You use the bonding screw (or bonding strap) only in the main service disconnect panel. Its sole purpose is to physically connect the neutral bus bar to the ground bus bar and the metal panel enclosure. In any downstream subpanel, the bonding screw must be removed or left out of the factory packaging. Getting this single piece of hardware right—or wrong—is the difference between a properly clearing ground fault and a lethal shock hazard on your appliances.
The Hazard: What Happens When You Get the Bonding Screw Wrong?
Before we look at the installation rules, you need to understand the physics of what goes wrong when the bonding screw is misplaced. The National Electrical Code (NEC) requires that the neutral (grounded conductor) and the ground (equipment grounding conductor) be bonded together at exactly one point in your electrical system. This is called the main bonding jumper, which in residential 100A to 200A panels is typically a green #10-32 machine screw or a copper strap.
Hazard 1: The Missing Bond in a Main Panel
If you fail to install the bonding screw in your main service panel, the neutral and ground systems remain isolated. If a hot wire inside your washing machine chafes and touches the metal chassis, the fault current has no low-impedance path back to the transformer. The breaker will not trip. The metal casing of your washing machine will sit there energized at 120V, waiting for you to touch it and complete the circuit to earth. This is a lethal shock hazard.
Hazard 2: The Left-In Bond in a Subpanel
If you leave the bonding screw installed in a subpanel (like one feeding a detached garage or a kitchen addition), you create parallel neutral paths. Under normal operation, return current flows back on the white neutral wire. But because the neutral and ground are bonded at the subpanel, current will also flow back on the bare copper ground wire, the metal conduit, and the panel enclosure. If that neutral wire ever breaks upstream, the full 120V return load will energize every metal surface connected to the ground system in that building.
Defining the Terms: Ground vs. Bond vs. Neutral
- Neutral (Grounded Conductor): The white (or gray) wire designed to carry normal return current back to the source.
- Ground (Equipment Grounding Conductor): The bare or green wire designed to carry current only during a fault to trip the breaker. It should carry zero current during normal operation.
- Bond: The physical connection (the screw or strap) that ties the neutral and ground systems together to establish a common reference point of zero volts.
Main Panel vs. Subpanel: A Decision Tree for the Bonding Screw
To determine whether the green screw should be driven into the neutral bar, identify the exact role of the panel in your electrical hierarchy. Use the decision matrix below to verify your setup.
| Panel Type / Location | Bonding Screw Status | Neutral & Ground Bars | Why? |
|---|---|---|---|
| Main Service Panel (First point of disconnect after the meter) | INSTALLED | Can be combined on one bar or separated but jumpered. | Establishes the single system bond to clear ground faults. |
| Subpanel (Same building, fed from main) | REMOVED | Must be strictly isolated. Add-on ground bar required (e.g., Square D HOM8GBCP). | Prevents neutral return current from flowing on ground wires/enclosures. |
| Subpanel (Detached building with 4-wire feed) | REMOVED | Strictly isolated. Requires its own grounding electrode system (ground rods). | Prevents parallel paths across the feeder cable between buildings. |
| Generator Interlock Panel (Backfed breaker with slide interlock) | INSTALLED (Usually) | Combined or bonded, as it acts as the main service disconnect. | Maintains the single bond point if the utility main is physically locked out. |
For a deeper look into the physics of why parallel neutral paths violate safety standards, refer to the extensive breakdown on grounding vs. bonding by EC&M.
How to Verify Your Panel's Bonding Status with a Tester
If you are auditing an existing panel—perhaps in a home you just bought or after a DIYer worked on it—you need to verify the bond. Do not guess based on wire colors alone. Follow these steps to confirm the physical and electrical status of the bond.
Step 1: The Visual Inspection (Live or Dead)
- Remove the panel dead front (cover). Warning: The main lugs remain live even if the main breaker is off. Keep your hands clear of the incoming service cables.
- Locate the neutral bus bar (where the white wires land). Look for a green machine screw driven directly through the bar into the threaded hole in the panel's metal backplane. Alternatively, look for a copper bonding strap wrapping around the bar.
- If it is a main panel, the screw/strap must be present. If it is a subpanel, it must be absent.
Step 2: The Multimeter Continuity Test (POWER OFF ONLY)
If the visual inspection is inconclusive (e.g., painted over, or hidden behind wires), you must test electrically.
- De-energize the panel. Turn off the main breaker for a main panel, or the feeder breaker in the main panel for a subpanel.
- Verify the bus bars are dead using a non-contact voltage tester (NCVT) and a digital multimeter (like a Fluke 117) set to AC Voltage. Measure between the hot bus bars and the ground bar. It must read 0V.
- Switch your multimeter to Continuity/Ohms (Ω) mode.
- Place one probe on the neutral bus bar and the other on the ground bus bar (or the bare metal panel enclosure).
- Interpretation: A reading of < 1.0 ohm confirms a physical bond exists. An 'OL' (Open Loop) or infinite reading confirms the bars are isolated.
When to Call a Licensed Electrician
You must bring in a licensed professional if you discover a subpanel that is improperly bonded and lacks a dedicated equipment grounding conductor run back to the main panel. Upgrading a 3-wire feeder to a 4-wire feeder requires pulling new cable, calculating voltage drop, and potentially upgrading the feeder breaker. Additionally, any work involving the service entrance cables upstream of the main breaker requires utility coordination and a licensed electrician, as those lugs are always live and carry massive fault current. For more on service entrance safety, review the NFPA 70 NEC guidelines and OSHA electrical safety standards.
Frequently Asked Questions About Panel Bonding Screws
What if my subpanel didn't come with a separate ground bar?
Most panels sold at big-box stores (like the Eaton BR2020B100V25) ship with a single, pre-bonded neutral/ground bar intended for main panel use. To use it as a subpanel, you must remove the green bonding screw and purchase an add-on ground bar specific to your panel brand. For Square D Homeline, use the HOM8GBCP; for Eaton BR, use the BRPG1. Do not mix brands, as the mounting clips and UL listings will not match. Land all bare/green wires on the new isolated ground bar, and all white wires on the factory bar.
Can I use a bonding wire instead of the green bonding screw?
Yes. The NEC permits a main bonding jumper to be a wire instead of a screw or strap, provided it is sized correctly for the service. For a standard 200A residential service, a #6 AWG or #4 AWG bare copper wire routed from the neutral bar to a dedicated grounding lug on the panel enclosure is code-compliant. However, in 99% of residential load centers, the factory-supplied green screw or copper strap is the standard, listed method and is much easier to install.
Why do some older panels have neutrals and grounds on the same bar without a visible screw?
In older installations, particularly from the 1960s through the 1980s, panels were often installed as the main disconnect but the bonding mechanism was hidden. Some Siemens and Murray panels use a 'push-in' bonding tab located on the side of the neutral bar that snaps into the panel frame, rather than a visible screw. Alternatively, if it is a subpanel and the grounds and neutrals are mixed on a single bar with no separate ground bar and no 4-wire feeder, you are looking at a legacy code violation that should be evaluated by an inspector.
Does a whole-house generator transfer switch need the bonding screw?
It depends on the switch type. If you are using a standard subpanel-style transfer switch fed by a breaker in your main panel, the bond screw must be removed. However, if you are installing a service-rated automatic transfer switch (ATS) that sits between the meter and your main panel, it acts as the new main service disconnect. In this scenario, the bond is established inside the ATS, and the downstream 'main' panel is effectively demoted to a subpanel, meaning its bonding screw must be removed. Always check the manufacturer's wiring diagram for the specific ATS model.
