Sub-Panel Wiring Diagram: 4-Wire Feeder, Neutral-Ground Separation, and Safe Installation

Sub-Panel Wiring Diagram: 4-Wire Feeder, Neutral-Ground Separation, and Safe Installation — interactive diagram. Open it in the editor to customise components and wiring.

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A sub-panel (secondary distribution board) receives a 4-wire feeder from the main panel and requires a fully isolated neutral bus — the neutral and ground must NOT be bonded together at the sub-panel, only at the main service panel.

A sub-panel (also called a secondary distribution board, load centre, or sub-board) is a distribution board fed from the main service panel (main switchboard) via a feeder circuit. It distributes power to a separate area — a garage, workshop, outbuilding, granny flat, or a section of a commercial building — through its own branch circuit breakers.

The critical safety principle that governs sub-panel wiring is the neutral-ground separation rule: the neutral conductor and the equipment grounding conductor (earthing conductor) are bonded together at exactly one point in the system — the main service panel (where the main bonding jumper connects them, and where the service neutral is grounded). At every sub-panel, the neutral bus and the ground bus must be isolated from each other. This is non-negotiable and is required by NEC 250.142(B), BS 7671, IEC 60364, and virtually every national wiring standard.

Why? If neutral and ground are bonded at the sub-panel, the neutral return current will have two parallel paths back to the main panel: the neutral conductor and the ground conductor. Current flowing on the ground conductor is dangerous — it energises metal equipment enclosures at a voltage above true earth potential, creating shock hazard and interfering with ground-fault protection.

The feeder from the main panel to a sub-panel in a detached building (garage, outbuilding) must be a 4-wire feeder: two ungrounded (hot) conductors, one grounded (neutral) conductor, and one equipment grounding (earth) conductor. The only permitted exception under NEC 250.32 for a 3-wire feeder (no separate equipment ground) is for existing installations where there is no continuous metallic path between buildings — new installations must use a 4-wire feeder.

At the sub-panel, the neutral conductor terminates on the isolated neutral bar. The equipment grounding conductor terminates on the ground bar. The ground bar is bonded to the sub-panel enclosure. The neutral bar must be insulated from the enclosure — verify there is no factory-installed bonding strap or green screw connecting neutral bar to enclosure, and remove it if present. A grounding electrode conductor from a local ground rod at a detached building also connects to the ground bar at the sub-panel in certain jurisdictions and code editions.

How to wire sub panel wiring diagram

Determine feeder size and sub-panel amperage rating Calculate total anticipated load using NEC Article 220 or equivalent local code demand factor method. Select a sub-panel with sufficient breaker spaces and an amperage rating at or above the feeder current. All work must be performed by a licensed electrician and permitted by the local authority.
Install the sub-panel enclosure Mount the sub-panel enclosure securely on a non-combustible surface. Ensure adequate clearance in front of the panel per NEC 110.26 (914mm / 36 inches minimum working space depth) or equivalent local requirement.
Remove the neutral-to-enclosure bonding strap Before installing any wiring, locate and remove the neutral bar bonding strap or green bonding screw that factory-connects the neutral bar to the enclosure. The neutral bar must be fully isolated from the enclosure in a sub-panel. Retain the bonding hardware — it documents that it was deliberately removed and is available if the panel is ever converted to main service use.
Run the 4-wire feeder from the main panel Run a 4-conductor cable or individual conductors in conduit from the main panel breaker to the sub-panel. The feeder consists of: two ungrounded (hot) conductors, one grounded (neutral) conductor (white or grey), and one equipment grounding conductor (green, green with yellow stripe, or bare copper). Size all conductors for the feeder ampacity.
Terminate the feeder conductors at the sub-panel The two hot conductors terminate on the main sub-panel breaker (or directly on the bus bars if no main breaker is fitted). The neutral conductor terminates on the isolated neutral bar. The equipment grounding conductor terminates on the ground bar. Verify neutral bar and ground bar are on separate, insulated buses.
Bond the ground bar to the enclosure The ground bar (not the neutral bar) bonds to the sub-panel metal enclosure. Most sub-panel kits have the ground bar factory-bonded to the enclosure — confirm this is correct for the ground bar only, and that the neutral bar remains isolated.
Install branch circuits and verify with inspection Install branch circuit breakers and wiring per local code. Have the installation inspected and approved by the authority having jurisdiction (AHJ) before energising. Do not energise the feeder until the sub-panel installation has passed electrical inspection.

Specifications

Number of feeder conductors (new installation)	4: 2 ungrounded (hot), 1 grounded (neutral), 1 equipment grounding (earth)
Neutral-ground bonding location	Main service panel ONLY — never at sub-panel
Minimum working clearance in front of panel (NEC 110.26)	914mm (36 inches) depth, full height of panel, 762mm (30 inches) wide
Minimum driven ground rod length (NEC 250.52)	2.4m (8 feet)
Feeder conductor ampacity standard (NEC)	NEC Table 310.12 for residential services; Table 310.16 for general use
Voltage drop recommendation for sub-panel feeders	Maximum 3% voltage drop on feeder recommended (NEC Annex D is informational, not mandatory)
Sub-panel neutral bar status	Must be isolated (insulated) from enclosure — bonding strap/green screw removed

Safety warnings

CRITICAL: The neutral bar and ground bar must NEVER be bonded together at a sub-panel. They are bonded at one point only — the main service panel. Incorrect bonding at the sub-panel creates parallel return current paths on grounding conductors, energises equipment frames above earth potential, and defeats ground-fault protection. This is a code violation and a serious shock and fire hazard.
Sub-panel installation must be performed by a licensed electrician and requires a permit from the local authority having jurisdiction (AHJ). The installation must pass an electrical inspection before being energised.
Always de-energise, verify dead, and apply Lockout/Tagout procedures on the main panel feeder breaker before working inside the sub-panel. Use a CAT IV rated voltage tester on all bus bars before touching any internal component.
The main panel feeder breaker does NOT make the main panel's bus bars safe to touch — the main panel service entrance conductors remain live at all times (controlled only by the utility). Never work inside the main panel near service entrance conductors without utility isolation.
Feeder conductors sized only for ampacity may still experience excessive voltage drop over long runs — size conductors for voltage drop as well as ampacity. Excessive voltage drop causes motor overheating, nuisance tripping, and poor equipment performance.

Tools needed

Calibrated voltage tester, CAT IV rated (for proving dead at all bus bars before any work)
Digital multimeter for continuity and resistance checks
Wire strippers for feeder conductor gauges
Knockout punch set for conduit entries
Torque screwdriver set (terminal torque is specified in the panel installation manual — undertorqued terminations cause arcing and fire)
Conduit bender (if using conduit)
Lockout/Tagout equipment

Common mistakes

Leaving the neutral-to-enclosure bonding strap installed in the sub-panel — this is the single most common and most dangerous sub-panel wiring mistake. The neutral and ground bars must be isolated from each other in the sub-panel.
Running a 3-wire feeder (no separate equipment ground) to a sub-panel in a detached outbuilding — this is not permitted under current NEC 250.32 for new installations and leaves the outbuilding without a proper equipment ground.
Terminating the equipment grounding conductor on the neutral bar instead of the ground bar — creates the same hazardous parallel current path as bonding the bars together.
Undersizing the feeder conductor for the planned load or for the cable run length, resulting in excessive voltage drop and nuisance breaker tripping under full load.
Installing a sub-panel without a permit or without passing electrical inspection — this is a code violation, voids homeowner's insurance in many cases, and creates liability if an electrical fire occurs.

Troubleshooting

GFCI breakers or outlets in the sub-panel nuisance-trip constantly: Cause: Neutral and ground are bonded at the sub-panel — return current is flowing on the ground conductor and being detected as ground fault current by the GFCI device Fix: Isolate the sub-panel feeder. Open the panel and verify the neutral bar and ground bar are on separate, isolated buses with no bonding strap or green screw connecting them to each other or to the enclosure. Remove any bonding connection between neutral bar and enclosure.
Voltage measured between neutral bar and ground bar in sub-panel: Cause: This is normal for a correctly wired sub-panel — there will be a small voltage between neutral and ground due to neutral return current flowing through the neutral conductor's resistance. Typically less than 1–3V under load. Excessive voltage (5V or more) indicates excessive neutral current or high feeder impedance. Fix: Verify the feeder conductor sizes are adequate. Measure feeder current with a clamp meter. If the voltage is within 1–3V under normal load, the installation is functioning correctly.
No voltage at sub-panel bus bars despite main panel feeder breaker being on: Cause: Open circuit in feeder (loose terminal in main panel, broken conductor, or open connection at sub-panel main breaker), or feeder breaker has tripped Fix: Verify the feeder breaker in the main panel is not tripped. Check both sides of the feeder breaker for voltage with CAT IV tester. Trace the feeder conductors for any damage, loose connection, or missing termination.

Frequently asked questions

Why must the neutral and ground be separated at a sub-panel?

Bonding neutral and ground at the sub-panel creates two parallel return paths: the neutral conductor and the ground conductor. Neutral return current flowing through the ground conductor energises metal enclosures and equipment frames above true earth potential, creating shock hazard. It also interferes with GFCI and ground fault protection which rely on all return current flowing on the neutral.

How many wires are in a sub-panel feeder?

New installations must use a 4-wire feeder: two ungrounded (hot/line) conductors, one grounded (neutral) conductor, and one equipment grounding (earth) conductor. A 3-wire feeder (no separate equipment ground) is only permitted for sub-panels in the same building under specific conditions in some codes, and is not permitted for sub-panels in detached structures under current NEC 250.32.

Where does the bonding strap or green screw go in a sub-panel?

The bonding strap or green screw that bonds the neutral bar to the enclosure must be REMOVED from the sub-panel. It is correctly installed only in the main service panel. In the sub-panel, the neutral bar must be fully isolated from the enclosure. Most sub-panel kits include a removable bonding strap — remove it before installation.

Does a detached outbuilding sub-panel need its own grounding electrode (ground rod)?

Under NEC 250.32 (and similar provisions in other codes), a separate structure served by a feeder must have a grounding electrode system (such as a ground rod) at the separate structure. The grounding electrode conductor from this local ground rod connects to the sub-panel's ground bar — not the neutral bar. Local code interpretation and jurisdiction may affect exact requirements.

What size feeder do I need for a sub-panel?

Feeder size depends on the sub-panel's anticipated load. Calculate the total connected load in watts and convert to amps by dividing by the voltage (240V for two-pole circuits in North America). Apply demand factors per your local code (NEC Article 220). The feeder conductors, breaker in the main panel, and the sub-panel's main breaker (if fitted) must all be rated to the calculated feeder current. A licensed electrician should perform this calculation.

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