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My gas furnace will not turn on. I verified 120V where the armoured cable enters the furance, so I was going to continue testing for voltage before and after the transformer (with the door safety switch pressed in and 24V call-for-heat manually triggered, flame safety module is good), but I noticed a broken bit of (aluminum) wiring at the green ground screw. The black and white wires are solid copper. I assume the aluminum wire inside the armoured cable is ground, and was wrapped a couple times around the outside of the metal sheath, and then finally connected to the green ground screw.

Is using a short strand of solid copper ground wire, with the copper wire screwed into the grounding screw, and pigtailed to the aluminum wire with an AlumiConn the best practice?

Quck note - I'm comfortable and confident with the gas-related safety components on a gas furnace, but aluminum wiring requires special knowledge and is a source of fires, so I'm super cautious.

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    If it doesn't work w/o that (the freaking strain relief as a ground?) then the furnace is either actually broken or you lost the neutral. – Mazura Oct 14 at 20:03
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    If you pigtail a piece of copper to the aluminum there is a special grease you should use to assure that the connection does not oxidize. (Though the grease is not needed for a temporary connection for testing purposes.) Eg: acehardware.com/departments/automotive-rv-and-marine/… – Hot Licks Oct 15 at 0:08
  • I suspect that the fitting is missing because the unit is up against a wall where the cable comes through, and there was no practical way to fasten the fitting to the box. – Hot Licks Oct 15 at 1:51
  • It's BX... nothing is not a possibility. I suspect it's missing because it was a Friday. That 'aluminum wire' IS NOT a conductor. – Mazura Oct 15 at 2:00
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Someone forgot a fitting

Normally, armored cables (like yours) are brought into a box using a fitting designed to clamp the the cable armor, grounding it to the box and also providing a strain relief for the cable inside. Your furnace installer didn't have that fitting on hand, though, so they simply shoved the cable through the hole and shoved a "redhead" bushing into the end of the cable to protect the wires inside, leading to the mess that you see here.

So, with the breaker off, I would undo that armored cable (including taking the "redhead" out), get the correct fitting, insert the fitting into the KO and hold it on with a locknut, cut the bonding strip off at the end of the cable armor and insert the cable into the fitting, insert the "redhead" into the armored cable again, and reconnect the hot and neutral wires. Then, you'll at least have this furnace properly grounded.

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    Yup, first thing I saw... furnace installers... – Harper - Reinstate Monica Oct 14 at 11:42
  • Yeesh, I'm not even an electrician and have literally zero certifications, but when I ran AC I figured out how to properly ground and terminate it. And when I found I was missing a bit, I put the project on hold, went and got the appropriate materials, and did it right. – Doktor J Oct 14 at 19:20

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