Detached Garage Sub-Panel Wiring Diagram
This is a free printable detached garage sub panel wiring diagram: download the diagram as SVG or open it and print to paper or PDF.
Wire a detached garage sub-panel correctly under NEC 250.32: a 4-wire feeder with isolated neutral, separate grounding electrode at the outbuilding, and no neutral-ground bond at the sub-panel.
Wiring a sub-panel in a detached garage — or any detached outbuilding — is governed by a specific set of rules that differ meaningfully from wiring a sub-panel in an attached structure. In the United States, NEC 250.32 (NFPA 70, National Electrical Code) establishes the requirements. Similar principles apply under other national standards: AS/NZS 3000 clause 5.4 (Australia/NZ), BS 7671 Section 544 (UK), and IEC 60364-5-54. Understanding why these rules exist is as important as knowing what they require.
The defining requirement for a detached structure sub-panel is the 4-wire feeder. This means the feeder cable from the main panel to the sub-panel must contain four conductors: two ungrounded hot conductors (Line 1 and Line 2), one grounded neutral conductor, and one grounding conductor (equipment ground). A 3-wire feeder — which combines the neutral and ground functions into one conductor — was permitted for detached structures under older NEC editions but is no longer acceptable for new installations under NEC 2008 and later.
The reason a 4-wire feeder is required is critical to understand. In a detached building served by a 3-wire feeder, the single combined neutral/ground conductor connects the two buildings' grounding systems together. If the neutral conductor breaks (an open neutral fault), the metal chassis of appliances in the outbuilding can become elevated to a dangerous voltage relative to the ground at the outbuilding — because the grounding path back to the main panel passes through the broken neutral conductor. A 4-wire feeder with a separate isolated neutral and a separate equipment grounding conductor avoids this hazard.
At the sub-panel in the detached structure, the neutral bar and the grounding bar must be separated — there must be no neutral-to-ground bond inside the sub-panel. This is opposite to the main panel, where the single main bonding jumper connects neutral to ground at the service entrance. In the sub-panel, neutral and ground are kept isolated.
Additionally, NEC 250.32(A) requires the detached building to have its own grounding electrode system — typically a driven ground rod (8-foot copper-clad rod, or two rods if resistance exceeds 25 Ω). The grounding electrode conductor connects the sub-panel's grounding bar to this local ground rod. This provides a local fault-current path and limits the voltage rise of the building's grounding system during a lightning or fault event.
How to wire detached garage sub panel wiring diagram
- Calculate feeder load and determine panel size List all loads planned for the detached garage: lighting circuits, receptacle circuits, EV charger, air compressor, welder, or other equipment. Calculate the total load per NEC Article 220. Size the feeder and sub-panel to accommodate the calculated load with reasonable headroom for future additions. A 60 A or 100 A sub-panel is typical for a residential garage.
- Obtain permits and confirm requirements with the Authority Having Jurisdiction (AHJ) Sub-panel installation in a detached structure is typically a permitted electrical project. Contact the local building department or Authority Having Jurisdiction before beginning work. Confirm which edition of the NEC (or applicable local standard) is in force, and whether any local amendments apply. Inspections are typically required at rough-in and at completion.
- Select feeder wiring method and route Common feeder methods from main panel to detached structure: (a) direct burial cable (UF-B or SER-type rated for direct burial) in a trench at the minimum required depth — 24 inches under NEC Table 300.5 for 120/240 V residential circuits (less depth with conduit or GFCI protection depending on conditions); (b) individual conductors in schedule 40 or 80 PVC conduit buried at the minimum required depth; (c) overhead service — must meet NEC 230.24 clearance requirements for span length and height. A 4-conductor cable or conduit with 4 conductors is mandatory.
- Install the sub-panel enclosure and verify it is equipped for isolated neutral Select a sub-panel (load centre) with a main breaker (optional but recommended for local disconnection) and sufficient spaces for planned circuits. Crucially, verify the panel is configured with the neutral bar insulated from the enclosure (sub-feed configuration) — or that the bonding screw or strap has been removed. Most sub-panels ship with the neutral and ground bars bonded together (for use as a main panel); this bond must be removed when used as a sub-panel in a detached building.
- Install the feeder breaker at the main panel Install a double-pole breaker in the main panel rated for the feeder ampacity (60 A, 100 A, or as calculated). Connect Line 1 to the left breaker terminal and Line 2 to the right terminal, neutral to the neutral bar, and the equipment grounding conductor to the main panel's grounding bar. The main panel is the only location where neutral and ground are bonded.
- Terminate feeder conductors at the sub-panel Connect Line 1 and Line 2 to the main lugs or main breaker of the sub-panel. Connect the neutral conductor to the isolated neutral bar — verify the neutral bar is insulated from the panel enclosure. Connect the equipment grounding conductor to the grounding bar — which should be bonded to the metal panel enclosure. Confirm no connection exists between the neutral bar and the grounding bar.
- Install the grounding electrode system at the detached structure Drive an 8-foot (2.4 m) copper-clad ground rod at the detached structure, at least 2 feet from the building foundation. Test resistance per NEC 250.56 — if resistance exceeds 25 Ω, drive a second rod at least 6 feet away and connect in parallel. Connect the grounding electrode conductor from the sub-panel grounding bar to the ground rod using an approved clamp. Size the grounding electrode conductor per NEC Table 250.66 (minimum 6 AWG copper for feeders up to 100 A).
Specifications
| Applicable code — USA | NFPA 70 National Electrical Code (NEC), Section 250.32 — Separate Buildings or Structures |
|---|---|
| Feeder conductor count | 4 — Line 1 (hot), Line 2 (hot), Neutral (isolated), Equipment grounding conductor |
| Neutral-Ground bond at sub-panel | PROHIBITED — neutral bar must be isolated from panel enclosure and grounding bar |
| Grounding electrode requirement | Mandatory per NEC 250.32(A) — minimum one 8-foot copper-clad ground rod; two rods if resistance exceeds 25 Ω (NEC 250.56) |
| Minimum feeder size for 60 A circuit (copper) | 6 AWG copper for ungrounded and neutral conductors; 10 AWG copper equipment grounding conductor (NEC Table 310.12) |
| Minimum burial depth — direct burial cable | 24 inches (610 mm) per NEC Table 300.5 for 120/240 V residential feeders (depth reduced with conduit — see Table 300.5) |
| Grounding electrode conductor size | Minimum 6 AWG copper for feeders up to 100 A (NEC Table 250.66) |
| Feeder voltage | 120/240 V AC single-phase (typical North American residential) |
Safety warnings
- This diagram is a general reference only. Electrical installations in detached structures are subject to local building codes, the applicable edition of the NEC (NFPA 70) or equivalent national standard (AS/NZS 3000, BS 7671, IEC 60364, SANS 10142-1), and may require permits and inspection. Always consult a licensed electrician and your local Authority Having Jurisdiction (AHJ) before proceeding.
- CRITICAL — Neutral-Ground separation: In the detached structure sub-panel, the neutral bar and grounding bar must NOT be bonded together. The neutral-to-ground bond exists only at the main service panel. Bonding neutral and ground at the sub-panel creates a parallel neutral return path through the grounding conductor, which is a code violation and creates a shock hazard.
- De-energise and verify dead at the main panel before any work on the feeder breaker or feeder conductors. The main panel busbars remain live even with the main breaker off — only the utility provider can de-energise the service entrance conductors. Do not open the main panel cover without confirming the appropriate circuits are de-energised.
- Direct burial cable and conduit must be buried at the minimum depths required by NEC Table 300.5 (or equivalent). Insufficient burial depth creates a risk of physical damage to the cable, which can cause a ground fault or fire. Mark the cable route clearly to prevent future excavation damage.
- The detached structure grounding electrode system (ground rod) is a mandatory safety requirement, not optional. It provides a local fault-current path and limits voltage rise during fault events. Do not omit or defer the installation of the ground rod and grounding electrode conductor.
Tools needed
- Licensed electrician (required in most jurisdictions for sub-panel installation)
- Multimeter with AC voltage measurement
- Non-contact voltage tester
- Wire stripper and cable cutter
- Torque screwdriver or torque wrench (for terminal torquing to specification)
- Ground rod driver or sledgehammer (for driving ground rod)
- Conduit bender and hacksaw or conduit cutter (if using conduit)
- Trenching shovel or mechanical trencher (for underground feeder route)
- Label maker or permanent marker (for conductor identification)
Common mistakes
- Leaving the neutral-ground bonding screw or strap in place at the sub-panel — most load centre panels ship bonded for use as a main panel. The bonding screw must be removed when the panel is used as a sub-panel in a detached structure. Failure to remove it creates a code violation and a safety hazard.
- Using a 3-wire feeder (two hots and a combined neutral/ground) for a new detached structure installation — this is not permitted under NEC 2008 and later editions for new installations. Only existing, previously approved 3-wire feeders may remain in service under a specific exception in NEC 250.32.
- Omitting the local grounding electrode (ground rod) at the detached structure — the ground rod is a mandatory NEC 250.32(A) requirement. Running the feeder grounding conductor back to the main panel is not a substitute for a local grounding electrode.
- Undersizing the feeder for anticipated future loads — plan for the maximum realistic load that might be connected in the future (EV charger, welder, air conditioning) and size the feeder accordingly. Upsizing a buried feeder after the fact is extremely costly.
- Failing to identify conductor polarity and function at both ends — colour code conductors consistently and label them at the main panel and sub-panel. Swapping Line and Neutral conductors at the sub-panel creates a dangerous condition with neutral-to-ground voltage present on the neutral bar.
Troubleshooting
- Breakers in sub-panel trip immediately when loads are energised
- Cause: Ground fault in branch circuit wiring; a crossed neutral and ground at the sub-panel; or a wiring fault creating a live-to-earth short in one of the branch circuits Fix: Disconnect all branch circuit loads. Test each circuit for insulation resistance with a megohmmeter. Verify neutral and ground are not transposed at the sub-panel. Check that the neutral bar is fully isolated from the enclosure (no bonding screw). Re-energise circuits one at a time to identify the faulted circuit.
- Voltage at sub-panel is significantly lower than at the main panel
- Cause: Excessive voltage drop due to an undersized feeder conductor for the run length, high-resistance feeder connections (particularly at aluminium terminations), or a loose connection at the main panel feeder breaker Fix: Measure voltage at the main panel feeder breaker terminals and at the sub-panel main lugs simultaneously under load. Calculate voltage drop using conductor resistance and load current. Tighten all feeder terminations to the torque values marked on the breaker and panel lugs. If conductors are undersized, upsize the feeder.
- RCD or GFCI devices in the sub-panel trip immediately at energisation with no load connected
- Cause: Neutral-to-ground bond left in place at the sub-panel — the parallel ground path causes GFCI/RCD devices to see an imbalance between Line and Neutral current even with no load, because some neutral current is returning via the grounding conductor Fix: De-energise the feeder. Open the sub-panel and confirm the bonding screw or bonding strap between the neutral bar and the panel enclosure has been fully removed. Check that no neutral wire is inadvertently landed on the grounding bar (neutrals to neutral bar only, grounds to ground bar only).
Frequently asked questions
Why does a detached garage sub-panel need 4 wires instead of 3?
A 4-wire feeder separates the neutral and equipment grounding conductors. This prevents a broken neutral conductor from energising the metal chassis of appliances in the outbuilding. Under NEC 250.32, a 3-wire feeder is only permitted for existing (previously installed) feeders — all new detached structure sub-panel installations require a 4-wire feeder.
Should neutral and ground be bonded together in the detached garage sub-panel?
No — this is the most critical rule. The neutral bar and grounding bar in the detached garage sub-panel must be separated and kept isolated from each other. The neutral-to-ground bond exists only at the main service panel. Bonding neutral to ground at the sub-panel creates a parallel neutral return path through the grounding conductor, which is a code violation and a safety hazard.
Does a detached garage sub-panel need its own ground rod?
Yes, under NEC 250.32(A). The detached structure requires its own grounding electrode system — typically one 8-foot copper-clad ground rod (two rods if resistance exceeds 25 Ω per NEC 250.56). The grounding electrode conductor connects the sub-panel's ground bar to this local rod. This is separate from the grounding conductor that runs in the feeder cable.
What size wire is needed for a detached garage sub-panel feeder?
Feeder size depends on the load and distance. A common residential detached garage installation uses a 60 A feeder — 6 AWG copper conductors (hot 1, hot 2, neutral) and 10 AWG copper equipment ground, minimum. A 100 A feeder requires 4 AWG or 3 AWG copper (or 2 AWG aluminium). Always calculate the actual load and voltage drop for the specific run length, and size per NEC Article 220 and Table 310.12.
Can I use aluminium wire for the detached garage sub-panel feeder?
Yes, aluminium conductors are acceptable for feeder runs and are common for longer distances due to lower cost. Use aluminium conductors rated for the application (XHHW-2 or USE-2 for direct burial). Termination points must be rated for aluminium connections — not all breakers and lugs accept aluminium. Apply anti-oxidant compound at all aluminium terminations and torque terminals to specification.
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