Okay, this is a classic and frustrating stray voltage problem. You've already done some excellent troubleshooting steps. Here's a breakdown of the likely culprits and some more suggestions beyond pulling new circuits (although that might be the best long-term solution)

Understanding the Problem:
Induced Voltage (Capacitive/Inductive Coupling)

The most probable cause is induced voltage from the bundled Romex cables. The magnetic field produced by current flowing in one conductor can induce a voltage in nearby conductors, especially when bundled tightly together. This is often referred to as capacitive coupling. The long runs of bundled cables act like a large capacitor.

Ground Potential Difference

Even with the power disconnected, a small voltage can appear on the ground due to potential differences. This can be from various sources, including ground loops, nearby electrical equipment, or even differences in soil conductivity.

Golf Cart Chargers

These are a major suspect. Many golf cart chargers are poorly filtered and can generate significant electrical noise and harmonics that can induce voltage in nearby wiring.

Troubleshooting and Solutions (Beyond Pulling New Wire Immediately)


Confirm the Source

You've isolated the problem by disconnecting everything, but let's confirm the bundling is the culprit:

Gradual Disconnection

If possible (and safe), carefully start separating the bundled Romex cables, starting with the ones closest to your thermostat cable and the 6/2 circuit. Check the voltage after each separation. This will help pinpoint which cables are the primary source of the induced voltage.

Measure Current in Ground Wires (Carefully!)

Use a clamp meter capable of measuring very low AC current (milliAmps, mA) on the ground wires (both the branch circuit and the thermostat cable if you re-attach it). This could confirm induced voltage is being passed to ground. Be extremely careful when working with ground wires. Never disconnect a ground without ensuring an alternate ground path is established.



Check for Harmonic Distortion

An electrician with an advanced multimeter or power quality analyzer can check for harmonic distortion in the electrical system. Excessive harmonics can exacerbate induced voltage problems.2.

Mitigation Techniques (If Pulling Wire is Difficult/Temporary)



Shielded Thermostat Cable

This is the easiest first step. Replace the existing thermostat cable with shielded cable. The shield must be properly grounded at ONE END ONLY. Grounding it at both ends can create a ground loop, making things worse. Often grounding at the air handler is easiest.

Grounding Improvement



Dedicated Grounding Conductor

Run a dedicated, insulated ground wire (green) from the air handler back to the main electrical panel. This provides a cleaner ground path and can help reduce ground potential differences.

Check Grounding Rods

Ensure the main electrical panel has proper grounding rods, and that the connections are clean and tight.

Supplemental Grounding

Consider adding supplemental grounding rods closer to the golf cart chargers if possible. This can help sink some of the noise they generate.

Ferrite Beads/Chokes

Install ferrite beads or chokes on the thermostat cable and/or the power cable to the air handler. These can help filter out high-frequency noise that contributes to induced voltage.

Twisted Pair Thermostat Cable

This can also help to reduce the induced voltage. The twisting helps to cancel out the magnetic fields.3.

Addressing the Golf Cart Chargers (Long-Term)



Filter the Chargers

If possible, investigate the possibility of installing power line filters on the golf cart chargers. This is the most effective way to eliminate noise at the source. Work with a qualified electrician and the golf cart charger manufacturer.

Dedicated Circuits

Ensure each golf cart charger has its own dedicated circuit. This helps prevent overloading and reduces the amount of current flowing through shared circuits.

Safety Considerations



Electricity is Dangerous! If you're not comfortable working with electricity, hire a qualified electrician.

De-energize Circuits

Always turn off the power at the breaker before working on any electrical wiring. Verify the circuit is de-energized with a non-contact voltage tester.

Grounding is Critical

Never disconnect a ground wire without ensuring an alternate ground path is established.

PPE

Wear appropriate personal protective equipment (PPE), such as insulated gloves and safety glasses.

Why Pulling New Wire is Often the Best Solution (Long-Term)



Eliminates the Root Cause

Moving the thermostat cable and power circuit away from the bundled Romex cables directly addresses the problem of induced voltage.

More Reliable

Mitigation techniques can sometimes be effective, but they don't always eliminate the problem completely. Pulling new wire provides a more reliable solution.

Future-Proofing

You can run the new wire in conduit to provide additional shielding and protection.

In summary: Your first step should be to try the shielded thermostat cable, and then run a dedicated ground. If that doesn’t fix the problem, pulling a new circuit and thermostat wire would be the best permanent fix. But, also address the golf cart chargers. Good luck! Flag for review