In any subpanel, the neutral bus bar and the ground (equipment grounding conductor) bus bar must be physically and electrically isolated from each other and from the panel enclosure. The main bonding jumper—the green bonding screw or metal strap that connects the neutral bar to the metal panel casing—must be removed before the subpanel is energized. This is the single most critical rule of subpanel bonding.
If you leave the bonding screw in place at a subpanel, you create a parallel path for normal return current to flow on the grounding system. This energizes appliance chassis, creates a lethal shock hazard, and can cause ground fault protection to misbehave. Below, we break down the physics of why this happens, how to size your feeder correctly, and how to verify your installation with a multimeter.
The Hazard: What Happens When Subpanel Bonding is Done Wrong
To understand the hazard, you have to clearly separate three terms that are often confused on the jobsite: neutral, ground, and bond.
- Neutral (Grounded Conductor): The white wire that carries normal, unbalanced return current back to the source during regular operation.
- Ground (Equipment Grounding Conductor / EGC): The bare or green wire that carries current only during a fault condition (like a hot wire shorting to a metal casing) to trip the breaker.
- Bond: The physical, intentional connection between the neutral and the ground system.
According to NEC-style guidance (specifically Article 250.32), the neutral and ground may only be bonded at the main service disconnect. If you bond them at a subpanel, the ground wire running back to the main panel becomes a parallel path for the neutral's return current. Because current divides across parallel paths inversely proportional to their resistance, a portion of your normal 15A or 20A load current will travel back along the bare ground wire. This puts a measurable voltage on every metal equipment enclosure, conduit, and appliance chassis connected to that subpanel's ground system.
Think of the neutral as the normal return highway, and the ground as the emergency shoulder. Bonding them at the main panel is the designated on-ramp. Bonding them again at a subpanel merges the shoulder into the highway downstream, putting heavy traffic (current) on the shoulder where pedestrians (people touching metal enclosures) are walking.
Subpanel Feeder Sizing and Grounding Conductor Requirements
Proper subpanel bonding relies on pulling a 4-wire feeder (two hots, one neutral, one ground) from the main panel. Sizing these wires correctly ensures the ground can safely clear a fault without melting, while the hots and neutral handle the continuous load. The table below outlines standard copper THHN feeder sizing based on the 75°C column of NEC Table 310.16, with the Equipment Grounding Conductor (EGC) sized per NEC Table 250.122.
| Subpanel Rating | Hot & Neutral Wire Size (Copper) | EGC Ground Wire Size (Copper) | Min. Conduit Size (EMT) |
|---|---|---|---|
| 60 Amp | #6 AWG | #10 AWG | 1 inch |
| 100 Amp | #3 AWG | #8 AWG | 1-1/4 inch |
| 125 Amp | #1 AWG | #6 AWG | 1-1/2 inch |
| 200 Amp | #2/0 AWG | #4 AWG | 2 inch |
Note: If your feeder run exceeds 100 feet, you must calculate voltage drop. A 3% maximum drop on a 240V feeder means you cannot lose more than 7.2V. For a 100A subpanel at 150 feet, you would need to upsize your hots and neutral from #3 AWG to #1 AWG to maintain acceptable voltage at the far end.
Step-by-Step: How to Verify and Correct Subpanel Isolation
If you are inspecting an existing subpanel in a new home, or verifying your own rough-in before the drywall goes up, you need to prove that the neutral and ground are isolated. Here is the exact bench-and-jobsite procedure to verify correct subpanel bonding.
- De-energize and Lockout: Turn off the main breaker feeding the subpanel at the main service panel. Use a lockout/tagout device on the breaker handle. Verify the subpanel is dead using a non-contact voltage tester and a multimeter checking hot-to-ground and hot-to-neutral.
- Visual Inspection for the Bonding Screw: Look at the neutral bus bar. In a main panel, you will see a green bonding screw or a metal bonding strap connecting the bar directly to the steel enclosure of the panel. In a subpanel, this screw must be backed out completely and removed, or the strap must be cut and removed. The neutral bar should be mounted on plastic or nylon insulators, floating completely free from the metal panel box.
- Multimeter Continuity Test: Set your digital multimeter to the continuity or resistance (Ohms) setting. Place one probe on the neutral bus bar and the other probe on the equipment grounding bus bar (or the bare metal enclosure of the panel).
- Expected Reading: "OL" (Over Limit) or infinite resistance. This proves they are isolated.
- Fault Reading: Any reading below 1 ohm (or a continuity beep) means the bars are bonded or touching.
- Check Wire Termination Segregation: Ensure no bare ground wires are terminated under the same screw as a white neutral wire. Neutrals go on the isolated neutral bar; bare/green grounds go on the ground bar (which is bolted directly to the panel enclosure).
Troubleshooting Decision Tree: Multimeter Readings
| Multimeter Reading (Neutral to Ground) | Diagnosis | Required Fix |
|---|---|---|
| OL (Infinite) | Correct isolation. Bonding screw removed. | None. Panel is safe to energize. |
| 0.0 to 0.5 Ohms | Hard bond present. Screw/strap left in, or bars touching. | Remove green bonding screw or bend back the metal bonding strap. |
| 10 to 50 Ohms | Downstream neutral-to-ground fault (a miswired outlet or appliance). | Disconnect all branch circuits and re-test to isolate the faulty downstream wire. |
When to Call a Licensed Electrician
While adding a circuit to an existing, properly configured subpanel is a standard DIY task for a competent hobbyist, the rules change when you are altering the feeder or the service entrance. The principles of grounding and bonding are unforgiving, and local Authorities Having Jurisdiction (AHJ) have the final say on code compliance in your specific municipality.
You must hire a licensed electrician in the following scenarios:
- Legacy 3-Wire Feeders: If you bought an older home (pre-1996) with a detached garage or workshop, it may be fed by a 3-wire cable (two hots and a neutral, with the neutral bonded to the ground at the subpanel). The NEC eliminated this exception for detached buildings in 2008. Upgrading this to a modern 4-wire feed requires pulling new cable and trenching, which requires a permit and professional execution.
- Service Entrance Upgrades: If your main panel lacks the physical breaker spaces or the ampacity headroom to add a 100A subpanel breaker, you are looking at a heavy-up (service upgrade). This involves the utility drop, the meter base, and the main bonding jumper at the service disconnect. This is strictly licensed work.
- Conduit Bending and Pulling: Pulling #2/0 AWG THHN through 2-inch EMT conduit requires specialized bending tools, fish tapes, and pulling lubricant. If you kink the conduit or damage the wire insulation during the pull, you create a hidden arc-fault hazard inside the walls.
A professional subpanel installation (including a 100A panel, 50 feet of 4-wire feeder in conduit, and drywall repair) typically ranges from $1,800 to $3,500 in 2026, depending on regional labor rates and whether trenching is required for detached structures. Paying for a licensed pull ensures the main bonding jumper is correctly placed at the service disconnect, and your subpanel bonding is correctly isolated, keeping your shop's equipment chassis safely at zero volts.
