Buying AC cable from buildings that are being rewired.

The quickest way to tell AC from MC is to look inside the armour at a cut end. AC has a flat strip of metal running along the inside of the interlock, bonded to the armour and folded back at terminations. MC generally does not, and carries a separate insulated grounding conductor instead.

Print helps where it survives. AC cable is marked as such along the armour, often with the conductor count and a temperature, and on older lengths that print is frequently the only marking still readable after decades overhead. A flashlight and a rag recover more of it than a photograph taken in a dark ceiling void.

Older commercial buildings are the source: small office buildings, schools, churches, clinics and the kind of light industrial building that has been added to three times. Rewires in those buildings are usually partial, which means a lot arrives as a mix of AC whips and modern cable from the newest work.

The mix is not a problem, but it does need separating. AC whips reviewed alongside modern MC are written as two lines, because the constructions are different and the copper inside each of them is only part of the story a buyer has to tell.

Paint on the armour is normal and mostly cosmetic, though it hides rust and can seal a cut end so the conductors are not visible. A buyer will ask for one whip opened up rather than judging a whole bundle from the outside, since the inside tells you whether moisture has been sitting in the armour.

Length is the harder limit. Whips from a ceiling are usually short, and short whips have less re-use value than long cut-offs, so a lot of a hundred pieces may review on count and copper content rather than on any prospect of the cable going back into a ceiling.

Cut one whip open and photograph the inside of the armour. It answers the bonding-strip question and the moisture question at the same time.

Mixing the two families in one bundle creates work at the scale and an argument at the terms. AC and MC are different constructions with different grounding arrangements, and a buyer sorting a mixed cart has to open enough pieces to be confident about how the pile divides. The modern construction is set out under MC cable.

Where a site holds both, keep the AC in its own cart or pallet and label it. The same applies to the older cloth-wrapped and fabric-insulated conductor that sometimes comes out of the same ceiling, which is handled as its own family and should not be coiled in with the armoured cable.