Extending electrical service to a detached garage or other outbuilding is not much different from adding a circuit inside the house, with one important exception: the cable has to cross the yard underground. If you are comfortable adding a circuit indoors, running a circuit to an outbuilding is a project you can reasonably take on yourself, but it involves work at the main breaker panel to install and connect one or more new breakers. Many homeowners hire an electrician for those panel connections and still save money on the most labor-intensive part of the job, laying the cable from the house to the garage.
By providing the sweat equity yourself, you limit the expense of the underground portion to the cost of materials. Water is the enemy of underground electrical work, and the drainage thinking that engineers apply to large-scale underground construction, including the story behind Miami’s deepest underground parking garage, applies in miniature to a yard trench: keep moisture away from the conductors and the installation lasts for decades instead of failing in the first wet season.
Check Local Requirements Before You Start Digging
The first call is not to a supplier; it is to the local building department. Permits, inspection schedules, and burial depths vary by jurisdiction, and an unpermitted underground run can be flagged during a future home sale. Ask which edition of the electrical code the town enforces, because ground-fault requirements for outbuilding circuits have tightened in recent code cycles and older tables no longer apply everywhere.
Call 811 First
Every state operates a one-call center, and 811 is the national number. A locate request is free, and utility crews mark gas, water, power, and communication lines within a few business days. Digging before the marks go down risks cutting a gas line or a buried fiber backbone, and in most places the homeowner carries that liability. Give the locate ticket a few days of slack in the schedule so the marks are fresh when the trench opens.
The Depth Comparison That Matters
Not every buried line runs deep, so a power trench must be planned around what else is already in the yard. A lawn irrigation system, for example, typically sits only 8 to 10 inches down; installing an underground sprinkler system for lawn irrigation involves shallow loops that a power trench can cross, provided the power line stays below and the sprinkler line is rerouted or sleeved where the two meet.
Choose the Right Wire, Conduit, and Burial Depth
Underground circuits have two legal paths: direct-burial cable, or individual conductors pulled through conduit. Direct-burial UF cable is simpler to install and cheaper per foot, but conduit protects the conductors, allows future wire changes without re-digging, and is required in many jurisdictions for outbuilding feeders. Buy the materials after the inspector confirms the depth and conduit rules for the site, because the table in the code book is a starting point, not a substitute for a local amendment.
Cable Types for Wet Locations
Wire rated for wet locations is non-negotiable underground. THWN-2 conductors carry a 90-degree Celsius rating and are made for conduit in damp soil, while UF cable has its own molded jacket and can be buried without conduit where code allows. Aluminum XHHW or SE feeders are common for long garage runs because the lower material cost offsets the larger gauge and the anti-oxidant paste required at the lugs.
Sequence the Work Like a Pro
The order of operations is the same on every underground feed, and it is the same discipline a crew applies when they install a pivot door like a pro: prepare the opening before you hang anything. Verify the route, mark the utilities, dig, lay the conduit, pull the wire, make the connections, and only then backfill, because every step after the trench is open gets harder once the dirt is back in place.
Typical Burial Depths
| Installation type | Typical minimum cover | Notes |
|---|---|---|
| Direct-burial UF cable | 24 inches | Simplest run, hardest to upgrade later |
| Schedule 40 PVC conduit | 18 inches | Common choice for residential feeders |
| Schedule 80 PVC conduit | 12 to 18 inches | Stronger walls, used near the surface |
| Rigid metal conduit | 6 inches under concrete, 18 inches in soil | Best physical protection of the group |
Local amendments override these figures. Confirm the depth, the conduit type, and the wire size with the inspector before you buy material, and keep the permit card accessible on site for the rough-in visit.
Pull the Wire and Prep the Ends
Conduit protects the conductors, but the wire still has to get through it, and a bad pull damages insulation invisibly. Use a fish tape or a vacuum-and-bag system for long runs, pull with steady tension, and never yank. Wire-pulling lubricant cuts friction dramatically on runs with bends, and it is cheap insurance against a stuck feeder halfway across the yard.
Keep the Bends Legal
Code limits the total bend angle between pull points to 360 degrees, which is four quarter-bends. Beyond that, add a pull box or an LB fitting to give the wire an access point. Long straight runs over about 100 feet also deserve a pull point, because dragging 150 feet of feeder by hand is a two-person job and a good way to stretch the insulation.
Strip and Terminate Cleanly
The ends of the conductors need clean, square cuts and the correct strip length for the lugs or breakers. Dedicated tools make this work consistent, and the cable stripping tools used for underground distribution line work, with their depth stops and rotary heads, prevent the nicks that later turn into hot spots. Ring terminals and a torque wrench are not optional extras on a feeder; loose connections are the most common cause of underground circuit failure.
Bring the Conduit Into the Building
Where the conduit enters the house and the outbuilding, the details decide whether the installation passes inspection and stays dry. Every above-ground stub must be protected from damage, which is why Schedule 80 PVC or rigid metal is required where conduit emerges from the ground. Seal the penetration through the foundation or wall so water and rodents cannot follow the wires in.
Wall and Foundation Penetrations
The entry through a basement or crawl space wall needs a hole drilled at a slight downward angle, a sleeve or fitting rated for the wall type, and sealant on both faces. The same water-management thinking that goes into building an underground basement wall, where drainage and waterproofing protect the whole structure, applies to a single conduit penetration: slope it, seal it, and keep soil and mulch below the siding line.
Grounding and Bonding
Outbuildings fed by a branch circuit need their own equipment grounding conductor run with the circuit, and separately derived systems need a grounding electrode at the building. The electrician making the panel connections will confirm the bonding requirements for your setup; ask for the grounding details in writing before you backfill so the trench does not need to reopen.
Connections, Inspection, and Backfill
Before the trench closes, schedule the rough-in inspection with the building department. Inspectors check depth, conduit support, fitting tightness, and the grounding path, and fixing a problem with the trench open costs minutes instead of a full re-dig. Leave the wire ends accessible, coiled neatly at both ends, and labeled for the electrician who makes the final hookups.
As-Built Records Matter
The trench disappears, but the record should not. Sketch the route with measurements from the house corners, note the depth and the location of any pull boxes, and keep the sketch in the home file. That drawing is a miniature version of underground surveying, where the location and depth of buried assets are documented so future digging does not find them the hard way.
Backfill in Layers
- Bed the conduit on 2 inches of sand or rock-free, compacted soil.
- Add the same material above the conduit before the native soil goes back.
- Tamp in 6-inch lifts instead of dumping the whole spoil pile at once.
- Lay warning tape about 12 inches above the conduit so a future shovel hits the tape before the wire.
- Water the trench lightly if the soil is dry, then let it settle before grading.
Common Mistakes That Fail Inspection
Most rejected underground installations share a handful of fixable problems, and knowing them in advance keeps the project on schedule. The list below covers the issues inspectors see most often in residential yard trenches.
- A shallow trench: a 12-inch run that measures 10 inches at the inspection is a full re-dig.
- Missing warning tape, or tape laid directly on the conduit instead of above it.
- Unsupported conduit: PVC needs support every 3 feet in the trench and at every fitting.
- The wrong wire: NM cable has no place underground; only UF or wet-rated THWN conductors belong in the dirt.
- Overtightened or cross-threaded fittings on Schedule 80 stubs.
- Splices buried in the trench instead of brought up into a listed box.
Craftsmanship in Hidden Work
The work that no one will ever see again still deserves the same care as the work on display. The care shown in installing tile murals in shower walls, where the substrate must be flat before the first tile goes down, has a direct parallel in the trench: the conduit bed, the sweep bends, and the connections were either done right the first time or they fail years later in the dark. Do the hidden work properly, label the panels, and hand the inspector a clean, documented installation. That is the difference between a feed that runs for 40 years and one that dies in the first thunderstorm.