peculiar wiring in residential switch box?

guys, I was going to replace a dimmer switch when I ran across an unusual wiring method in the single-gang receptacle box. The dimmer controls a chandelier in the dining room, it's the only switch for the fixture.
Two runs of NM cable enter the box just like a typical switch. The two leads on the old Lutron dimmer are connected to the white and black on the first cable. Black on the second cable is connected to the *white* on the first cable but the white on second cable is capped off with a wire nut. When I saw this mess I put the plate back on until I can understand it. The wiring makes no sense to me unless the switch is placed at the end of the run and the white on the first cable is being used as a hot, in which case it should've been marked with some black tape. But then why is the black from the second cable wired to the same nut (nut connects the black from second cable, white from first cable, and the lead from dimmer) and why is the white capped off? I can see several scenariors to explain this, none pleasant.
The house is 35 years old and the whole subdivision supposedly had aluminum wiring originally. All the wiring I've run across is copper so it's likely that someone, sometime replaced all or most of the wiring...and who knows how qualified that party was.
I'm thinking that I should (1) disconnect the black on the second cable (the one with the disconnected white) and see if the dimmer and ceiling fixture operate, then (2) determine if there is current in the black wire of the second cable, then go from there.
Any thoughts on this wiring setup? Thanks!
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<< why is the black from the second cable wired to the same nut (nut connects the black from second cable, white from first cable, and the lead from dimmer) and why is the white capped off? >>
Might have been part of a now defunct 3-way switch set up. I'd disconnect the second cable entirely and see if everything works. Standard admonishments apply here when working on your own electrical system. Good luck.
Joe
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thanks Joe!
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David Jones wrote:

I think you're right on target that this was done when the Alum. wiring was replaced. They probably had a nightmare job trying to pull new NM cable thru all the wall/ceiling spaces.
In the case of this box, they needed a Hot and didn't have it so they pulled a cable and only used one conductor.
Can you DO that?? Wellllllll......it's a very fine point. There's nothing inherently wrong with only using one conductor, but Code does require that currents cancel each other where a conductor exits thru a hole in a metal box. (There was concern about the box overheating due to induced eddy currents.) The currents DO cancel if both conductors run thru the same hole and carry opposite flow (the usual case).
DO boxes overheat in cases like this? Not likely.
That's a long-winded way of saying that, IMHO, put it all back together and don't worry about it.
Jim
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Thank you Jim.
I am always impressed with your knowledge and responses and general gentlemanly ways. You are a real asset to the many who come here with questions!
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Just wondering, *is* there a 3-way switch in the vicinity of this mess? In some older homes, it was not uncommon to use 2-wire cable to grab just a hot from a local outlet, to feed the begining of a 3-way set. Then, 2-wire cable is again used to bring the 2 travellers to the 2nd 3-way switch, another 2-wire cable is used to bring the switched hot only from the 2nd switch to the light fixture, and another 2-wire ran from the fixture to any local outlet for neutral.
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I wouldn't guarantee anything with this mess. There are no other switches in that room but I suppose they could be plastered over. Thanks for the ideas.
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David Jones wrote:

I wouldn't think they would at 20 amps or less pulled through a single hole.
But, it's definitely important never to pull hot and return leads (or the three wires of a 3-phase circuit) through separate pieces of metal conduit between two metal boxes. that can create significant current loops in the conduits and make them and the boxes get HOT, particularly on higher power industrial stuff.
Jeff
--
Jeff Wisnia (W1BSV + Brass Rat '57 EE)

"As long as there are final exams, there will be prayer in public
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