You've hit on a very common point of confusion in electrical work, especially when dealing with subpanels. Here's a breakdown to clarify what's happening and why running separate neutrals and grounds to a subpanel is crucial

Understanding the Main Panel (Service Entrance):
Bonding

At the main service panel, the neutral bus

is bonded to the ground bus and the metal enclosure. This creates a single point where neutral and ground are electrically connected. This is only permitted at the service entrance. Why bond at the service entrance? This bonding is essential for safety. It ensures that in the event of a fault (e.g., a hot wire touching the metal enclosure), a low-resistance path to ground is created. This allows a large fault current to flow, tripping the circuit breaker and quickly de-energizing the circuit, preventing a dangerous shock hazard.

Why Separate Neutrals and Grounds at a Subpanel? Preventing Neutral Current on Ground Wires

The primary reason for separating neutral and ground in a subpanel is to prevent neutral current from flowing on the ground wires.

How neutral current gets on ground

If you were to bond the neutral and ground buses in a subpanel, neutral current (which is the return path for the hot wire) would have multiple paths back to the source (the main panel). Some of that current would flow through the ground wire, which is unintended.

Why is neutral current on ground bad? Shock Hazard

Ground wires are meant to be at or near 0 volts relative to earth. If they carry neutral current, they'll have a voltage potential. This can create a shock hazard if someone touches a grounded appliance or enclosure and a path to earth.

Electromagnetic Interference (EMI)

Neutral current on ground wires can create unwanted EMI, potentially interfering with sensitive electronic equipment.

False Readings

It can cause misleading readings when testing electrical circuits.

Dedicated Ground Path

By running a separate ground wire (Equipment Grounding Conductor or EGC) to the subpanel, you create a dedicated, low-impedance path for fault current to flow back to the main panel. This ensures that in the event of a fault, the breaker will trip quickly, protecting people and equipment.

What You Need to Do in Your Friend's Installation


Inspect the Main Panel



Verify Bonding

Confirm that the neutral and ground buses are bonded together only in the main panel. This is usually accomplished by a bonding screw or strap that connects the neutral bus to the metal enclosure.

Identify the Bonding Screw/Strap

Make sure you can clearly identify the bonding screw or strap in the main panel.2.

At the Subpanel



Separate Neutrals and Grounds

The neutral bus in the subpanel must not be bonded to the enclosure. Remove any bonding screw or strap that connects the neutral bus to the subpanel enclosure. The neutral bus should be isolated.

Connect Neutral Wire

Connect the neutral wire from the main panel to the neutral bus in the subpanel.

Connect Ground Wire

Connect the ground wire (EGC) from the main panel to the ground bus in the subpanel. The ground bus in the subpanel should be bonded to the subpanel enclosure.

Correcting the Main Panel (If Needed)

If, as you say, the neutral and ground are landed on each other's bars, you will need to correct this at the main panel after the subpanel is correctly wired. Move all the neutral wires to the neutral bus. Move all the ground wires to the ground bus.The only connection between the neutral and ground should be the bonding screw/strap.

Important Notes



NEC (National Electrical Code)

All of this is based on the NEC. Consult the NEC (or your local electrical code) for specific requirements and regulations in your area.

Local Inspectors

Electrical codes vary by location. It's highly recommended to consult with your local electrical inspector or a licensed electrician to ensure your installation complies with all applicable codes.*

Safety First: Electrical work can be dangerous. If you're not comfortable or confident, hire a qualified electrician.In summary: bonding is permitted only at the main panel. The subpanel must have separate neutral and ground paths. Flag for review