Detached Garage Feeder Sizing: Wire, Voltage Drop and the Separate Grounding Rules
Size a detached garage feeder the right way: ampacity first, voltage drop second, and the detached-building grounding and disconnect rules that a normal subpanel guide does not cover.
Step-by-Step Procedure
Fix the load, voltage and real route length
Write down the feeder load, the voltage and the actual route length, including bends and the drop into each panel. In this example the load is 40 A, the voltage is 240 V and the one-way route is 180 ft. A straight-line distance on a map is not the wire length.
Check ampacity in the correct temperature column
8 AWG copper THHN in conduit is 50 A at the 75 C column, so it passes ampacity for a 40 A feeder. That pass does not end the job, because voltage drop over 180 ft is a separate check.
Check voltage drop with the round-trip length
180 ft one-way is 360 ft of conductor path. With 8 AWG copper at 0.6282 ohms per 1,000 ft, the drop is 40 A x 0.6282 x 360 / 1,000 = 9.05 V, which is 3.77% of 240 V and fails a 3% branch-circuit guide. With 6 AWG copper at 0.3951 ohms per 1,000 ft, the drop is 5.69 V, or 2.37%, which passes.
Use a four-wire feeder and keep neutral and ground separate
A detached garage feeder normally needs two hot conductors, a neutral and a separate equipment grounding conductor. In the garage panel, keep neutral isolated from the enclosure and bond the grounding conductor to the enclosure. Do not re-bond neutral to ground in the detached panel.
Add the detached-building grounding and disconnect rules
A detached building supplied by a feeder normally needs its own grounding electrode system and a disconnect at the building. Burial depth, wiring method and disconnect type depend on the locally adopted code and the route, so check them before digging. This guide is a planning estimate only; final design and installation should be verified by a licensed electrician.
Worked Practical Example
40 A garage feeder, 240 V, 180 ft one-way
A detached garage workshop fed at 40 A, 240 V, 180 ft from the main panel, copper THHN in conduit.
- •Ampacity check: 8 AWG copper THHN at 75 C is 50 A, so it passes ampacity for 40 A
- •Voltage drop, 8 AWG: 40 A x 0.6282 ohms per 1,000 ft x 360 ft / 1,000 = 9.05 V, which is 3.77% of 240 V and fails a 3% guide
- •Voltage drop, 6 AWG: 40 A x 0.3951 ohms per 1,000 ft x 360 ft / 1,000 = 5.69 V, which is 2.37% of 240 V and passes
- •Feeder: four-wire, with neutral isolated and ground bonded in the garage panel, plus a grounding electrode system and disconnect at the garage
Common Mistakes to Avoid
- ✕Passing ampacity and never checking voltage drop on a long garage run.
- ✕Running only three wires to a detached garage and bonding neutral to ground in the garage panel.
- ✕Forgetting the grounding electrode system and disconnect at the detached building.
- ✕Measuring the straight-line distance instead of the actual wire route length.
Frequently Asked Questions
What size wire do I need for a detached garage 180 ft away at 40 A?
As a planning example at 240 V, 6 AWG copper passes both ampacity and a 3% voltage-drop guide, while 8 AWG passes ampacity but fails voltage drop at 3.77%. Always check your actual load, route and locally adopted code.
Does a detached garage need its own ground rod?
A detached building supplied by a feeder normally needs its own grounding electrode system in addition to the equipment grounding conductor in the feeder. The exact electrode, bonding and disconnect requirements depend on the locally adopted code.