Detached Garage Wiring Diagram

Wiring a detached garage is mostly about getting the feeder and subpanel right before you even think about outlets.

The whole thing starts with figuring out what you actually need to power. A couple of lights and some general-purpose outlets is very different from running a workshop welder or charging an EV. Once you know your load, you pick the wire size and conduit. Most residential garages end up on a 60-amp or 100-amp feeder. That usually means 4/0 aluminum SER cable for 60 amps or 3/0 aluminum for 100 amps, though some people prefer THHN in EMT for flexibility and easier future upgrades. EMT in 1¼ inch or 1½ inch is my go-to. It's cheap, you can see what's inside, and a bent conduit run looks like you actually know what you're doing when an inspector walks by. The real confusion people hit is the subpanel bonding. Here's the thing that catches most DIYers: your detached garage subpanel is a separately derived system in every practical sense, which means the ground and neutral bus bars must be isolated from each other. There is no bonding screw. You do not splice grounds to neutrals. The ground wire runs from the feeder, lands on the ground bus only, and the neutral lands on the neutral bus only. If you bond them like you would in a main service panel, you'll create a parallel path for fault current and trip breakers or worse, energize the case.

Detached Garage Wiring Diagram

A basic diagram breaks down into three sections. First, the supply side: your main panel feeding the garage through a double-pole breaker, with the correct conductor count based on whether you need a dedicated equipment grounding conductor. Second, the subpanel: incoming feeder terminates on the hot bus bars, neutral on the isolated neutral bar, ground on the isolated ground bar. Third, the branch circuits: standard 15-amp or 20-amp circuits branching off the subpanel for lighting, outlets, and any dedicated loads like a dryer or EV charger. I drew one of these out recently for a job where the homeowner wanted to eventually run a 240-volt MIG welder. The diagram helped me size the feeder correctly from the start instead of pulling wire back out later. A Detached Garage Wiring Diagram doesn't have to be fancy. A single line showing the main panel, the disconnect or subpanel, and the branch circuits is enough to keep you from making costly mistakes mid-installation.

Feeder wiring choices and why they matter

You have two main approaches here: SER cable or THHN in conduit. SER is faster. It comes pre-assembled with the conductors bundled together, and you terminate it at both ends. The downside is that once it's in the ground or running through a wall, you cannot pull it out and swap it for a larger size. THHN in EMT costs more upfront because of the conduit and fittings, but you gain the ability to upgrade conductors later by simply pulling new wire through the same raceway. I prefer THHN in EMT for anything above 60 amps or when I suspect the homeowner might add load later. The conductor count depends on your grounding strategy. A 4-conductor feeder includes two hots, one neutral, and one equipment ground. A 3-conductor feeder relies on the metal raceway as the ground path. Metal conduit as a ground is perfectly legal under NEC 250.118, but you lose that option with non-metallic cable. If you're running EMT or rigid steel, the 3-wire approach works and saves you one conductor. With SER or Romex, you need 4 wires. I've seen people cut corners here and end up with ungrounded receptacles, which is an immediate failure on inspection and a shock hazard nobody wants to explain to their insurance adjuster.

Running the feeder through the foundation

This is the part that always takes longer than expected. You need a penetration from your house's main panel area into the garage. If the garage sits a few feet away, you're dealing with an underground raceway or direct-burial cable through the trench. If it's attached but detached electrically, you're drilling through the foundation wall. Foundation walls are rarely kind to conduit. Concrete block is manageable. Poured concrete with rebar is another story. I once spent nearly two hours trying to cut a 3-inch hole through an 8-inch poured slab with rebar every six inches. The core bit kept binding. Eventually I switched to a hammer drill with a masonry bit and pecked around the rebar rather than straight through it, then used a chisel to knock out the concrete between bars. Not elegant, but it worked. From there I threaded a 2-inch Schedule 80 PVC conduit below grade and pulled the THHN through it. Use Schedule 80 underground if the conduit goes through any concrete or is subject to physical damage. Schedule 40 will crack if the ground settles even slightly.

Subpanel installation details

Mount the subpanel on a solid surface, preferably on a weatherproof enclosure rated for the environment. Garage panels should be mounted at a height that allows comfortable servicing but keeps the bottom of the panel at least 4½ feet from the floor in most jurisdictions. Secure it with lag bolts into a structural stud or blocking, not just into siding or sheathing. Inside the panel, label every breaker before you power it up. I learned this the hard way on a job where I installed four circuits and forgot to mark which breaker controlled which leg. Six months later the homeowner called because an outlet was dead and I had no idea which circuit to trace without pulling every device off the wall. Five minutes of labeling at install time saves hours of troubleshooting later. The grounding electrode is another detail people miss. A detached structure requires a grounding electrode system per NEC 250.32. This usually means a ground rod driven at least 8 feet into the earth near the subpanel, bonded to the ground bus with a #6 copper minimum. Some inspectors also want a Ufer ground or concrete-encased electrode if the slab has rebar, but a single ground rod is the standard and simplest approach for most garages. Bond the electrode conductor to the ground bus, not the neutral bus.

Branch circuit planning

Start listing what you're powering. Every circuit needs a purpose. A standard 120-volt, 20-amp circuit with 12 AWG wire on GFCI-protected receptacles covers general outlets. Lighting can run 15 amps on 14 AWG, though some inspectors prefer 12 AWG for garage lighting as a safety margin. Dedicated circuits need their own breakers and wire sizing: a 240-volt dryer needs 30 amps on 10 AWG, an EV charger might need 40 or 50 amps on 8 AWG or 6 AWG depending on the load, and a HVAC unit follows the manufacturer's MCA and MOP ratings, not a guess. GFCI protection is mandatory for all receptacles in detached garages under current code. AFCI protection is increasingly required for branch circuits too, though not universally enforced in all jurisdictions. Check your local amendments. A single dual-function GFCI/AFCI breaker on the subpanel simplifies wiring because you don't need individual GFCI outlets, though some electricians still prefer outlet-level GFCI for cost reasons on simple lighting circuits.

Inspection considerations and common failures

Inspectors fail installations for a predictable set of issues. The biggest ones I see: bonded grounds and neutrals in the subpanel, undersized feeders for the calculated load, missing or improperly installed grounding electrodes, unmarked breakers, and exposed conductors where the raceway enters the panel without a fitting. A missing bushing or grommet where THHN enters the panel can nick the insulation during pull-in, creating a latent fault that won't show up until something trips. Load calculations matter more than people think. You don't need to do a full manual calculation for a simple garage, but you do need to verify the feeder can handle the connected load. A 60-amp feeder with three 20-amp branch circuits is reasonable. A 60-amp feeder feeding a 50-amp welder plus a 20-amp circuit plus a 20-amp circuit is pushing into continuous load territory where the 80% rule applies and you should probably be running a 100-amp feeder instead. The math is straightforward but easy to overlook when you're in the middle of pulling wire.

Practical tips from actual install work

Leave yourself a foot or two of extra conductor at both ends of the feeder when you pull it through conduit. I've been cursed more than once by pulling wire through a long run only to find I had exactly zero slack at the panel end, which meant standing on a ladder with a stripped wire trying to make a termination while holding my breath. Coiling the extra wire neatly and taping it to the conduit near the termination point prevents this entirely. Mark both ends of every conductor before you pull. I use colored electrical tape: black for hot, white for neutral, green or bare for ground. Without those marks, you'll spend time with a multimeter testing continuity on a bundle of identical-looking wires, and you'll still guess wrong at least once. The tape costs pennies and saves minutes.

Consider adding a J-box with a disconnect nearby the subpanel if you plan to do any work on the garage later. A visible disconnect lets you isolate the garage power without shutting down your whole house. It's not always code-required but it's practical. I installed one on a recent job where the homeowner wanted to be able to cut power to the garage for storm season without touching the main panel. The wire gauge and length relationship is worth noting. Voltage drop becomes a factor past 100 feet on a feeder. If your garage is 120 feet away and you're running a 60-amp circuit, a 4/0 aluminum SER might drop more than 3% under load. Upgrading to 350 kcmil aluminum or 250 kcmil copper brings the drop well under 3%. This isn't always a code requirement for residential, but it affects how well your equipment runs, especially motors and chargers. A welder on a deeply undervolted feeder won't perform at rating, and an EV charger will throttle its charge rate. Factor this into your initial design rather than discovering it after the concrete is poured.

When to call a professional

If your main panel doesn't have space for another double-pole breaker, or if the service entrance is already at capacity, you may need a main Lug panel upgrade or a subpanel fed from a different location. This gets into utility coordination and permit work that most homeowners shouldn't attempt alone. Similarly, if you're running anything above 100 amps or dealing with 3-phase equipment, a licensed electrician is the right call. The wiring diagram itself is straightforward, but the execution around panel capacity and utility service limits is where things go wrong. A proper Detached Garage Wiring Diagram serves as both a planning tool and a reference during installation. Draw it on paper before you buy anything. List every circuit, every wire size, every breaker rating, and every conduit run. When you're standing in a driveway at 6 PM with a hole cut in the foundation and a coil of THHN in the back of your truck, that diagram is the only thing keeping you from making a mistake that costs a day to fix.