David Jones wrote:

"Speedy Jim" wrote in message ...

David Jones wrote:

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!

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.

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
schools"