Subpanels Explained: When Your Shop or Garage Needs Its Own

August 23, 2026 · Family-owned electricians in North Idaho

Public domain, via Wikimedia Commons

If your garage or shop needs more than a couple of lights and one outlet circuit, it needs a subpanel. Anything with a welder, air compressor, table saw, mini-split, or a car charger should get its own panel instead of running everything off one long circuit from the house. Most detached shop subpanels in this area run $2,000 to $6,000 installed, depending on trench length, panel size, and how much the main panel needs to be upgraded to feed it.

What you need

• Confirmation your main panel has spare capacity, both physical breaker slots and amperage headroom
• A subpanel sized for the load: 60 amp for basic lighting and outlets, 100 amp for a shop with tools and a compressor, 125-200 amp if you're adding a mini-split, EV charger, or a lot of 240-volt equipment
• Feeder cable or conduit sized for that amperage and the distance run (100 amp at 100 feet needs bigger wire than 100 amp at 30 feet, due to voltage drop)
• A disconnecting means at the detached structure if it's not right next to the house, per NEC 225
• A grounding electrode system at the detached building, usually a ground rod or two, separate from the house grounding
• A permit from Bonner or Kootenai County, or your local jurisdiction, and an inspection before it gets covered up or energized

Step by step

1. Add up your loads. List everything that will run in the shop or garage: lighting, outlets, a welder, compressor, heater, EV charger, mini-split. Note the amperage or wattage on the nameplate of the big items. This tells you whether 60 amps covers it or whether you need 100 amp or larger.
2. Check the main panel. Look at the amperage on the main breaker (usually 100, 150, or 200 amp) and count open slots. A 100 amp main panel that's already full of circuits for the house often can't feed a 100 amp subpanel without an upgrade. This is where a lot of projects hit a wall before they even start.
3. Plan the run. Measure the distance from the main panel to where the subpanel will sit. Decide if it's underground (direct burial or conduit) or overhead. Distance affects wire size, and in this climate, buried cable needs to be at the right depth to survive frost and equipment traffic.
4. Pull the permit and get it inspected. The electrical inspector checks grounding, wire sizing, breaker sizing, and the disconnect before the trench gets backfilled or the wall gets closed up. Skipping this step is the single most common regret homeowners have later, especially when selling the property.

Where this goes wrong

The most common mistake is undersizing. Someone runs a 60 amp subpanel to a shop thinking it's just for lights and an outlet or two, then adds a welder and a compressor a year later and the panel can't handle it. Upsizing after the fact means redoing the whole feeder, not just swapping a panel, because the wire gauge has to match the amperage.

Another common problem is treating a detached garage like it's part of the house electrically. Detached structures need their own grounding electrode system. Skipping this, or running only a 3-wire feeder from an older era without a separate ground, creates a shock hazard if something goes wrong on the neutral. This shows up in older cabins and outbuildings around the lakes fairly often.

Voltage drop over long runs is another one people miss. A shop 150 feet from the house pulling 60 amps on undersized wire will show dim lights and underperforming tools even though everything looks fine on paper. The fix is bigger copper or aluminum conductors sized for the actual distance, not just the amperage.

Buried cable that's too shallow, or not rated for direct burial, is a recurring failure in this region because frost heave and future digging (fence posts, drainage work, tree planting) find it eventually. Depth and cable type matter more here than in milder climates.

Finally, some people run a subpanel without a permit to save time or money. If there's ever a fire, an insurance claim, or a home sale, unpermitted electrical work becomes a real liability. Inspectors aren't looking for reasons to fail a job, they're checking that the basics are safe.

When to stop and call someone

The planning steps above, figuring out your loads and roughly what size panel you need, are fine to do yourself with a pen and paper. Everything past that is licensed electrician territory. Sizing the feeder conductors correctly for distance and load, tying into the main panel safely, setting a proper grounding electrode system at the detached building, and getting the permit and inspection done right all require someone who does this work regularly.

If your existing main panel is full or undersized for what you want to add, that's also a sign to bring in a professional early, before you dig a trench or pour a slab around a plan that won't work. Sageland Electric handles subpanel installs for shops, garages, and outbuildings across Bonner and Kootenai counties, and can size the system correctly the first time so you're not redoing it in five years.