
and blow the feeder double pole 30's.
That's true too, of course. But if you have a roll of UF 8-3 w/G,
you DON'T need to buy new wire.
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