You've uncovered a classic, and potentially dangerous, wiring problem. Here's a breakdown of what's likely happening and why it's problematic

Understanding the Problem: Bootlegged Neutral and Shared Neutrals
Bootlegged Neutral

This is where a neutral is connected to a grounding conductor (equipment ground) to create a return path for current. This is dangerous because the equipment ground is not designed to carry normal circuit current.

Shared Neutral

This is where two or more circuits share the same neutral conductor. This can be done properly under specific conditions (multi-wire branch circuits) to save on wiring but must be done carefully to avoid overloading the neutral.

Unbonded Neutral Bar

Your added neutral bar must be bonded to the main service neutral. If it's not, it is not a safe neutral point. The main neutral bar is the only place that's directly connected to the utility neutral, and therefore the only place where a neutral connection provides a low-impedance return path back to the source.

How Your Circuits are (Likely) Working - The Dangerous Path1. Improper Neutral Connection

Since your added neutral bar isn't bonded, the only way circuits connected to it can work is if they are finding a return path through one (or a combination) of these dangerous means:

Grounding conductors (the most dangerous): This is the most likely scenario. The circuits on your added neutral bar are finding a return path through the equipment grounding conductors, back to the panel, and then relying on a bootlegged neutral to complete the path back to the source. This means that under normal operation, current is flowing on the equipment ground. Shared neutral with another circuit

You could have circuits on the original neutral bar that share a neutral at some point in the house (intentional or unintentional).

Other unknown connections: It's possible that there are other unintentional connections between the added neutral bar and the main neutral somewhere in the house.2. Voltage Drop and Ground Faults

This situation creates several problems:

Elevated Voltage on Equipment Grounds

Any current flowing through the grounding conductors will cause a voltage drop along the wire. This means that the metal enclosures of your appliances and equipment are no longer at true ground potential. You could get a shock if you touch a faulty appliance and a grounded object.

Overloaded Neutrals

Shared neutrals (intentional or not) can become overloaded, leading to overheating, insulation damage, and fire hazards.

GFCI Issues

GFCIs work by detecting an imbalance between the current flowing through the hot and neutral conductors. If the neutral current is returning through a path other than the neutral wire, the GFCI will trip. This is why your new GFCI breaker might be tripping or not working as expected.

Inaccurate Voltage Readings

You may get incorrect voltage readings at outlets and lights.

Why the Bonding Jumper is Critical

A properly installed bonding jumper between your added neutral bar and the main service neutral bar will: Establish a Safe Return Path

Provide a direct, low-impedance path for neutral current back to the source.

Equalize Potential

Ensure that the added neutral bar is at the same potential as the service neutral, preventing voltage differences that can cause shocks.

Enable GFCI Protection

Allow GFCIs to function correctly by ensuring that the neutral current flows only through the intended path.

Immediate Actions and Troubleshooting1. Safety First

This is a serious safety hazard. Proceed with extreme caution. If you are not completely comfortable working with electrical systems, stop and call a licensed electrician.2.

Install the Bonding Jumper

This is your immediate priority. Connect a wire-type bonding jumper (sized appropriately for the service) between the added neutral bar and the main service neutral bar.3.

Verify Voltage

After installing the bonding jumper, check the voltage between the added neutral bar and a known good ground (like the metal enclosure of the panel, properly grounded). It should be very close to zero volts.4.

Trace Circuits

Once the immediate safety issue is addressed, systematically trace the circuits connected to the added neutral bar. Identify where they originate and where they run. Look for any evidence of shared neutrals or connections to grounding conductors.5.

Inspect Junction Boxes

Open all junction boxes along the affected circuits and carefully inspect the wiring for any signs of damage, loose connections, or improper wiring practices.6.

Professional Help

Given the complexity of the problem, I strongly recommend consulting with a licensed electrician. They can perform a comprehensive inspection of your wiring and identify any hidden issues. They can also correct any wiring errors to ensure your electrical system is safe and code-compliant.

Important Considerations NEC Compliance

Electrical work must comply with the National Electrical Code (NEC) and any local codes.

Grounding and Bonding

Understanding the principles of grounding and bonding is essential for safe and effective electrical work.

Testing

Use appropriate testing equipment, such as a multimeter, to verify voltage levels and continuity.

In summary, your circuits are likely working in a dangerous and potentially fatal way. Install the bonding jumper immediately and then systematically troubleshoot the wiring to identify and correct any improper connections. Flag for review