I recently acquired a lot of older BX/armored cable. Many have a little
aluminum wire inside which I see wrapped around the ends of the cable on
some of them where they were taken out of service.Some of the cables have
white, green and black wires in addition to this aluminum wire, many are
just white, black and the little wire. What is the purpose of this little
aluminum wire ? Is this a primitive ground or bonding wire?
Yes. The idea is to fold and twist that drain wire back along the spiral in
the armor. The screw in the box connector then contacts that drain wire to
make a solid ground. It's not as good as a conductor-sized ground but it
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