100-Amp Sub Panel Trenched Out to a Backyard Shed in Woodland, WA

You can’t really work in a shed that runs off one extension cord. On this rural property in Woodland, WA, we trenched from the house out to the backyard shed, put a 100-amp subpanel inside with surge protection, and added a 20-amp circuit on the outside of the garage while the trench was open.

Why a subpanel instead of one long circuit

Running one circuit out to an outbuilding is the cheap option, and people usually regret it. One circuit means one thing at a time, voltage drop over the distance, and no way to add more without digging again.

A subpanel gives you:

• Several circuits at the shed, so the lights, outlets and a tool circuit aren’t fighting each other
• Room to add more without another trench, which is the expensive part
• A shutoff at the shed
• One properly sized feeder instead of a bunch of undersized runs

The panel is rated for 100 amps, which is the most the shed can ever carry. How much it actually gets depends on the house. On a property that’s still on a 100-amp house service, we size the feeder breaker to what the house can really spare. Getting that balance right is the main decision on subpanel and feeder work, and you want to make it before anybody starts digging.

The trench

The trench is the part nobody sees and the part the inspector cares about most. NEC 300.5 sets how deep you have to bury it depending on what’s going in the ground, and PVC conduit rated for direct burial has its own number.

Two things decide whether a trench passes or gets dug back up:

1. Measure before you backfill. We put the tape in the trench and photograph the depth while it’s open. Once the dirt’s back in, proving the depth means digging again.
2. Go deeper than the minimum. The ground settles, landscaping changes, somebody puts in a garden bed later. A few extra inches costs nothing on the day.

We dig these runs deeper than code requires for that reason. Our guide to wiring a detached shop or shed covers how the feeder, the trench and the panel all tie together.

Surge protection out there too

The subpanel went in with surge protection. On a rural property that matters more than in town. The shed ends up holding the tools, the freezer, the well controls and the chargers, and it’s usually the last place anybody protects.

Putting surge protection on the subpanel while we’re building it costs very little. Adding it later means opening a panel that’s already live and already full.

Got a shed, shop or barn in Cowlitz County running on an extension cord? Call 360-828-7143 or walk the trench route with us, and we’ll price the run.