[antipode12]

Kiss, another expert here, can actually show the formulas to calculate this voltage drop using real values and show exactly how it works mathematically using Ohms Law.

I don't have the patience to do it.

That's OK because I don't have the mind to understand it.

It's good enough to know that it's a legitimate possibility.

Why you want to get this fixed besides not having use of the dehumidifier is that a bad connection under these conditions can be what is called a "glowing connection", where there is heat like this there can be fire.

So discontinue any use of this circuit until it can get repaired.

It's been disconnected ever since it was discovered.

[antipode12]

You might have to go do things very unconventional.

Locate the drop non-destructively.

You can use a breaker finder or other AC detection system and trace the circuit in the wall. Might cost you $40 or so.

I tried a non-contact detector, but no luck through the wall. I did not see any other alternative tool at my big box.

Epilogue: I can't deal with the possibility of ripping open the walls to find it is one or multiple (or none) breaks in the neutral line, nor having a good way to repair it.

Soooo, I've disconnected the suspect cable entirely. I will tie the receptacle to a different circuit that I can get to from an adjacent room.

It's not the way I'd liked to have done it (its a pretty busy circuit I'll be tying into), but I don't feel like I have another choice. Old house. Multiple handy-man renos. Bound to be complications, right?

[Stratmando]

If you don't want to give up, disconnect one of the hots until the power goes out to the defective receptacle, check the neutral connection on the white that runs with that black.

[KISS]

This voltage detector isn't so bad. Craftsman AC Voltage/Current Detector, Non-Contact

Hopefully, you have no reason to suspect that there is a splice within the wall?

[fixitguy60]

Turn of the breaker to this circuit only. Inspect thoroughly for all outlets, lights or other appliances that no longer function. Each outlet and switch location as well as fixture location is also a potential for poor connections. If that still fails, then inspect attic or crawl spaces for j-boxes as well as any "mystery" blank plates covering poorly done remodel work randomly about the house... Very seldom is a bad connection due to a breakdown of a wire internally. If someone has been doing any work with a stapler one might re-think that but it is my experience that most of the time it's a bad connection in one of the box locations.

[antipode12]

This voltage detector isn't so bad. Craftsman AC Voltage/Current Detector, Non-Contact

Hopefully, you have no reason to suspect that there is a splice within the wall?

No. It looks like relatively new work, and there's nothing that might tap into it.

I really think that someone sunk a drywall screw into the 2x3 that the wire goes through, and never knew because it only powered low loads.

Thanks everyone!