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

A shed with one extension cord running to it is not a shed you can work in. On this rural property in Woodland, WA, the fix was a proper underground feeder and a panel of its own: Newman Electric trenched from the house out to the backyard shed, landed a 100-amp sub panel inside with surge protection, and added a 20-amp circuit on the outside of the garage while the trench was open.

Why a sub panel instead of one long circuit

Running a single circuit to an outbuilding is the cheap answer, and it is the one people regret. One circuit means one thing at a time, a long voltage drop, and no way to add anything later without digging again.

A sub panel changes the arithmetic:

• Multiple circuits at the shed, so lights, receptacles and a tool circuit are not fighting each other
• Room to grow without another trench, which is the expensive part of this job
• Local disconnect, so the shed can be shut off at the shed
• One feeder to size properly instead of a handful of undersized runs

The panel here is rated for 100 amps, which sets the ceiling for what the shed can ever carry. What it actually draws is a different question, and on a property still on a 100-amp house service the feeder breaker gets sized to the capacity that is genuinely available. That balance is the real design decision on sub panel and feeder work, and it is worth getting right before the shovel comes out.

What goes in the trench, and how deep

The trench is the part nobody sees and the part an inspector cares about most. NEC 300.5 sets minimum cover by what you are burying and what you are burying it in, and PVC conduit listed for direct burial has its own number in that table.

Two things make the difference between a trench that passes and one that gets reopened:

1. Measure before you backfill. The tape goes in the trench and the depth gets documented while it is still open. Once the dirt is back, proving cover means digging again.
2. Dig past the minimum, not to it. Grade settles, landscaping changes, someone puts a garden bed in later. The extra few inches cost nothing on the day.

We dig this kind of run deeper than the table requires for exactly that reason. Our guide to wiring a detached shop or shed walks through how the feeder, the trench and the panel decisions connect.

Surge protection belongs on the outbuilding too

The sub panel went in with surge protection on it. On a rural property that matters more than it does in town: outbuildings collect the tools, the freezer, the well controls and the chargers that nobody thinks about until a surge takes them out, and the shed is usually the last place anyone protects.

Putting surge protection on the sub panel while the panel is being built is a small line item. Adding it later means opening a panel that is already energized and already full.

Rural place in Cowlitz County with a shed, a shop or a barn running on an extension cord? Call 360-828-7143 or walk the trench with us and we will price the run properly.