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Can I use the existing 10/3 steel armored cable as feeder for a 240v 30 amp subpanel in my garage (detatched)?

That's 3 wires and a grounded metal jacket. 2 hots (red and black) which connect to a double 30a breaker at the main panel, along with a neutral wire(white) which is bonded to the main panel.

I've seen similar things discussed here so I know I'm missing a wire somewhere. The Does not the metal jacket substitute for the lack of a bare ground wire? Suppose I use a ground rod at the subpanel. Any reason not to ground cable at both ends? Does a 30 amp GFI at the main panel mitigate concerns about the missing wire (?) be it green or bare copper? That's a lot of questions so I'll summarize. How can I di this without replacing the 10/3 armored feeder?

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    Is there a bonding strip in the cable? – ThreePhaseEel Nov 23 '16 at 3:50
  • Also, where/how will the cable be run? Whether it is subject to damage or suitable for burial may also be considerations. – statueuphemism Nov 23 '16 at 4:02
  • Also, what are you running off the garage subpanel? – ThreePhaseEel Nov 23 '16 at 4:34
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    There is indeed a bare wire coming out of the metal jacket but it is not heavy gauge like the other three. Its at least a couple of gauges finer. It's clearly exposed to the inside of jacket along its entire length. – jon jay Nov 23 '16 at 5:41
  • The cable is attached by brackets to a fence every 2 feet or so for about 16 ft. The fence is sturdier than most, more of a solid garden wall (7 feet high with 6x6 posts and 4 stringers.) As fences go, it's fairly permanent. The cable runs along the underside of the third stringer. I think it wasn't buried because of a patio and a flagstone sidewalk in the way. – jon jay Nov 23 '16 at 6:05
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Armored cable can't defend itself from the rain

You'll need to re-run this cable in order to meet Code. While you have enough wires there for your proposal as the armor on modern type AC can be used as a grounding conductor as per 320.108, AC cable can't be run in damp or wet locations (such as outside attached to a fence) as that violates 320.12 point 2. In fact, I would not be surprised if the cable already has water in it, or is even rusting away as we speak, depending on how long it's been out there.

Replacing the cable with a PVC-jacketed (wet location rated) type MC or MCI-A cable is your best bet here as a result.

  • Agreed, even if this is in a desert, condensation and the occasional rainstorm are still possibilities which can be easily defended against. – wallyk Nov 24 '16 at 1:12

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