A.Rex Cauldwell responds:
An outbuilding should be treated as a separate building, as
opposed to an extension of the house. The takeoff point for the
cables to the shop should be the home's main service panel or a
subpanel (see illustration, below). There should be four cables
feeding the shop: hot, hot, insulated neutral, and ground.
(There are times when code allows you to use three cables by
combining the neutral and ground in one conductor. I don't
recommend that, however, and many electricians, including me,
refuse to do it because combining the two conductors could
result in a hazardous condition, especially for an electrician
working on the circuit.)
The code requires an insulated neutral, which means you can't
use common USE (underground service entrance) cable with a bare
stranded neutral. You should use four individual insulated
cables approved for direct burial. Bury the cables at least 2
feet deep or whatever your local codes and utilities
require.
The size of service depends on what loads you're going to have
in your shop. I would recommend at least a 50-amp/240-volt
service (the same size as your electric stove circuit).
Therefore, you'll need two open slots in your service
panel.
Typically, you would come out of the back of the panel, through
the exterior wall, down along the exterior wall, and then
underground. Since you are running individual cables, they need
to be in conduit from where they leave the panel until they're
underground. I would use plastic conduit, not metal, because
plastic is a nonconductor and easier to work with, since you
glue the joints. Use at least 1-inch-diameter conduit; that
allows extra room inside, which makes handling and pulling the
cable easier. To make the turns from horizontal to vertical,
use an LB, or elbow. Once the cables are underground, they
don't need to be in conduit.
Try to pick an approach to the shop that doesn't take you under
sidewalks and paved driveways. Always contact the
utility-location company to verify that you're not crossing any
underground services.
Once at the shop, the cable must again be in conduit as it
comes out of the ground, up the wall, and makes a turn into the
shop service panel. The shop service panel must have a ground
rod (or two) at its location, just like the main service panel
at the house.
Rex Cauldwell
is a master electrician in Rocky Mount,
Va., author of Wiring a
House, and a frequent contributor
to
JLC