A garage subpanel is one of the most common — and most commonly botched — residential electrical jobs. The failure points are always the same: undersized feeder, neutral and ground bonded together, and no grounding electrode at a detached garage. This guide does all three right, with the load calc and feeder sizing that beat the thin guides currently ranking.
Garage subpanels serve real loads: EV chargers, welders, air compressors, heaters, and workshop tools. Those loads need a proper load calculation and a properly sized feeder — not a spare 30-amp breaker and a prayer. Get the feeder right and the panel right, and the garage is set for decades.
This guide covers both attached and detached garages, because the grounding rules differ and the difference matters. Work to the NEC as adopted locally, pull the permit, and if any of this is outside your training, hire the licensed electrician.
Subpanel and feeder work is licensed-electrician work in most jurisdictions and requires a permit and inspection. If the feeder crosses to a detached structure, burial depths and grounding electrode rules apply — get them right the first time. Work to code; hire a licensed pro where required.
Subpanel rules live in NEC 250 (grounding and bonding — the separated neutral/ground is 250.24(A)(5)), 310 (conductor ampacity), 408 (panelboards), and 250.32 (grounding at separate structures). Feeder sizing follows NEC 310.16 at the 75°C column for most terminations. References are to the 2023 NEC — check your local amendments, since some jurisdictions enforce the 2026 edition.
List every load the garage will serve, honestly: EV charger (often 40–60A continuous), welder, air compressor, electric heat, lighting, and general receptacles. Apply NEC Article 220 — continuous loads at 125% — and total them. The result sets your feeder size: a workshop with a welder and a charger lands very differently from a garage with lights and a door opener. Size for what is actually coming, not what is there today.
Take the calculated load and select conductors from the ampacity table at the 75°C column (the rating of most modern terminations). Common pairings: 60A on #6 copper, 100A on #3 copper — but verify against the table, apply derating for ambient temperature and conduit fill where required, and check your local amendments. The feeder breaker in the main panel protects these conductors, so the breaker and the wire must agree.
For an attached garage, SER cable or conduit through the structure works; for a detached garage, plan the underground run in PVC conduit at the NEC 300.5 burial depth (typically 18 inches for PVC under residential property, deeper under driveways). Size the conduit for fill per Chapter 9 tables — pulling four fat conductors through an undersized conduit is a miserable afternoon you only do once.
Pull two ungrounded (hot) conductors, one grounded (neutral), and one equipment grounding conductor — all four, full size. The equipment ground is what clears faults; without it, a fault in the garage has no low-impedance path home and the breaker may never trip. Label the feeder at both ends before you terminate anything.
In the subpanel, the neutral bar and the ground bar stay isolated from each other and from the enclosure — remove the bonding jumper or screw that ties them. Neutral and ground bond together at the main service disconnect and nowhere else (NEC 250.24(A)(5)). Bonding them in the subpanel creates parallel neutral paths, objectionable current on the grounding system, and a guaranteed inspection failure.
A detached garage is a separate structure and needs its own grounding electrode system — typically one or two ground rods with a grounding electrode conductor to the subpanel's ground bar. The electrode connects to the equipment grounding conductor arriving in the feeder, giving fault current its path home. Bond any metal water piping or structural steel in the garage to the grounding system as well.
A detached garage needs a disconnecting means for the feeder — the common compliant method is a backfed main breaker in the subpanel, sized to the feeder, with a proper hold-down kit. Label it clearly as the building disconnect. The inspector will look for it, and the next electrician to work in that garage will bless you for it.
Torque every termination to manufacturer spec (NEC 110.14(D)), fill out the panel directory per NEC 408.4, and verify: correct voltage phase-to-phase and phase-to-neutral, neutral-to-ground voltage near zero with loads running, and proper GFCI protection on garage receptacles (NEC 210.8). A subpanel that tests clean on day one stays clean for decades.
Every one of these has cost someone a callback. Learn them here instead of on the job.
The single most common subpanel violation. Neutral and ground bond at the main service only (NEC 250.24(A)(5)). In the subpanel they stay separated — always.
Running #10 copper for a 60-amp welder circuit because the wire was on hand. Size the feeder from the load calc via NEC 310.16, not from the scrap bin.
A detached structure needs its own grounding electrode system (NEC 250.32), bonded to the equipment ground from the feeder. Skipping it fails inspection.
Two hots and a neutral with no equipment ground — a violation since the 2008 NEC for new work. Run the fourth wire.
A detached garage needs a disconnecting means at the structure. A backfed main breaker in the subpanel is the common compliant answer.
Old subpanels are copper busbars waiting to be sold: the busbar itself, the feeder tails, and the branch-circuit copper pulled during the upgrade. A 100-amp feeder pull leaves heavy copper cutoffs at both ends, and the old panel's guts strip out in minutes with a screwdriver.
Most electricians keep the old panels and feeders for themselves — ask your journeyman who gets the cutoff bucket on your crew. When the pile builds up, call Sonny at 312-880-8174. Old panels are regular work, which means the copper is regular too.
No minimums — every bucket counts. On bigger lots, we handle the freight so you don't lift a finger.
Work to code. Hire a licensed pro where required. This guide explains how the work is done — it does not replace training, apprenticeship, or a license.