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Why are Neutral and Ground Wires Separated in a Subpanel?

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Why Do Neutral and Ground Conductors Need to Be Separated in a Subpanel?

According to NEC Article 250, neutral and ground wires must remain separate in subpanels. Bonding (connecting) the neutral and ground should only occur in the main panel or at the first service disconnect. In a service equipment (main panel) and remote distribution panel (subpanel), the ground terminal must be connected to a ground rod using an equipment grounding conductor. If the subpanel is installed on the same premises, you only need one ground rod with a properly sized grounding conductor, as specified in NEC Table 250.122. An additional ground rod is required if the subpanel is located in a separate building.

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.
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.

We know that neutral and ground should not be tied in the subpanel, but why? What is the reasoning behind this?

If neutral and ground conductors are bonded downstream of the main panel such as in a subpanel, several safety risks arise:

Table of Contents

Parallel Path for Return Current

If neutral is bonded to ground in the subpanel, return current from the neutral conductor may travel not only on the neutral wire but also on any bonded ground wires and metal parts of the main and subpanel’s enclosure. This parallel path for current can lead to:

- Shock Hazards: Metal parts bonded to ground (like conduits, enclosures, or equipment) can become energized, increasing the risk of electric shock. The Ground wire carries electric current to the main panel even during normal operation which poses electric shock hazards.
- Fire Hazards: Excessive current on the ground wire may overheat wiring and metal components, creating a fire risk.
- Interference with Grounding: The ground wire should carry current only in a fault condition. If it carries neutral current, it compromises the grounding system, reducing its effectiveness in clearing faults by tripping breakers and other protective devices.

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In case of ground faults on the double bonded panels, the equipment chassis will become energized with 120V (see below example). If someone touches the metal body, he may experience a mild or even serious electric shock.

See the next section below for the risks associated with parallel paths for return current.

False Sense of Safety

If a fault occurs in the subpanel, the breaker may not trip as expected due to return current traveling through unexpected paths (such as ground wires). This can delay the breaker’s response and leave the system energized, compromising the effectiveness of GFCI, AFCI, or RCBO devices.

Standard breakers may also fail to trip if fault current flows incorrectly through the ground to neutral. The ordinary 15 or 20-Amp breaker may fail to operate in case of 2 to 10-Amp fault current flowing through the ground conductor to the transformer and back to the panel.

Code Violations and Inspection Failures

The NEC mandates that neutral and ground remain separate in subpanels to ensure a dedicated path for neutral current and a separate fault ground path. Failure to comply can result in code violations and failed inspections. For further details, refer to NEC Article 250 (2023) by NFPA.

This is why it’s essential to separate ground and neutral in a subpanel.

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.

What Happens If You Bond Neutral and Ground in a Subpanel?

The neutral and ground should never be bonded together in a facility, except at the main panel. If this occurs, let’s examine what happens in double-bonded panels during ground faults and risks associated with parallel paths for return current.

Case 1: Hot-to-Ground Fault

In a ground fault condition where the hot wire accidentally touches a metallic component, fault current may return to the main panel via parallel paths through ground and neutral (since they’re bonded in the subpanel). As metal parts are connected to ground, all metal parts, including raceways and equipment, can become energized, leading to potential electric shock.

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The ground wire should hold fault current until it reaches the main panel, grounding any surges, lightning currents, or static charges as added protection. In a double-bonded panel setup, the ground wire carries current during normal operation. Hence, the safety wire compromises its role as a protective conductor and creating a risk of electric shock to the personnel.

If a ground fault occurs in double-bonded service equipment and remote distribution panels, the ground wire intended for safety now carries electric current, turning it into a hazardous conductor. It becomes a current-carrying wire with a potential risk of electric shock.

Additionally, the neutral carries unbalanced current, causing all circuits connected to the GFCI to trip immediately.

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

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

Hence, all the metallic parts of the circuits are electrified with 120V.

Good To Know: The Ground wire should not carry any current back to the earth during normal conditions.

Case 2: Touching Both Panels

If the subpanel is near the main panel and both are accidentally bonded, touching both panels could cause an electric shock due to parallel paths for return current as shown in the following figure.

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Case 3: Absence of Neutral in the Subpanel

In cases where the neutral is disconnected in the subpanel, the circuit might still complete by improperly returning through ground. This can allow appliances like lights to continue operating but creates an unsafe return path. Hence, it potentially triggers and trip GFCI or AFCI devices due to false current imbalances.

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Why Do We Need to Separate the Ground and Neutral in a Subpanel?

We need to separate the ground and neutral in a subpanel for the following critical safety and functional reasons:

Prevent Parallel Paths for Current

Neutral carries current, while ground is a safety path meant only for fault conditions (when something goes wrong, like a short circuit). If neutral and ground are bonded (connected) in the subpanel, neutral current can flow through the ground wire and conductive components (like metal enclosures or other grounding paths). This creates a parallel path for the neutral current, which:
- Increases the risk of electric shock because metal parts that should only be grounded could become energized.
- Can lead to overheating in the grounding conductors, which are not designed to carry regular current.

Maintain Ground as a Dedicated Safety System

The grounding system is designed to safely carry fault current back to the main panel, where the breaker can trip, protecting people and equipment.
If ground and neutral are connected in the subpanel, the ground may carry normal neutral current, so when a fault happens, the ground path might not work correctly. This could delay or prevent a breaker from tripping, leading to shock hazards or electrical fires.

Avoid Shock Hazards

If neutral and ground are bonded in the subpanel, any grounding conductor (like metal conduit or the subpanel enclosure) can carry live current. If someone touches those components, they could receive a dangerous electric shock.
The ground wire should only carry current in the case of a fault, but if it’s tied to neutral in the subpanel, it could have live current on it all the time, making normally safe components unexpectedly dangerous.

Prevent Electromagnetic Interference

With ground and neutral separated, all normal current flows only through the neutral conductor. This minimizes electromagnetic interference (EMI) with other sensitive electrical equipment.
If ground and neutral are bonded in the subpanel, stray currents can flow through unintended paths (ground wires, building steel, or plumbing), which can cause voltage fluctuations and interference in electronics or communication systems.

Code Compliance and System Design

The National Electrical Code (NEC) requires neutral and ground to be separated in subpanels to ensure each system works independently. In the main panel, neutral and ground are bonded together, but in any downstream subpanel, they must be separated.
This is a fundamental aspect of electrical system design to ensure safety and proper function of overcurrent protection devices (breakers and fuses). Failing to separate them would violate code and could result in inspection failure or fines.

Ensure Proper Function of GFCIs and AFCIs

Ground Fault Circuit Interrupters (GFCIs) and Arc Fault Circuit Interrupters (AFCIs) work by detecting imbalances or faults in the neutral and ground wires. If neutral and ground are bonded in a subpanel, these devices might not function properly, leading to failure in protection against electrical shocks and arcs, which are critical for safety in modern installations.

How to Separate Neutral and Ground in a Subpanel

Many subpanels come with a green bonding screw or strap that connects the neutral bus to the panel’s metal frame. Remove this bonding screw or strap so that the neutral is not bonded to the panel’s metal enclosure.
If there is a bonding jumper between the Neutral and Ground, just remove the screw of the strap to separate them.
The neutral bus bar, where neutral (white) wires from branch circuits connect, should be isolated from the panel’s metal enclosure.
Install a separate grounding bus bar if needed, and connect the neutral and grounding wires from the main panel to the isolated neutral bus and grounding bus in the subpanel.

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.

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Why are Neutral and Ground Wires Separated in a Subpanel?