1

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 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 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 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 at 21:32
2

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.

2

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.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.