Digging the Trench for Your Solar Array and Battery Bank: A Landowner's Guide

A ground-mounted solar array almost always ends up somewhere other than right next to the house. You put it where the sun is: a south-facing slope, a clearing past the tree line, an open corner of the pasture. That leaves one job between the panels and the battery bank that a lot of off-grid planning skips over: getting the power from one to the other, underground, safely, and in a way you will not have to dig up again.

We sell compact excavators to landowners, and power trenches are one of the most common reasons people buy one. Here is what we have learned about planning and digging the trench so it is a weekend job instead of a month of frustration.

Start with the route, not the shovel

Before anybody digs, walk the line from the array to wherever your batteries and inverter will live, whether that is the house, a shed or a dedicated power building. A few things decide the route:

• Shorter is cheaper. Every extra foot of trench is extra wire, and on a long run the wire has to get heavier to keep voltage drop in check. Ask your electrician or system designer to size the conductors for the actual distance before you lock in a route.

• Straight is easier. Each bend makes it harder to pull wire through conduit. If you must turn, plan gentle sweeps rather than sharp corners.

• Stay out of the low spots. Water collects in low ground, and a trench that runs through it becomes a drain. Follow the higher ground where you can.

• Avoid future work. Think about where the driveway, septic field, garden or next building might go. A trench under a future septic lateral is a problem you are creating for yourself.

• Mark both ends and the bends with stakes and flagging so the line is obvious from the operator's seat.

Call 811 first, even on your own land

In the United States, 811 is the free call-before-you-dig line. Utilities will come mark their buried lines, usually within a few business days. On rural property it is easy to assume there is nothing underground, but older water lines, phone cable and electric service drops show up in surprising places. Make the call, wait for the marks, and keep the machine well clear of them.

How deep does the trench need to be?

Burial depth is set by the National Electrical Code (NEC Table 300.5) and whatever your local inspector adopts on top of it. The minimum cover, measured from the top of the cable or conduit to finished grade, depends on what you bury:

• Direct-burial cable (no conduit): 24 inches of cover.

• PVC conduit approved for direct burial: 18 inches of cover.

• Rigid metal or intermediate metal conduit: 6 inches of cover.

• Residential 120-volt branch circuits, 20 amps or less, GFCI protected: 12 inches of cover.

Most off-grid builders run PVC conduit at 18 inches or more. Conduit protects the wire, lets you pull new or bigger conductors later without digging again, and gives you room to add a data cable for monitoring. Digging a little deeper than the minimum costs almost nothing once the machine is in the ground, and it buys margin for settling and for anyone who drives a tractor across the line later.

Always confirm with your local inspector before you dig. Some jurisdictions add requirements, and solar DC circuits can carry their own rules.

How wide?

Wider than you think you need, but not by much. Conduit is only a few inches across, but you want room to lay it flat on a smooth bottom and to get a shovel or tamper in for backfill. A 6 to 12 inch bucket covers almost every residential power trench. If you are laying more than one conduit, for example separate runs for DC from the array and AC back to the house, a 12 inch bucket gives you room to keep them apart.

Picking the equipment

There are three realistic ways to dig a power trench:

By hand. Fine for a short run in soft soil. Past about 50 feet, or in clay or rocky ground, it becomes a back-breaking week.

A walk-behind trencher. A chain trencher cuts a narrow, clean slot quickly in open, rock-free soil. It struggles with roots, buried rock and wet clay, and it will not help you with anything else on the property.

A mini excavator. A 1 to 2 ton compact excavator digs well past the depth a cable trench needs, handles roots and rock better, and lets you move spoil, set the conduit, and backfill with the same machine. It is also the tool you will want for everything else an off-grid build needs: footings for the array racking, the water line, septic work and site cleanup. If you are weighing options, this overview of mini excavators for landowners shows the size range most people use for this kind of work.

For most landowners with more than one project on the list, the excavator is the machine that pays for itself across the whole build rather than just the trench.

Digging it, step by step

1. Strip the topsoil along the line and set it to one side. Put the subsoil from the trench on the other side. When you backfill, the topsoil goes back on top and grass comes back much faster.

2. Dig to depth plus a few inches. You will lay a bedding layer, so go a little deeper than your target cover.

3. Keep the bottom smooth. Pull out sharp rocks and roots. A rock pressing on conduit for twenty years can crack it.

4. Bed the bottom with 2 to 3 inches of sand or screened fill.

5. Lay the conduit with the right fittings and sweeps, and glue the joints. Cap the ends so dirt and water stay out until you pull wire.

6. Have it inspected before you cover it, if your jurisdiction requires a trench inspection. Many do.

7. Lay warning tape. Backfill partway, then lay underground electrical warning tape roughly a foot above the conduit, so anyone digging later hits the tape before the wire.

8. Backfill in layers and tamp each one, then finish with the topsoil you set aside. Expect some settling over the first wet season and top it off.

Planning for the battery bank end

The trench ends where your battery bank lives, so plan that end carefully. Bring the conduit up into the building through a proper entry, seal it so moisture and rodents cannot follow the pipe inside, and leave enough slack for the conductors to reach your combiner, charge controller or inverter without strain. LiFePO4 batteries are forgiving compared with older chemistries, but they still like a dry space with steady temperatures, so this is a good moment to make sure the building is ready for them.

If there is any chance you will add panels later, now is the time to lay a second, empty conduit in the same trench. A spare pipe costs very little today and saves digging the whole line again when your system grows.

The short version

Plan the route on the high ground, call 811, bury PVC conduit at 18 inches or deeper, keep the trench smooth and bedded, lay warning tape, and backfill in layers. Do those things and the line between your array and your battery bank is something you will never have to think about again, which is exactly what you want from the part of your power system nobody sees.

About the author: Braden Chupp works with Chupp Industrial in northeast Oklahoma, which has sold compact equipment and attachments to landowners and contractors for more than 15 years. Learn more at chuppindustrial.com.