When I went to add a light switch near my attic access for an existing light with a pull cord, I found that the light was wired to the disconnect box that feeds the air handler. This particular circuit is fed by a 60A breaker in my main panel (I assume it is so large due to the emergency heater if the heat pump is not working).
The light is wired with a 10 gauge cable, but this just seems really wrong to me. I suspect this was put in by the AC guys before we owned it since it was attached to a truss with hex head screws. It also didn't even have a junction box! The fixture was directly attached to the truss with enough clearance to wire it from the back. The terminals were exposed to open air and someone could have have touched a live 60A circuit. These are the same AC guys that cut 3 truss members so I don't have a lot of faith in what they did.
I have fixed the junction box situation, but before I add a switch to this I wanted to confirm that I need to move this to another circuit. Maybe there is another breaker inside the disconnect box that is only 15/20A? I didn't see anything…
This light fixture also has a receptacle built into it. Like this:
Update
I looked a little further into it and the neutral for the light is connected to the ground and it measures 120V at the receptacle. There is what looks like a 2 gauge aluminum coming in and a 6 gauge aluminum going out to the heater/ air handler, plus the 10 gauge copper going to the light/ receptacle. There is no separate 15A for the air handler. Side note, why does the tester show hot/ground reversed for this situation? My pictures don't stay rotated properly on the phone app…
Best Answer
This is an unmitigated disaster
Firstly, 60A auxiliary heater circuits are normal enough, but they don't have neutral. The wires are L1-L2-ground or black-red-bare. There is no way to get 120V out of that circuit, except by bootlegging neutral.
Bootlegging neutrl means misusing ground as neutral, intentionally connecting a "hot" wire (via a light bulb or tool) to the ground. If the ground-neutral bond in your panel has any sort of problem, it will electrify every ground in your house. Bad idea.
Nothing prevents someone from plugging in a power strip and four 1500W heaters. That will overheat the wiring and start a fire.
This is terrible work!
Must be more going on
Thing with heat pumps is there are usually other circuits involved. I am surprised that the air handler would share the 60A aux heat circuit, I would expect there to be a 15-20A circuit there for the air handler. I would think it might also be 120V. If so, it may be possible to borrow it for the attic light. The attic light must be on a 15 or 20A 120V circuit. It cannot use part of a 120/240 circuit.
The total of all hard-wired loads on a circuit must be less than 80% of total circuit capacity.
You cannot have any receptacles on a circuit unless the hard-wired loads are less than 50% of total circuit capacity. So you may need to change lamp fixtures.
Or, go 24V lighting
In this day and age, LEDs are so efficient that 5W of lighting is a fair bit, and two would light up an attic decently enough. Most furnaces have a 24 volt AC, 40VA (similar to watts) transformer with surplus capacity. You could grab 24VAC off the transformer and take it to LED lights. You would need to find LED fixtures that have no replaceable bulb, and work a range of voltages including 24V AC. You can use common thermostat wire to hook them up. Aim for an even number of lights, and hook each one reverse polarity of the next. Sometimes LEDs will tolerate AC but only light on half the AC cycle, by reversing polarity to every other one, you assure the full cycle is used.