How-To Guide
When adding a subpanel to a detached garage, modern code requires running a 4‑conductor feeder from the main service: two ungrounded conductors (hots), the grounded conductor (neutral), and an equipment grounding conductor (EGC). The EGC must be run with the feeder back to the main service rather than relying on a local ground rod alone.
Do not assume a 3‑wire run (hot/hot/neutral) is acceptable for a new detached‑garage subpanel; a separate equipment grounding conductor back to the main service is required.
The neutral (grounded conductor) in a subpanel must not be bonded to the panel enclosure or to grounding conductors; neutral‑to‑ground bonding is only at the service disconnect or main service equipment — the neutral bus in the subpanel must be isolated.
If a detached building is supplied by a feeder or branch circuit, NEC requires installing a grounding electrode system (for example, ground rod(s)) at that building and bonding any electrodes present together to form the building grounding electrode system.
The grounding electrode system at the detached garage must be bonded to the subpanel grounding bus, but that electrode system does not replace the requirement to run an equipment grounding conductor with the feeder back to the main service.
Running individual THHN/THWN conductors inside PVC conduit is a common, code‑appropriate approach for feeders to detached buildings; install the conduit and bury it to the minimum depth required by NEC Table 300.5 for your chosen wiring method (for example, individual insulated conductors in PVC conduit commonly require 18 inches, while direct‑buried UF cable typically requires 24 inches). Consult the exact table and any local amendments before digging.
THHN/THWN is commonly specified for conductors run in conduit to detached structures; verify the conductor's insulation rating and temperature marking match your application and wiring method.
NEC Part III of Article 250 covers grounding electrode types and required testing; two rods are commonly used and must be bonded together unless a single rod can be shown to have a resistance‑to‑earth of 25 ohms or less. Because a single rod rarely measures 25 ohms or less in practice, most installers drive two 8‑foot rods (bonded together) spaced per local practice to satisfy inspectors rather than rely on a 25‑ohm test.
After installing ground rod(s), bond the grounding electrode conductor to the garage subpanel grounding bus so the building grounding electrode system and the feeder EGC form the complete grounding/bonding network required by code.
Inspectors commonly check three things for detached‑garage feeders: (1) the neutral bus in the subpanel is isolated from the enclosure, (2) the equipment‑grounding conductor is continuous to the main service, and (3) the garage has a bonded grounding electrode system. These are frequent causes of rework when omitted or incorrectly executed.
Plan loads and subpanel ampacity with a licensed electrician — this determines feeder size and overcurrent protection.
Contact your local building department to confirm which NEC edition and local amendments are enforced and to obtain required electrical permits and inspections.
Decide route and wiring method: overhead vs underground, THHN in conduit vs UF direct‑burial, and confirm burial depth from NEC Table 300.5 for your chosen method.
Run a 4‑conductor feeder (two hots, neutral, and equipment grounding conductor) from the main service to the garage subpanel; do not rely on the ground rod as the only equipment grounding path.
Install a grounding electrode system (for example, two bonded 8‑ft rods) at the garage and bond that electrode system to the subpanel grounding bus.
Keep the neutral bus isolated in the subpanel — neutral‑to‑ground bonding belongs only at the main service disconnect.
Plan for required GFCI protection for garage receptacles and other circuits per the NEC edition adopted locally; provide GFCI breakers or GFCI receptacles as applicable.
For long feeder runs evaluate voltage‑drop and consider upsizing conductors if necessary to keep combined feeder and branch voltage drop near typical industry practice targets (about 3%) so equipment performs properly.
Verify conductor ampacities, conduit fill, termination ratings, and any manufacturer instructions and follow the authority having jurisdiction (AHJ) guidance during inspection and installation.
Costs vary widely with amperage, run length, trenching or conduit work, and local labor rates. Consumer cost sources place many typical detached‑garage subpanel jobs in the approximate $800–$2,000 range in many U.S. markets for a licensed electrician performing a typical 100 A install; longer runs and extras push costs higher — use these figures only as a starting estimate and get an on‑site quote.
Electrical feeder installations, alterations, or extensions generally require a permit and inspection; confirm permit specifics and fees with your local authority before work begins.
Ask the electrician to show you that the feeder includes a continuous equipment‑grounding conductor back to the main panel and that the neutral bus in the subpanel is isolated — these are quick visual checks an inspector will also perform.
Request documentation of the grounding electrode installation and bonding to keep with your permit/inspection paperwork.
If the project involves long trenches or deep burial, confirm the planned trench depth and wiring method against NEC Table 300.5 and any local amendments before final trenching and pulling conductors.
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Electrical