How to run underground electrical wires
1 of 1GFCI tester
Use a GFCI tester to make sure the outlet at the house is GFCI protected.
If you need to install an outdoor electrical outlet, because you need to power a pond pump or other device, think safety. There are several different methods
for safely running the underground
wiring, but the 12-in. deep PVC
conduit method we show is about
the easiest and most practical for
running a typical 15-amp line. A
second method, 12-in. deep direct
burial of type UF (underground
feeder) cable, is another easy alternative.
We recommend the PVC
conduit because it offers more protection
against physical damage.
Both methods have to be GFCI
(ground fault circuit interrupter)
protected before the wires enter the
ground to guard against electrocution
in case the wire is accidentally
cut while you’re digging.
No matter which method you
choose, you’ll have to get an electrical
permit first. Review your plan
with the inspector and get instructions
for inspections.
Tying into an existing garage outlet
is usually the simplest way to
power your outside outlet for two
reasons. First, the electrical boxes
are usually exposed for easy hookups,
and second, any outlets in
garages built after 1981 are supposed
to be GFCI protected, which would
automatically protect the new outside
outlet as well. Unfortunately,
there are plenty of unprotected
garage outlets out there, so you’ll
have to check the outlet before
beginning your project. The safest
way to check is to buy a GFCI
receptacle tester and
test the garage outlets. If they’re not
protected, you’ll need to install
a GFCI as part of the job.
You’ll find all the
electrical supplies
you need for the
new outlet project
at a home center.
Begin by installing
a permanent post
for mounting the
pond outlet box and digging the
12-in.-deep trench. You’ll need
these electrical supplies and parts
for doing the electrical work:
- Two weatherproof electrical
boxes: one for mounting the
receptacle at the pond and a second
for the garage to splice the
transition between the cable and
individual wires. Metal electrical
boxes must be grounded to the
bare or green insulated ground
wire(s) contained in the box.
- A cable clamp for running cable
through the back of the box from
the garage outlet. Use duct seal
putty to seal the wall penetration
hole to keep moisture out of
the box.
- “Schedule 80” PVC (1/2-in.) conduit,
elbows and couplings for
containing the wires and connecting
the exterior boxes. Use PVC
cement for joining the parts.
- PVC adapters for connecting the
conduit to the weatherproof
boxes.
- Type THWN moisture-resistant
insulated wire to pull into the
PVC conduit between the two
weatherproof boxes. You’ll need
three different colors (green for
the grounding wire, white for the
neutral wire, and black or red
for the hot wire). Match the wire
gauge to the existing wire in the
electrical box you’re
powering the new
outlet from.
- Conventional cable (Type NM-B)
to connect the garage outlet to the
individual THWN wires within
the outside garage splice box.
(Again, match the existing garage
wiring when choosing the gauge.)
- Rain-tight weatherproof covers to
protect the plug and receptacle.
In addition, you’ll need a receptacle,
cable staples, conduit straps and
wire connectors. Mark the garage
outlet plate cover you’re tying into
with a “GFCI Protected” sticker (or
handwritten notice). The inspector
will want to examine the below-grade
depth and connections, so
don’t backfill the trench until your
work is examined. Otherwise you
may be forced to redig the trench.
Underground electrical details
Underground Electrical Wires in PVC Conduit
Bury PVC conduit at least 12 inches down. Check all details with a local building inspector.
Note: You may view and print a larger version of this image. See Additional Information below.
