This is indeed a perplexing situation, but here's a breakdown of possible explanations and things to consider

Possible Explanations:

1. Shared Neutral / Multi-Wire Branch Circuit Gone Wrong (Highly Likely)

Even though the circuit is labeled as a single circuit, the symptoms strongly suggest a failed attempt or misconfiguration of a multi-wire branch circuit (MWBC). Here's why:

Two Hots, Same Circuit Breaker

The fact that both the red/black and purple wires go dead when you shut off one breaker indicates they're fed from the same phase. In a properly wired MWBC, they should be on different phases (120 degrees apart in a 3-phase system, 180 degrees apart in a split-phase system). This is critical for the neutral to function correctly.

Zero Volts Between the Hots

This confirms that the wires are on the same phase.

Shared Neutral Overload

This is the most dangerous consequence. The white (neutral) wire is likely overloaded. In a properly working MWBC, the neutral only carries the difference in current between the two hot legs. With both hot legs on the same phase, the neutral will carry all the current of both circuits. This is a fire hazard.2.

Failed 3-Way/4-Way Lighting Circuit

Less likely, but possible if someone repurposed the wiring and didn't remove the unused conductors. In this scenario, purple could have been a traveler wire in an old lighting configuration.3.

Old Control Circuit

Possibly, the purple wire was used for some kind of control circuit that is now defunct. However, the presence of 120V on it makes this less likely without more context.4.

Mis-Wiring/Accidental Parallel Wiring

It's possible that the wiring was done incorrectly, and the purple wire is essentially wired in parallel with the red or black wire. This is a mistake and not an intentional configuration.5.

Specialized System (Less Likely)

There's a very slim chance that this is part of some old, specialized system that used a unique wiring configuration. But without more context, this is difficult to assess.

Why the "Future Modular Furniture" Label Doesn't Necessarily Matter

Labels can be misleading or outdated. What the circuit was intended for and what it's actually wired for can be different. The original intention might have been for a proper MWBC to serve modular furniture, but it was wired incorrectly.

Troubleshooting Steps (Proceed with Extreme Caution)


DO NOT USE THESE CIRCUITS UNTIL THE PROBLEM IS RESOLVED. The overloaded neutral is a serious fire hazard.2. Take the Dead Front Off the Panel (Safety First)

This is essential. If the screws are seized, try using penetrating oil or a screw extractor.

Ensure the main breaker is OFF before working inside the panel. Wear appropriate PPE (insulated gloves, safety glasses).3. Trace the Wires in the Panel

Carefully trace the red/black and purple wires back to their respective breakers.

Are the red/black and purple wires connected to the same breaker? This is the most likely scenario. Are there two breakers tied together with a handle tie (breaker tie)? This could be a way of using one breaker for two hots on the same phase, but it's still wrong for an MWBC.4. Inspect the Neutral Bus Bar

Look for any signs of overheating on the neutral bus bar, especially around the neutral wires associated with these circuits. Discoloration or melting indicates a potential problem.5.

Map Out the Circuit Completely

You need to understand where everything is connected. Start at the panel and follow the wires to each receptacle, light fixture, or junction box connected to this circuit. Document your findings.6.

Consider Calling an Electrician

If you are uncomfortable working inside the panel or diagnosing the problem, it is best to call a qualified electrician. Electrical work can be dangerous.

What to Look for in the Panel (Assuming You Get the Dead Front Off)



Missing or Incorrectly Sized Neutral

The shared neutral may be too small of a gauge for the load.

Multiple Neutrals on a Single Terminal

If the neutral bus bar is crowded, someone might have tried to put multiple neutral wires on a single terminal, which is often against code.

Loose Connections

Check all connections at the breakers and neutral bus bar to ensure they are tight.*

Signs of Overheating

Discoloration, melting, or burning around the breakers, wires, or bus bars.

In Conclusion:

Based on your description, a miswired multi-wire branch circuit is the most probable cause. Correcting this will require identifying the source of the problem in the panel, ensuring the wires are properly phased, and that the neutral is properly sized and connected. Calling a qualified electrician would be best in this scenario. Flag for review