Adding power to a shed transforms it from simple storage into a functional workspace. With electricity, you can add lighting, outlets for power tools, and even heating and cooling.
But running power to a detached structure involves trenching, underground conduit, grounding, and installing a subpanel. From determining how many volts and amps fit your needs to whether the existing electrical service can provide them to digging a code-compliant trench for conduit, powering an outbuilding is typically best handled by a licensed electrician. In this video, Ask This Old House’s master electrician Heath Eastman shows us how he does the job.
How to Run Electrical Power to a Shed
The following is an overview of how to run power from a home electrical panel to a subpanel installed in an outbuilding. However, this project should only be handled by a professional electrician, so specific details are limited.
1. Determine Power Needs & Check Main Panel Capacity
Before any digging begins, determine the anticipated electrical usage the outbuilding will need. Consider:
- Will it only have lighting and general-use receptacles?
- Will it be a workshop? What kind of tools and machines will be run? How many will be used at once?
- Will it have heating and cooling?
- Will anything be run for three hours or more? This is considered a continuous load and is calculated at a higher rate.
- A load calculation will be needed to determine the type of breaker, subpanel, and feed wire gauge to use.
Next, check the home’s main electrical panel to confirm:
- There’s physical space for a new double-pole breaker.
- The main breaker’s amp rating and determine if it can handle the additional load.
To check your current service capacity, look at the main breaker, which is normally located at the top of the panel. As This Old House Magazine notes, if your panel contains screw-in or cartridge-type fuses rather than breakers, you should seriously consider upgrading to a safer and more convenient breaker system before adding an outbuilding circuit. A telltale sign that your house may already be underpowered: the lights dim when a big appliance goes on.
Pro Tip: Heath Eastman, This Old House electrician, advises checking your panel carefully before adding circuits: “It looks like we’re pretty full, but we see a tandem in here. And a lot of times with these panels, even though we have a certain number of spaces, it may take more circuits than it actually shows.” He recommends turning off the main breaker and removing the panel cover to see if you can actually add more circuits without having to add a subpanel.
2. Plan the Route
Map the route between the home and the shed to determine where the underground conduit will run. This will include where it enters the shed, where it travels underground, and where it enters the home.
Mark the path with white marking paint or white flags for 811. Choose the most direct route when possible. Avoid large trees, patios, or hardscaping.
Important: Call Before You Dig
Before digging, call 811 to locate underground utilities. This is required and helps prevent hitting:
- Gas lines
- Water lines
- Electrical lines
- Communication cables
3. Dig the Trench
Using a mini excavator or shovels, dig a trench for the conduit. This trench was dug 24 inches deep.
4. Install the LB Fittings & Expansion Fittings
Install the conduit bodies on the home and shed.
- Heath installed LB fittings, which are L-shaped conduit bodies that change the direction of the conduit by 90 degrees. LB fittings are used to run underground electrical service into and out of a building. The expansion fittings are self-adjusting, allowing them to shrink and expand with the weather.
- Heath installed four of these, two on the exterior walls and two on the interior walls of the house and shed.
5. Lay First Layer of Sand
Add about 3 inches of all-purpose sand to the bottom of the trench. This will act as a cushion to protect the conduit from rocks and allow it to move during freeze-thaw cycles.
6. Run the Conduit
After the first layer of sand, run the conduit. Heath used 1-¼” gray electrical PVC schedule 40 conduit.
Installation Tips:
- The bell end of the conduit should be facing downhill. So water will flow around the bell connection and continue downhill rather than inside the conduit.
- Use PVC caps when installing conduit in the dirt. This will block dirt and debris from entering the pipe.
- If a gentle bend in the PVC is required, use a PVC heating blanket to warm up the pipe enough to slightly bend it. Once the conduit cools, the new shape is permanent.
- Use PVC cleaner and cement to secure the conduit together.
- Important: PVC electrical conduit should be buried at least 18 inches deep. The conduit’s material will determine how deep it needs to be buried. Measure from the top of the pipe to the finished grade.
7. Add Grounding Rods
- Detached buildings typically require ground rods. Heath drove two 8-foot copper grounding bar rods into the ground, 6 feet apart. He used a rotary hammer with a ground rod driver.
- Then, Heath connected the rods with bare #6 copper wire, secured with ground rod clamps. The ground wire was then run to the subpanel.
8. Add 2nd Layer of Sand
Add another layer of sand on top of the conduit in the trench. Just enough to cover the conduit.
9. Add Warning Tape & Backfill
- Next, backfill with some dirt and place warning tape along the trench, about 6 inches from grade. Heath used detectable tape, but non-detectable tape is fine. This helps prevent future accidental digging into the conduit.
- Next, backfill the trench completely.
Pro Tip: After the conduit is laid and the trench is partially backfilled, add a layer of caution tape on top of the sand before completing the backfill. As demonstrated on a This Old House project, the crew backfills the trench with a few inches of sand to act as a warning layer, then places caution tape on top of that. This alerts anyone digging in the area in the future that electrical conduit is buried below.
10. Install Subpanel
- In the outbuilding, install the subpanel. Pick a location near the conduit’s entry point and mount the subpanel. Run the PVC conduits with the electrical and ground wire into the subpanel.
- In this project, Heath installed a 125-amp subpanel for future expansion. The subpanel installed should be rated for more amps than the breaker feeding it.
Pro Tip: On a recent This Old House project, electrician Heath Eastman explained why a subpanel makes sense for outbuildings far from the main house: “With 100-plus feet, we have so much going on on that side, we don’t want to run a ton of wires out of that panel. It’s much easier and probably more cost effective to put a sub panel on this end and use that for that end of the house.”
11. Pull Wires Through Conduit
To pull wire:
- Feed electrician fish tape from the house end of the conduit to the shed end. It’s helpful to have a helper for this step.
- Once the fish tape pops up at the shed end of the conduit, tie pull tape to the fish tape and pull it back to the house.
- At the house end, have the wire ready. In conduit, code requires the use of individual conductors rather than sheathed cable, which is typical house wiring where two or three conductors plus a ground wire are wrapped together in an outer shell. Heath used four wires for two conductors, one neutral, and a ground (black, red, white, and green) of 6-gauge copper THHN.
- Take the pull tape and tie a few half hitch knots around the wire. Then use electrical tape to tape the ends of the wires and pull tape together to avoid the tip of the wires catching on anything inside the conduit.
- At the shed end, pull the pull tape through to get the wires through the conduit.
- Pull the wire and ground wire into the subpanel.
- Add covers to the LB fittings.
12. Wire the Subpanel and Shed
When electrical service is run to an outbuilding, it is generally required by code to install at least one GFCI receptacle and one lighting outlet controlled by a wall-mounted switch. Check with your local building department for any local requirements.
13. Connect to the Main Panel
- At the house’s main electrical panel, shut off the main power. Then connect the feeder wires to the new breaker. Heath installed a 2-pole 6-amp breaker.
- Once complete, turn the power back on and test the new system.
14. Seal Exterior Openings
Use exterior-grade window and siding sealant to seal around the LB fittings to prevent rainwater and pests from entering.
Resources
Dig Trench
- White marking paint
- Call 811 before digging to check for underground utility lines.
- Mini excavator: This trench was dug about 2ft.
- Shovels: To smooth out the trench and spread sand.
- All-purpose sand: Heath added about 3” of sand before laying conduit. Sand adds a cushion for rock movement during the freeze-thaw cycle.
Run Conduit
- 1-¼” x 10ft PVC schedule 40 conduits: The bell end should be facing downhill.
- 1-¼” 90-degree schedule 40 standard radius electrical plain end elbow
- 1-¼” PVC caps: Gently place on the end of the 10ft conduit while connecting the opposite end to the LB fitting. This will prevent dirt from entering the conduit.
- PVC cleaner
- PVC cement
- PVC heating blanket: Use to slightly bend the PVC conduit if needed.
- PVC cutter
Mounting LB Fittings
- 1-¼” LB conduit fitting: Heath used 4 total.
- 1-¼” schedule 40 electrical PVC expansion fitting: Use these in areas where conduit is run outside of the ground, exposed to the elements. This type of fitting compensates for length changes caused by temperature variations.
- Pipe strap 1-¼” 2-hole
- Drill/driver
- 2” hole saw w/ arbor and pilot bit
- Torpedo level
- Exterior grade, paintable window, door, and siding sealant
Add Grounding Rods
- 5/8in. x 8ft. copper grounding bar rod: After adding conduit and before the 2nd layer of sand, Heath placed 2 vertical posts in the trench, 6ft apart.
- SDS-Max rotary hammer
- Bare #6 copper: Used as ground wire to connect ground rods and the electrical panel.
- 5/8” ground rod clamp: Used to connect the ground wire to the rod.
- Ratchet and socket: Used to tighten ground rod clamps.
Fill Trench
- Shovel: Heath used a shovel laid across the trench and measured from there to the conduit to check that the conduit would be buried at least 18” deep.
- All-purpose sand: Heath added another 3” of sand on top of the conduit before backfilling with soil.
Add Caution Tape
- Scotch detectable buried barricade tape, “Caution Buried Electric Line Below”
Add Electrical Panel
- Siemens SN series 125 amp 20-space 20-circuit: The subpanel installed should be rated for more amps than the breaker feeding it.
Pull Wire
- Electrical steel fish tape
- 6 AWG standed copper THHN wire: 100ft of black, green, red, and white on a spool
- Mule tape / pull tape
- Electrical tape
