How to install electric wiring in an outside building

Updated February 21, 2017

When an exterior building is constructed on previously developed property, such as a pole barn or work garage, builders must plan how to install electrical wiring in the building. When an outbuilding is assembled, the home's existing power supply must also be run to the new building, and then the building is wired for use. The most important aspect of this project is planning ahead, and contacting local building code enforcement organisations to inquire about electrical codes.

Call the local utility companies before digging a trench for the new line. Ask them to visit the property, and mark the locations of any existing underground power, water, telephone or sewer lines. Plan to dig a trench that avoids the existing installations.

Dig a trench between the main house and the outside building. The trench should be at least 12 inches below grade level, and large enough to accept a conduit for ROMEX® electrical cable from the main house to the outbuilding. Typically, a 2-inch conduit will be large enough for 1-2 circuits between the home and the outside building.

Drill a hole through the home's basement foundation wall, and insert the PVC conduit into the home. Caulk the joint between the PVC conduit and the home's foundation wall with silicone caulk to prevent any water or insect penetration.

Drill a hole through the wall in the outside building, and install the conduit into the building. Each 90 degree fitting should be at least a 24" radius, so the wire can be pulled through the conduit, and not become jammed when turning the corner. One trick is to pull a small rope through the conduit before installing it. The rope serves as an electrician's fish tape, and can be used to pull the wiring from the home to the outbuilding after the conduit is installed. Bury the conduit when complete.

Install the new electrical load centre on the inside of the building at the point where the new conduit enters the building. The new load centre should be located within 12-18 inches of the conduit's entrance point. The power supply line should be inside the conduit until it reaches the auxiliary load centre.

Pull the ROMEX® supply line from the inside of the home to the out building using the rope inserted in the conduit when it was installed. Connect the supply line to the new load centre, but do not connect the power supply to the home's main power centre until finished with the next step.

Identify the location for each electrical socket, light fixture, and light switch in the new building. Identify the proper gauge ROMEX® wire required to run each device or circuit. Homeowners should consult electrical guidelines, or have an electrician install the wiring if they are unsure of regulations. Many safety codes must be followed when installing new electrical service. If a homeowner is not familiar with electrical code, he should consult an electrician or hire one to complete the job.

Install ROMEX® between the light fixtures, the duplex receptacles, and the light switches and the new load centre. Approximately 6 inches of ROMEX® should extend from each electrical box so that electrical fixtures and switches can be connected. The ROMEX® can be attached to the internal ceiling joists and wall studs with screw-in or hammer-in ROMEX® staples. The ROMEX® should be firmly attached to the building's interior structure and not allowed to dangle, which would cause a safety hazard.

Connect the electrical fixtures to the electrical boxes according to local electrical code. Screw the fittings fixtures and wall plates in place before connecting to the main load centre. Connect to ROMEX® wires for each circuit to the load centre, and complete all the circuits for the new building. When all the wires are connected according to local code, turn off the light switches, and open all the circuit breakers in the out building before connecting to the main power supply line in the home.

Connect the power supply line into the main electrical distribution panel in the home. The heavy gauge ROMEX® supply line should be connected securely to the lug bar that is in the distribution panel, and not spliced into an axillary line. When all the connections are complete, turn on the power to the outside building from the main distribution panel.

Return to the outside building, and close the main circuit on the auxiliary distribution panel. Then close the circuits in the circuit panel one at a time, and verify that each circuit stays closed without tripping the breaker. Test the circuits in the outside building one at a time. Turn on each light and verify that the switches work. Test each duplex receptacle and electrical appliance to verify that the power is flowing to each outlet.

Things You'll Need

  • Mechanical trencher, or shovel and work gloves.
  • PVC conduit rated for underground
  • Silicone caulk
  • A 50 to 100 amp auxiliary breaker service box
  • ROMEX® electrical wire: 10-3 w/ ground, 12-3 w/ ground, 12-2 with ground, and 14-2 with ground
  • Electric drill, with various size hole saws and auger bits.
  • ROMEX® staples
  • Electrical boxes and fixtures, including duplex plugs, light switches, and light sockets
  • Miscellaneous hand tools, including hammer, wire strippers, screwdrivers, flashlight, etc.
  • 6' or 8' fibreglass step ladder
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About the Author

Since 2003, Timothy Burns' writing has appeared in magazines, management and leadership papers. He has contributed to nationally published books and he leads the Word Weavers of West Michigan writers' group. Burns wrote "Forged in the Fire" in 2004, and has published numerous articles online. As a trained conference speaker, Burns speaks nationally on the art, science and inspiration of freelance writing.