The direct answer to the most common bench and jobsite question: your main service panel must have the neutral and ground bonded together via a green bonding screw or copper strap, while every subpanel downstream must have them strictly isolated. Getting this backward creates an immediate, lethal shock hazard. This guide breaks down the physics of why this rule exists, how to test your existing setup with a multimeter, and the exact hardware you need to correct a deficiency.

The Lethal Mistake: Why Incorrect Panel Bonding Energizes Your Appliances

To understand electrical panel bonding, you must first understand the specific hazard it prevents: touch potential. When a hot wire (120V) inside your refrigerator or water heater frays and touches the metal chassis, a ground fault occurs. The circuit breaker must trip instantly to clear this fault. For the breaker to trip, the fault current needs a low-impedance (very low resistance) path back to the transformer source.

HAZARD ALERT: The Missing Main Bond
If your main service panel lacks a main bonding jumper, the fault current has no direct metallic path back to the neutral source. It attempts to return through the dirt via your grounding electrodes (ground rods). Dirt has extremely high impedance. The fault current will be too low to trip the breaker, leaving the entire metal chassis of your appliance energized at 120V. The next person to touch it becomes the return path.

Conversely, if you incorrectly bond a subpanel (connecting the neutral and ground bars together in a detached garage, for example), you create parallel neutral paths. Normal, everyday neutral return current will split and flow backward along the bare copper ground wire. This energizes the grounding system, meaning the metal enclosures of your tools, the conduit, and appliance chassis in the subpanel can carry a measurable, shock-inducing voltage.

Grounding vs. Bonding vs. Neutral: Clearing the Bench Confusion

Even experienced DIYers mix up these three terms. They serve entirely different physical functions in a circuit:

  • Grounding: Connecting the electrical system to the earth (dirt) via ground rods or ufer grounds. This stabilizes voltage from lightning strikes and line surges. It does not trip your breakers during a standard ground fault.
  • Bonding: Connecting all non-current-carrying metal parts (panel enclosures, conduit, appliance chassis) together to ensure they are at the same electrical potential (equipotential). This provides the low-impedance metallic highway that allows fault current to surge back to the panel and trip the breaker.
  • Neutral (Grounded Conductor): The intentional, current-carrying return path for normal 120V circuit operation. It is white or gray and carries load current every time you turn on a light.

Bonding is what saves your life during a fault. Grounding is what protects your house from a lightning strike. The main service panel is the only location in a standard residential system where the Neutral and the Bonding/Grounding system are intentionally connected.

The Decision Tree: Main Service Panel vs. Subpanel Bonding Rules

Use this decision matrix to determine the correct action for the panel you are working on. This guidance reflects standard NEC-style practice (specifically NEC Articles 250.24 and 250.32); however, always remember that your local Authority Having Jurisdiction (AHJ) or electrical inspector has the final legal authority on code compliance in your municipality.

Panel Type Neutral-to-Ground Bond Status Action Required Concrete Hardware Pick
Main Service Panel (First point of disconnect after utility meter) Missing bond (neutral bar is floating from the metal enclosure) Install main bonding jumper or green bonding screw to tie neutral bar to the grounded metal can. Factory copper strap, or panel-specific green screw (e.g., Siemens EQGB1 or standard #10-32 green machine screw if tapped).
Subpanel (Fed by a breaker from the main panel, e.g., detached garage or addition) Bonded (Green screw is installed on the neutral bar) Remove and discard the green bonding screw immediately. Neutral and ground must be isolated. N/A (Discard screw). Ensure neutral wires are on the floating bar, ground wires on the chassis-mounted bar.
Subpanel (Missing a dedicated ground bar) Ground wires are illegally landed on the neutral bar Install a separate, isolated equipment grounding bar directly to the metal enclosure. Siemens ECGSB1 (Copper, 14 positions) or Eaton GBK2 (Copper, 14 positions).

How to Verify Your Panel Bonding with a Multimeter

You do not always need to pull the panel cover to verify if a subpanel is illegally bonded. You can diagnose parallel neutral paths from a standard receptacle using a digital multimeter (DMM) and a plug-in load.

  1. Establish a Load: Plug a high-draw resistive load (like a 1500W space heater or a hair dryer) into the receptacle fed by the subpanel you want to test. Turn it on to draw ~12 Amps.
  2. Set the DMM: Set your multimeter to AC Volts (V~).
  3. Measure Hot-to-Neutral: Insert probes into the hot (short slot) and neutral (long slot). You should read roughly 118V to 120V.
  4. Measure Neutral-to-Ground: Move the red probe to the ground slot (the U-shape). Keep the black probe in the neutral slot.
  5. Interpret the Reading:
    • 1.0V to 3.0V (Correct for Subpanel): This is the voltage drop across the neutral wire caused by the 12A load. Because the ground wire carries zero normal current, there is no voltage drop on it. The difference proves they are isolated at the subpanel.
    • 0.0V to 0.2V (Illegal Subpanel Bond): If the reading is near zero under load, the neutral current is splitting onto the ground wire. The subpanel is illegally bonded, creating parallel paths and eliminating the voltage differential.
    • < 1.0V (Correct for Main Panel): At the main panel, the neutral and ground are bonded, so the voltage difference between them at a nearby receptacle will be virtually zero, even under load.
Bench Tip: If your multimeter reads 0.0V N-G at a subpanel receptacle, but the space heater is unplugged, you are getting a false pass. The voltage drop only exists when current is actively flowing on the neutral wire. Always test under a minimum 10A load.

Sizing the Main Bonding Jumper and Picking the Right Hardware

If you are replacing a missing main bonding jumper in an older main service panel, you cannot simply use any piece of copper wire. The National Electrical Code (NEC Table 250.102(C)(1)) mandates specific sizing based on the size of your largest ungrounded (hot) service entrance conductors.

For a standard modern residential service:

  • 200-Amp Service (2/0 AWG Copper Hot Wires): The main bonding jumper must be at least 4 AWG copper or 2 AWG aluminum.
  • 200-Amp Service (4/0 AWG Aluminum Hot Wires): The main bonding jumper must be at least 2 AWG aluminum or 1/0 AWG copper.
  • 400-Amp Service (600 kcmil Copper Hot Wires): The main bonding jumper must be at least 1/0 AWG copper.

In modern load centers, this jumper is usually replaced by a green main bonding screw. This screw threads through the neutral bus bar and bites directly into the grounded metal enclosure of the panel. Do not substitute this with a standard zinc-plated wood screw or drywall screw; the threads (usually #10-32 or #12-24) and the corrosion-resistant green coating are specifically engineered to maintain a low-impedance fault path over decades of thermal expansion and contraction. If your panel manufacturer specifies a part number (like the Square D HOM15GFI bonding kit or Siemens EQGN95), use the exact OEM part.

When to Call a Licensed Electrician (and When You Can DIY)

Knowing your limits keeps you alive and keeps your home insurable. Here is the definitive breakdown of when to pick up the tools and when to pick up the phone.

You can DIY this if:

  • You are adding an isolated equipment grounding bar (like the Siemens ECGSB1) to an existing, de-energized subpanel.
  • You are removing an illegally installed green bonding screw from a subpanel neutral bar.
  • You are performing the multimeter N-G voltage drop test at receptacles.

You must hire a licensed electrician if:

  • The main bonding jumper is missing in your main service panel and you are unsure how to safely de-energize the bus to install it.
  • You need to upgrade your service entrance conductors or pull the utility meter to safely work on the line-side of the main breaker.
  • Your panel is an older Federal Pacific (FPE) or Zinsco panel. These panels have known, documented failure modes where the bus stabs fail to grip breakers, causing fires. Do not attempt to modify the internal bus bars; the entire panel requires replacement by a professional.

Electrical panel bonding is not a matter of preference; it is the fundamental physics of fault clearing. Ensure your main panel is bonded, ensure your subpanels are isolated, and always verify your work with a meter under load before closing the cover. For further reading on residential electrical safety standards, consult the National Fire Protection Association (NFPA) electrical safety guidelines and review manufacturer-specific grounding FAQs from major panel builders like Schneider Electric (Square D). For workplace and general shock prevention data, the OSHA electrical safety standards provide excellent baseline metrics on touch potential hazards.