[In Ontario, Canada]

I have a "furnace room" (it's a room that contains my furnace, and also washer / dryer, water softener and water heater). Recently I had the furnace replaced and the contractor used the armoured/metallic BX cable for wiring the switch for it: BX cable

I also have this wiring: NM cable

Exposed NM cable doesn't seem right, but it's at ceiling height (~8 ft), safe from flooding and out of the way, and it's been like that for as long as I can remember, without much issue.

Should that exposed NM cable be replaced with BX cable? I'm assuming it's not code, as the furnace contractors probably wouldn't have spent the extra time and money using BX for the furnace switch if it were. Searching online I didn't find anything conclusive about it.

  • 3
    Not sure on Canadian code, but the contractor may simply prefer to use armored cable, in part because it means less callbacks when mice (don't) get into it - or because it suits commercial work and is perfectly fine for residential as well (you can always exceed code minimum requirements) Or because it means no need to add protection for the parts that come down off the ceiling. It's more likely to be MC or MC-lite (aluminum armor) than BX, this era.
    – Ecnerwal
    Oct 27, 2020 at 19:46

1 Answer 1


They have more flexibility in how the AC is installed than if they'd used NM.

For example, in most areas, the NM-B cable you have stapled to the bottom of your joists is not allowed; it should be running through the joists like the other one. I wouldn't suggest changing it; it just doesn't meet today's standard.

But if that NM-B on the bottom of the joists were AC, it would pass an inspection.

The HVAC contractor would probably rather carry one box of AC than several different materials to minimize material costs but taking up more room in their van, or causing more trips to the supply house and running up miles & hours.

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