Trying to understand wiring to old heat lamp in bathroom

Question

I am replacing a very old heat lamp in a bathroom ceiling with a fan/light/nightlight.

The house is over 50 years old but the heat lamp may have been added in the 80's, when a previous owner did some remodeling. The original house wiring did not use junction boxes in the attic or at light fixtures, but there is one junction box in the attic above this fixture.

I am trying to understand how that wiring worked.

There is 12-3 romex running from the electrical panel into that junction box above the old light.
There are three 12-2 romex lines coming out of that junction box: One 12-2 runs to the switch that controlled the heat light. That line is controlled by breaker 10 in the panel. I understand that. A second 12-2 runs from the junction box directly to the heat lamp. It is also controlled by circuit breaker 10. The third 12-2 runs out from the junction box to a different bathroom. That line is controlled by breaker 12 in the panel.

I am assuming that 10 and 12 share the 12-3 at the electrical panel.

I do not understand why one wire fed the light directly while the other ran through the switch. I would be grateful if someone could explain to me how that works. How does the switch cut the power when there is a second line running directly into the fixture that is always on?

The 2x6 is charred where the heat lamp was attached (4 or 5 inches). Is that likely to have been caused by the heat from the fixture or the wiring? The white romex jackets close to the fixture are browned.

What should be done with the line that ran directly from the junction box to the heat lamp? Should the junction box be rewired so that it can be removed?

I am planning to call an electrician, but I want to understand it before I get an estimate.

Answer 1

Since you are on breakers 10 & 12 it is probably a multiwire branch circuit sharing the neutral, today's code requires handle ties if this is the case. Verify the hot feeding the light and switch. A noncontact tester may show a hot line or a phantom induced voltage. Depending on the wiring it could be a switched neutral this would be a code violation and is common in home owners doing there own wiring. The correct way would be to take the black wire that feeds the light and connect it to the black that goes to the switch, in the switch box the white wire that comes in with the black needs to be marked with paint or electrical tape a color other than white or green. The white in the light on the same cable feeding the switch needs to me marked also. Heat lamps usually take special fixtures that have high temp wires and many of the heat lamp fixtures have a maximum wattage of 300w some much less. Excess wattage lamps can be the cause for the discoloration of the insulation. The wiring needs to be evaluated and possibly repaired with proper junction boxes added. It would be a good idea to add handle ties to breaker 10-12 if it is a multiwire branch circuit. They may not have been required when the home was wired but are a good safety feature now. This or replacing the 2 breakers with a double pole breaker with 1 handle.

Answer 2

"I do not understand why one wire fed the light directly while the other ran through the switch."

The cable from the junction box to the switch carries the hot voltage both ways, and does not include a neutral. One wire -- properly the white -- carries the switched hot back to the junction box, whence it is passed through to the heat lamp. You should see the white from the switch nutted to the black to the lamp.

(White wires used for other than neutral must be marked at both ends with colored paint or tape.)