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I would like confirmation on the following: Is any metal box automatically grounded if it is supplied by an armored cable (without a ground wire) that is properly connected to the box and back to the panel? If a metal box downstream on the same circuit is only supplied by Romex, I believe to ground that box I must pigtail the ground wires to a lug on the box - correct?

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  • Can you tell approximately when said armored cable was put in? – ThreePhaseEel Aug 18 at 14:27
  • Can you shoot a photo of it? There are at least 2 kinds of armored cable (to say nothing of FMC) and some are, some aren't. Does it have paper wadding? Does it contain a green ground wire? – Harper Aug 18 at 18:10
  • yes it's electrically grounded, metal conducts electricity. use your multi-meter if in doubt. keep in mind though that we need to follow the laws of man, not just physics. – dandavis Aug 19 at 16:46
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Armored cable without a ground connected to a metal box is not a proper ground. The Romex downstream would have it's ground connected to a lug or screw to the metal box but it still wouldn't be properly grounded because of the armored cable. Voltage testers would show a ground but it wouldn't be a proper ground.

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    It depends on the type, without that info we don’t know. – Ed Beal Aug 19 at 14:42
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    The NEC 250.9 (b) only allows armored cable in lengths of six feet or less to be used as a ground and both connectors on either end must be a grounding type connector. – Retired Master Electrician Aug 19 at 15:58
  • @ Retired Master Electrician But then can you extend the circuit? – JACK Aug 19 at 16:47

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