How to Ground a Subpanel: 4-Wire Feeder and Isolation Rules

The Lethal Mistake: Why Subpanel Grounding is Different

If you wire a subpanel exactly like a main panel, you create a severe shock hazard. The most common and dangerous mistake DIYers make when learning how to ground a subpanel is leaving the neutral and ground bars bonded together.

To understand the hazard, you must understand the difference between three terms that are often incorrectly used interchangeably:

• Neutral (Grounded Conductor): The intentional, current-carrying return path for normal circuit current. It is insulated and typically white or gray.
• Ground (Equipment Grounding Conductor / EGC): The emergency, non-current-carrying fault path designed solely to trip the breaker during a short circuit. It is bare copper or green.
• Bonding: The physical, intentional connection between the neutral and the ground system.

The Hazard: NEC-style guidance mandates that neutral and ground are bonded exactly once—at the main service disconnect. If you also bond them at a subpanel, normal 120V return current will split. It will travel back to the main panel on both the white neutral wire and the bare copper ground wire. Because the ground wire is connected to appliance chassis, metal conduit, and plumbing, those surfaces become energized with parallel current. Worse, if the neutral feeder wire breaks upstream, the entire subpanel grounding system (and every metal appliance plugged into it) will elevate to 120V, creating a lethal shock hazard.

Sizing and Running the Equipment Grounding Conductor (EGC)

Modern electrical practice (reinforced since the 2008 NEC cycle) strictly requires a 4-wire feeder for subpanels: two ungrounded 'hot' conductors, one grounded neutral conductor, and one equipment grounding conductor (EGC). You can no longer rely on a 3-wire setup or the metal panel chassis for new installations.

The EGC must be sized according to the rating of the upstream feeder breaker, not the load calculation. Below is the standard copper EGC sizing reference based on NEC Table 250.122.

Note: If you are using aluminum feeder wire (like 2-2-2-4 SER cable), the EGC included in that cable bundle is typically 4 AWG aluminum, which is code-compliant for a 100A feeder. Always check the cable jacket printing.

Step-by-Step: Terminating Grounds and Neutrals in a Subpanel

When your subpanel arrives from the factory, it often comes with a green bonding screw or a copper bonding strap pre-installed, connecting the neutral bar to the metal panel chassis. Your first job is to remove it.

1. Verify De-energization: Turn off the feeder breaker in the main panel. Use a non-contact voltage tester, then verify with a multimeter set to AC Voltage across the hot bus bars and from hot to ground. It must read 0V.
2. Remove the Bonding Jumper: Locate the green bonding screw or metal strap tying the neutral bar to the panel enclosure. Back it out completely. Keep it in your toolbox; do not leave it resting inside the panel where it could short against a bus bar.
3. Install an Accessory Ground Bar (If Needed): Many subpanels only ship with one bar (intended for the isolated neutral). You must buy and install an accessory grounding bar that mounts directly to the metal chassis. Ensure it matches your panel brand (e.g., Square D PK series, Eaton GB series) so it fits the pre-punched mounting holes.
4. Land the Feeder EGC: Terminate your incoming bare or green feeder ground wire to the new accessory ground bar. Torque the lug screw to the manufacturer's specification (typically 20 to 30 in-lbs for smaller lugs). Use a calibrated torque screwdriver; loose ground lugs cause high-resistance faults that prevent breakers from tripping.
5. Terminate Neutrals and Branch Grounds: Land the white feeder neutral wire on the isolated neutral bar. Land branch circuit white neutrals on the neutral bar, and branch circuit bare grounds on the chassis ground bar.
   Pro-Tip: NEC 408.41 strictly prohibits 'double-tapping' neutrals (putting two neutral wires under one screw). One neutral per lug. Ground wires, however, can often share a lug if the panel's wiring diagram explicitly permits it.

Disclaimer: This is NEC-style guidance for educational purposes. Your local Authority Having Jurisdiction (AHJ) or city inspector has final legal authority over code compliance and permit requirements in your area.

Verifying the Installation: Testing Before Energizing

Before you flip the feeder breaker on, you must verify that the isolation is complete. Skipping this step risks energizing your grounding system the moment the panel is turned on.

Test 1: The Continuity Check (Power OFF)
Set your multimeter to the Ohms (Ω) or continuity setting. Place one probe on the isolated neutral bar and the other on the ground bar (or the bare metal panel chassis).
Expected Result: The meter should read 'OL' (Open Line) or infinite resistance. If it reads less than 1 ohm, or beeps, you have a bond. You either missed the bonding screw, or a downstream branch circuit has a neutral-to-ground fault that you need to hunt down before energizing.

Test 2: The Voltage Check (Power ON)
Once the inspector approves and you energize the feeder breaker, set your meter to AC Voltage.
• Hot to Neutral: ~120V (or ~240V across both hots)
• Hot to Ground: ~120V
• Neutral to Ground: ~0V to 2V. (A reading of 0.5V to 2V is normal due to voltage drop on the neutral wire under load. If you read 120V from Neutral to Ground, your neutral feeder is disconnected or broken).

When to Call a Licensed Electrician

While terminating a subpanel is a common advanced DIY task, you must hire a licensed electrician if:

• You need to upgrade the main service panel to accommodate the new 2-pole feeder breaker.
• The main panel lacks a proper Grounding Electrode System (e.g., ground rods, UFER, or metal water pipe bond). A subpanel relies entirely on the main panel's grounding electrode system.
• You are trenching and running underground PVC conduit to a detached structure, which requires specific depth, warning tape, and pull-string practices.

Subpanel Grounding FAQ

Do I need a ground rod for a subpanel in the same building?

No. If the subpanel is located in the same building as the main service disconnect (for example, a basement subpanel fed from a main panel in the garage), you do not need a grounding electrode (ground rod). You only need the 4-wire feeder. According to NFPA 70 (NEC) Article 250.32, a grounding electrode system is only required for a subpanel located in a separate structure (like a detached garage or shed). In that detached scenario, you still run the 4-wire feeder, keep the bars isolated, but you also bond the ground bar to two 8-foot ground rods driven into the earth outside the detached building.

Can I use metal conduit (EMT) as the ground wire for a subpanel?

Technically, yes. NEC Article 250.118 allows rigid metal conduit (RMC), intermediate metal conduit (IMC), and electrical metallic tubing (EMT) to serve as the Equipment Grounding Conductor, provided the fittings are tight and listed for grounding. However, in practical jobsite and DIY applications, pulling a separate copper EGC wire inside the conduit is vastly superior. Metal conduit relies on setscrew or compression couplings for ground continuity; if a coupling loosens over time due to thermal expansion or vibration, your ground path is broken. A pulled copper wire guarantees a continuous, low-impedance fault path.

What happens if I accidentally leave the bonding screw in a subpanel?

If the bonding screw is left in, you create parallel neutral paths. You will likely experience two immediate symptoms. First, any GFCI or AFCI breakers feeding the subpanel (or located inside the subpanel) will nuisance-trip constantly, because the GFCI sensor will detect current returning on the ground wire and interpret it as a ground fault. Second, you create a severe shock hazard. As noted by industry experts at EC&M, if the neutral feeder wire ever becomes disconnected upstream, the metal chassis of every appliance plugged into the subpanel will carry full line voltage, waiting for a person to touch it and complete the circuit to earth.