A subpanel in a detached garage or shed is fed by a 4-wire feeder (two hots, a neutral, and a ground) — and the two things inspectors fail are: the neutrals and grounds must be kept separate at the subpanel (the 3-wire allowance was removed in 2008), and the building needs its own grounding electrode (usually two ground rods). You also need a disconnect at the detached building. A 60 A feeder is typical (≈ 6 AWG copper / 4 AWG aluminum); 100 A (≈ 3 Cu / 1 Al) for a serious shop or EV charging. The dwelling “83% rule” does not apply to this feeder.
Running power to an outbuilding? The garage subpanel calculator sizes the feeder and walks the code chain that fails inspections — 4-wire feeder, separated grounds, ground rods, disconnect, burial — free, no signup. This guide explains each requirement.
🔌 Size the feeder
Pick the feeder amperage for what the building will actually run. A 60 A feeder is the typical garage/shop subpanel — enough for lights, outlets, and a couple of tools. Step up to 100 A for a heavy shop, a welder, or EV charging out there; drop to 30–50 A for a light-duty shed.
No 83% dwelling discount here. The reduced-conductor allowance in Table 310.12 applies only to a dwelling's main service/feeder — not to a subfeeder to a detached building. Size this feeder from the standard ampacity table (310.16): about 6 AWG copper (or 4 AWG aluminum) for 60 A, and 3 AWG copper (or 1 AWG aluminum) for 100 A at 75°C.
🔀 4-wire feeder, grounds separated
This is the single biggest detached-building mistake. The feeder must be four wires — two hots, a neutral, and a separate equipment ground — and at the subpanel the neutral bar and ground bar must be isolated from each other (250.32(B)). The neutral bus is lifted off the panel can; the ground bus is bonded to it.
The old 3-wire feeder (neutral doubling as ground) was allowed before 2008 and is why so many older garages are wrong. On any new install, grounds and neutrals share a bus only at the main service — never at a subpanel.
⏚ Grounding electrode at the building
A detached building with a subpanel needs its own grounding electrode system (250.32(A)) — typically two ground rods 6 feet apart, unless a single rod tests at 25 ohms or less (250.53(A)(2)), which almost nobody measures, so two rods is the default. That electrode connects to the ground bus in the subpanel.
This is separate from the equipment ground you ran in the feeder — the building gets both. The feeder ground bonds the panel back to the house; the ground rods give the building its own earth reference.
🔦 Disconnect & burial
The detached building needs a disconnecting means (225.31/.32) so power to the whole building can be shut off at the building — for an outbuilding, a snap switch is acceptable (225.36 Exception). The subpanel's own main breaker often satisfies this.
For the underground run, follow Table 300.5 burial depths: 24 inches for direct-burial UF cable, 18 inches for PVC conduit, 6 inches for rigid/intermediate metal conduit. Most people run PVC at 18 inches.
✅ The bottom line
Size the feeder for the building's real loads (60 A typical, 100 A for a shop or EV), run four wires, and keep neutrals and grounds separated at the subpanel. Drive the building its own ground rods, provide a disconnect at the building, and bury the run to the Table 300.5 depth for your method. Don't apply the dwelling 83% conductor discount to a subfeeder.
This is a planning tool; adopted NEC editions and local amendments vary. Size the feeder with the garage subpanel calculator, confirm the wire with the wire size calculator on long runs, and have a licensed electrician and your AHJ sign off.
Estimate your Garage Subpanel materials
What wire to a detached garage or shed? Free NEC calculator for the 4-wire feeder — size by amps and distance, plus ground rods, disconnect, and burial.
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