it’s been 30 yrs since i used bx cable and i don’t miss it at all. i was at the gym the other day and they were building a knee wall and were running bx thru the steel studs. my question is there was a 3″ red stripe and a 3″ black stripe about every 3 feet. is this marker there to indicate wire gauge? just wondering.
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I believe it denotes the number of conductors. Black=2, Black&Red=3, Black&RedBlue=4, etc.
That identifies the circuit conductors in the "MC cable" (BX is an obsolete product).
Here is a handy cheater
http://www.afcweb.com/pdfs/afc_cable_catalog/afc_cs_identifier_0704.pdf