Installing an EV Charger in a Detached Garage

Start with the question that decides the price

A detached garage install is really two projects bolted together: getting power to the building, and mounting a charger once it is there. The second part is the same job it would be anywhere. The first part is where the money goes, and it turns on a single question: are you running one circuit, or feeding a subpanel?

One dedicated circuitis the minimum. It runs from your main panel, across or under whatever is between the buildings, and terminates at the charger. It is the cheapest way to get a car charging, and it is a dead end — the day you want a light, a socket or a second charger out there, you are digging again.

A subpanelmeans running one larger feeder to the garage and putting a small breaker panel inside it. The charger circuit then originates a few feet from the charger. You pay more up front for the feeder and the panel; you pay almost nothing for every circuit after that. If the garage is genuinely empty and will stay that way, a single circuit is defensible. In every other case — a workshop, a freezer, a door opener, lighting, a second EV later — the subpanel is the one that stops the conversation happening twice.

There is a second, less obvious argument for the subpanel. It gives the garage a local disconnect, which makes any future work in that building a small job rather than one that involves the house.

Voltage drop: the reason distance is not linear

This is the part that surprises people who priced a 30-foot install and then measured the actual distance to the garage.

Conductors have resistance. Over a short run, the voltage lost to that resistance is negligible. Over a hundred feet at 40 or 48 amps, it is not. The consequence is real: a charger fed noticeably under its rated voltage delivers less power than its badge claims, so your 9.6 kW charger quietly becomes something less, every night, forever.

The remedy is to upsize the conductor — thicker wire, less resistance, less drop. And copper is priced by weight. That is why the quote for a long run climbs faster than the tape measure does: you are paying for more feet and more copper per foot at the same time. It is also the strongest argument for the subpanel, because you upsize one feeder rather than upsizing three separate circuits.

You do not need to run this calculation yourself — your electrician does it, and it is one of the things you are paying them for. What is worth doing is measuring the real route before you ask for quotes. Not the straight line between buildings: the path the cable can actually take, around obstacles, up and down the walls at each end. Quotes given against a guessed distance get revised, and never downwards.

Overhead or underground

Both are legitimate. The trade is money against appearance and disruption.

Undergroundis invisible when finished and protected from weather and ladders. It is also excavation, which means the cost swings wildly with what is in the way. A grass run is straightforward. A run under a concrete drive means either cutting and reinstating it or boring beneath it, and both of those are line items larger than the charger. Anything already buried on that route — gas, water, irrigation, telecoms — has to be located first, not discovered.

Overhead removes the excavation entirely and is usually the cheaper quote. It has to meet clearance rules over ground and over any vehicle route, it needs proper support at both ends, and it is visible. Some HOAs care about that; our condo and HOA guide covers how to put that request in a form that gets approved.

Ask for both on the quote. The gap between them on a specific property is often large and never predictable from the outside.

Sizing the feeder for the charger you will own

Under NEC Article 625, EV charging is a continuous load, so the circuit is sized to 125% of the charger’s draw: 40 amps needs 50, 48 amps needs 60. Our how many amps guideworks through the vehicle side of that decision, and it is worth reading before you commit, because the car’s own onboard charger often caps the useful number below what the wall can supply.

The detached-garage-specific advice is simple: size the feeder generously even if you buy a modest charger. The marginal cost of a slightly larger feeder, decided once while the trench is open, is small. The cost of discovering in four years that the feeder cannot carry a second charger is another trench. If your main panel cannot spare the capacity for a generous feeder today, read our panel capacity guide and our load management guide— load management is frequently the cheaper answer to a full panel than a service upgrade.

Choosing the charger once power is there

With a subpanel installed, the charger choice reverts to a normal one, with two detached-garage wrinkles.

Enclosure rating matters more than you think.Detached garages are frequently unheated, sometimes damp, and the charger often ends up on an exterior wall so the cable reaches a car parked outside. Buy something that publishes a rating and a temperature range rather than something that stays silent about both — the WOLFBOX WE48 publishes NEMA 4X, the Leviton EV40PNEMA 3R with a cable rated to −30°C, and our outdoor roundup ranks on exactly that.

Check the Wi-Fi actually reaches. A smart charger in a building forty feet from your router may not have a usable signal, and a charger that cannot reach the network loses its scheduling and its energy reporting. Either verify coverage first, or buy a charger whose value does not depend on it. Our Wi-Fi setup guide covers the fixes, and our smart versus basic comparison covers whether you needed the app in the first place.