Why Must Neutral and Ground Wires Be Bonded in the Main Panel?

Electrical Technology

Why Do Neutral and Ground Conductors Need to Be Bonded in the Main Panel?

According to NEC Article 250, both the neutral and ground wires must be connected only in the main panel or at the first service disconnect. They should never be connected together downstream of the service equipment, such as in subpanels or other parts of the circuits. This practice is essential for maintaining safety and ensuring the proper functioning of electrical circuits.

The ground busbar terminal in the service equipment (main panel) should be securely connected to the grounding rod using a properly sized equipment grounding conductor, as specified in NEC Table 250.122.

We understand that neutral and ground wires must be connected only at the main service panel. But why is this required? What is the reasoning behind this strategy? Let’s explore what happens if the grounding and grounded conductors are not connected in the main panel.

Before diving into the details, we must understand the roles of neutral and ground wires in a power and distribution system.

Neutral Wire: The neutral wire completes the electrical circuit and provides a return path for current back to the power source. It carries current under normal operating conditions and is usually insulated.
Ground Wire: The ground wire is a safety conductor that provides a path to the earth, or “ground,” to prevent electric shock. It does not carry current during normal operation but is designed to carry fault current in the event of a ground fault.

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Why Do We Need to Bond the Ground and Neutral in the Main Panel?

Consider the following properly grounded and bonded main and subpanel in accordance with NEC 250. The ground and neutral wires must always be bonded in the main panel or service disconnect and kept separate in the subpanel, as shown in the figure below.

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During normal operation, current flows through the hot wire and returns via the neutral wire to the transformer. The circuit is complete and functioning as intended; hence, the light is glowing, as shown in the figure below.

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If there is no subpanel used, the circuit will look like this:

Now, Suppose a hot (live) wire comes into contact with the metallic bulb holder. Since there is no load during this ground fault condition, an instantaneous surge in current will cause the breaker to trip, disconnecting the circuit.

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The case will be similar without a subpanel used in the premises.

In the next section, consider a dangerous scenario in the case of a ground fault when there is no bonding between the neutral and ground in the main panel or load center.

Safety and Fault Current Pathway: The bonding provides a path for fault currents to return to the main panel and ultimately to the utility transformer. In the event of a short circuit or ground fault, this direct path allows the excess current to flow back through the neutral-ground bond, enabling the circuit breaker to trip and stop the flow of current, preventing electric shock or fire.
Completes the Electrical Circuit: In normal operation, the neutral wire completes the electrical circuit by carrying return current back to the source. Grounding the neutral at the main panel ensures that this return path remains at a stable voltage relative to the earth, reducing the risk of electrical shock.
Prevents “Floating” Neutral Voltage: Without bonding, a floating or “unreferenced” neutral can occur, which may lead to unpredictable voltages and cause damage to sensitive equipment. This may also cause the unwanted tripping of the AFCI or GFCI breakers and associated circuits. By grounding the neutral at the main panel, it stabilizes the voltage and prevents fluctuations.

This is why the neutral and ground must be bonded in the main panel or main disconnect.

Now, If neutral and ground conductors are not bonded in the main panel, several safety risks arise. See the following scenario for clarification.

What Happens If You Do not Bond Neutral and Ground in the Main Panel?

Consider a scenario where there is no bonding between neutral and ground in the main panel. In a similar ground fault condition shown in the above fig, the fault current returns through the ground conductor to the ground terminal busbar in the main panel. Without a connection between the neutral and ground conductors, the circuit cannot be completed.

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In this case, the only available path for the fault current is through the grounding rod. The fault current will flow through the grounding conductor to the ground rod and then to the transformer.

Here, a fault current of around 4 to 10 amps may flow in the circuit. This fault current could end up flowing through the grounding conductor, creating a potential hazard.

In a short-circuit ground fault, there is no load resistance. Assuming the ground path has 30Ω of resistance, the amount of current flowing back to the supply source forms a series circuit:

I = V ÷ R
I = 120V ÷ 30Ω
I = 4 Amp

However, in most cases, we use a 60-amp breaker for subpanels and 15 to 20-amp breakers for household appliances. This 4 to 10-amp fault current may not be enough to trip a standard 15-20 amp breaker in the main or subpanels.

Additionally, since the metal enclosure of the equipment is bonded to the grounding terminal busbar, the grounding conductor carries the fault current. As a result, all metal components connected to the ground wire, such as metal raceways, the equipment’s metallic body, and other connected devices, become electrified with 120V.

If, V_In = supply voltage, V_Out = Electrified Voltage in the Circuit, R₁ = Load Resistance, R₂ = Ground Path, then: resistance back to the transformer

V_Out = (V_In × R₂) ÷ (R₁ + R₂)
V_Out = (120V × 30Ω) ÷ (20Ω + 30Ω)
V_Out = (120V × 30Ω) ÷ (0Ω + 30Ω)
V_Out = (3,600) ÷ (30Ω)
V_Out = 120V

To ensure the breaker trips in such a scenario, even with a small current differential, we need GFCI (Ground Fault Circuit Interrupter) circuit breakers.

If we do not connect the neutral and ground at the first main entrance, several potential issues could arise, compromising safety and system performance.

Dangerous Electrical Shock Risk: Without the bond, fault current cannot flow back to the panel and to ground effectively, leaving conductive surfaces like metal appliances or equipment housings live if a fault occurs. This can lead to severe shock hazards if someone touches the exposed metal.
Unreliable Path for Fault Current: The absence of a neutral-ground bond prevents the fault current from taking a low-resistance path back to the source (utility transformer), which is essential for tripping circuit breakers in case of a ground fault. Without this path, breakers may not trip, allowing fault currents to flow unchecked, potentially causing overheating and fire hazards.
Potential for “Floating Neutral” Issues: Without bonding at the main panel, the neutral wire might have unpredictable voltages relative to the ground, causing “floating neutral” conditions. This instability can lead to erratic voltage behavior in circuits, damaging sensitive electronics or causing unexpected tripping.
Risk of Equipment Damage: Some electrical devices, especially sensitive electronics, rely on a stable, grounded neutral for proper operation. A floating or improperly grounded neutral can cause voltage spikes or drops, leading to equipment malfunction or permanent damage.
Violation of Electrical Code: The National Electrical Code (NEC) mandates bonding of neutral and ground at the main panel specifically. Not following this guideline is a code violation and can result in failed inspections, increased liability, and possible legal or insurance issues if electrical faults cause damage or injury.

Therefore, if the ground and neutral wires are not bonded in the main panel during a ground fault:

The ground wire carries electric current, posing potential hazards.
The metallic parts become energized, creating a risk of electric shock.
The standard circuit breaker may not trip properly.

Why is Bonding in Main Panel or Service Disconnect Necessary?

Bonding the ground and neutral in the main panel is mandatory to create a safe and reliable electrical system. Here’s why this bonding is necessary:

Establishes a Low-Resistance Fault Path: Bonding ground and neutral at the main panel provides a direct path for fault currents to return to the source (utility transformer). This low-resistance path allows circuit breakers to detect the fault quickly and trip, cutting off the flow of electricity and preventing hazards like electrical shock or fire.
Stabilizes System Voltage: The ground-neutral bond ensures that the neutral wire is at or near the same voltage as the earth (ground), which stabilizes the voltage levels in the system. This consistency prevents floating or fluctuating voltages that could cause damage to devices or pose a risk of electric shock.
Prevents Parallel Current Paths: By bonding only at the main panel, the neutral and ground are correctly referenced, and current is prevented from traveling along unintended paths. This reduces the risk of stray currents running through metal pipes, framing, or other conductive building parts, which could lead to shock hazards.
Facilitates Overcurrent Protection: The bond at the main panel provides a clear path for ground faults, which is essential for circuit breakers to function correctly. Without the bond, fault currents could take unpredictable routes, preventing the breaker from sensing the excess current and tripping properly.
Meets Code Requirements for Safety: The National Electrical Code (NEC) mandates that ground and neutral be bonded at the main panel, but nowhere else in the system. This single bonding point establishes a reliable grounding system that meets safety standards and helps protect against electrical failures.

What is the Role of Bonding in the Main Service Disconnect?

Reasons for the Single Connection Point

The National Electrical Code (NEC) standards mandate that the neutral and ground wires must be bonded together only at the main service panel. There should be no other points of connection between the neutral and ground wires elsewhere in the system, such as in subpanels. This is to ensure a clear and controlled path for current, both under normal and fault conditions.

Prevention of Parallel Paths: If the ground and neutral are connected in multiple locations, they could create parallel paths for return current. This could lead to unexpected and unsafe current flow along the ground wire, increasing the risk of electric shock or fire.
Control of Fault Current: By bonding neutral and ground only in the main panel, it provides a controlled path for fault current to return directly to the source. This helps to ensure that the current flows safely back to the main panel and trips the breaker to isolate the fault.

Safety Aspects of Bonding in the Main Panel

Ground Fault Protection: When there’s a fault (e.g., a “hot” wire touches a metal appliance case), the ground wire provides a direct path for fault current. Because the ground and neutral wires are bonded at the main panel, fault current can safely return to the power source, allowing circuit breakers to trip and isolate the fault, protecting the equipment and preventing electric shock.
Voltage Stabilization: Bonding the neutral to the ground at a single point stabilizes the voltage in the system. This prevents voltage fluctuations in the neutral wire, which could otherwise cause equipment malfunctions, flickering lights, and potential shock hazards. It keeps the neutral at the same potential as the ground, effectively grounding the entire electrical system.
Reduction of Electromagnetic Interference (EMI): Multiple neutral-to-ground connections could create ground loops, resulting in undesirable currents that generate electromagnetic interference (EMI). This can affect sensitive electronic devices, degrade performance, and potentially damage components over time. By bonding only at the main panel, EMI risks are minimized, enhancing the stability and safety of the system.

NEC Standard Code Compliance

The NEC strictly mandate a single-point neutral-to-ground bonding at the main panel. This guideline:

Ensures that electrical installations follow standardized practices for safety.
Helps maintain a consistent safety protocol across different installations.
Prevents the risks associated with poor grounding, which can lead to equipment damage, fires, or electric shock.

Good to Know:

All metal parts, including metal raceways, panelboard cabinet, frames, equipment, should be properly grounded i.e. connected to the ground rod via the grounding conductor. NEC – 408.40.
The Ground wire should not carry any current back to the earth during normal conditions.
The main panel needs a dedicated neutral busbar terminal connected to the main neutral busbar located in the main panel.
The grounding electrode conductor shall be connected to the grounded (neutral) service conductor in the main panel or first service disconnect. NEC – 250.24(A).
Grounding conductors (Ground Wires) and grounded conductors (Neutral Wires) are not to be connected together anywhere on the load side of the main service disconnect, 250.24.
In a subpanel, only the grounding conductor is permitted to connect back to the the grounding terminal bar in the main panel. NEC – 408.40.
Bonding the neutral and ground in the subpanel may pose serious electric shock and fire risks for handymen or anyone working on the appliance. Ensure that they are separated in subpanel and only bonded in the main panel. If you are unsure, contact a licensed electrician to ensure the job is done correctly.

Why Not Bond Neutral and Ground in Subpanels?

We have posted a detailed post on “separation of neutral and ground wires in a subpanel” with associated risks shown in pictorial views.

Eliminates Parallel Ground Currents & Shock Hazards: If the neutral and ground were bonded in subpanels, current could inadvertently flow back through the ground wire, resulting in unpredictable circuit conditions and increased risk of electric shock.
Prevents Unwanted Tripping of (GFCIs): Ground-fault protection devices such as GFCIs are sensitive to leakage currents that should not exist on the grounding system. Bonding neutral and ground in subpanels could create ground loops, which may falsely trip GFCIs or AFCIs and interrupt the circuit without a legitimate fault.

How the Neutral-Ground Bond Prevents Electrical Hazards

In cases where there is a fault—such as insulation failure or accidental contact with conductive surfaces—the ground wire provides a path for excess current. With the neutral and ground bonded at the main panel:

The current from a fault is directed back to the main panel and to the earth, tripping the breaker and preventing electrical shock.
Electrical “noise” and stray voltages are reduced, ensuring system stability and reducing equipment wear.

Installation Best Practices for Bonding Neutral and Ground

Use Proper Bonding Jumpers: Ensure the bonding connection in the main panel is made with an appropriate bonding jumper. This guarantees a secure and low-impedance connection between the neutral bus and the ground bus.
Avoid Bonding in Subpanels: Subpanels should have an isolated neutral bus to prevent accidental bonding. This avoids accidental creation of multiple bonding points, which can lead to ground loops and safety issues.
Check Continuity of Grounding System: A consistent and continuous ground is essential to maintain the safety provided by the neutral-ground bond. Periodically inspect and ensure all connections remain intact and free from corrosion or damage.

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