My house was built in 1940 and all original wiring is metal clad / BX cable. With Romex, yellow vs white can tell me wire gauge. Is there any similar tell for MC? I suspect it's all 12 awg but I'd like to double check, only thing I know to do is use calipers to measure diameter of the conductor. Thanks

  • Even if there was a way to tell by measuring the outer diameter of the armored cable, that would be easier then prying open gangboxes and tearing apart old wiring to measure a conductor. – DrTarr Jul 21 '19 at 19:44
  • Found this guide: google.com/url?sa=t&source=web&rct=j&url=http://… But all mine is dirt-grey. Can I trust those OD's? – DrTarr Jul 21 '19 at 19:51
  • Can you get some calipers on stripped wire ends (with the power off?) Also, is it old-style BX (no bonding strip), new style AC (skinny bonding strip), or MC (no bonding strip, with a separate green wire for grounding)? – ThreePhaseEel Jul 21 '19 at 19:54
  • Never knew the difference. It is old style BX, no bonding strip. I can get calipers on it if I absolutely must. Sounds like there's no easy identifier. – DrTarr Jul 21 '19 at 21:32

Take a piece of #12 scrap wire and compare it to what you have. If the wire is the same it is 12, If the wire is larger usually #10 if smaller #14 other than an electric range # 14,12 & 10 will be everything except the range it will be much larger. These are the sizes used in a 1940’s era homes for the most part.

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If you have a hard time visually comparing the wires, go buy a standard, cheap light switch or outlet that has a back-stab connection (the insert spring clamp kind, not the screw down kind).

These connectors are normally labeled "14GA ONLY" and are designed to only fit 14GA wire. A strand of 12GA will not fit in the hole. Even with poor lighting this should be a simple, foolproof way to identify 14GA and 12GA wire.

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