How deep does an underground electrical line to an ADU need to be buried?

It depends on what you bury. The cover table in Section 300.5 of the electrical code, as summarized in California building department handouts, calls for at least 24 inches of cover over direct-burial cable, 18 inches over PVC conduit approved for direct burial, and 6 inches over rigid metal conduit, with separate values under driveways, under concrete slabs and under buildings. ADU feeders are often run in PVC conduit, which makes 18 inches of cover a common design target.

Cover is not the same as trench depth

Cover is measured from the top of the conduit or cable to finished grade. If you run a 2-inch conduit with 18 inches of cover, the trench has to be deeper than 18 inches. Allow for landscaping, pavers or a future patio that could change finished grade.

Common minimum cover values

• Direct-burial cable: 24 inches.
• PVC (rigid nonmetallic) conduit approved for direct burial: 18 inches.
• Rigid metal conduit: 6 inches.
• Residential branch circuits of 120 volts or less, GFCI-protected, with 20-amp maximum overcurrent protection: 12 inches. This reduced depth is meant for small circuits such as yard lighting or an outdoor receptacle, not for an ADU feeder.

The table also has separate rows for one- and two-family dwelling driveways and parking areas, for runs under a concrete slab, and for runs under a building. Your designer should check the row that matches each segment of the route.

Other trench rules that come up at inspection

• Conductors coming up out of the ground need protection from physical damage where they emerge.
• Backfill should not contain rocks or debris that could damage the conduit.
• Underground service conductors that are not encased in concrete and are buried 18 inches or more below grade need a warning ribbon placed in the trench about 12 inches above them.
• Ask whether your building department needs to inspect the open trench before you backfill.

Utility-owned lines are different

If your ADU gets its own utility service, the service lateral follows your utility's trench and conduit standards, not just the electrical code. LADWP's service requirements, for example, call for a single service lateral and service point per lot. Ojai's burial-depth handout likewise notes that its depths do not apply to utility company gas and electric lines.

What this means for your electrical plan

The plan should show the trench route, conduit type and size, minimum cover, where the conduit rises at each end, and any crossings with sewer, water or gas lines. Call 811 before anyone digs. For the rest of the feeder design, see how a detached ADU subpanel works and ADU sewer connections, which often share the same yard, plus our electrical plans page.

Sources: ojai.ca.gov, ecmweb.com, webprod.ladwp.com, dgs.ca.gov.