If you do not have a ground wire installed with the other wires do not hook up the ground wire without knowing how the wires were carried to the box. If the house is wired with flexible metallic tubing (flex) your grounding the fixture can create an extremely hazardous situation. Older homes are sometimes wired this way. I have run into this situation a lot in my field work. Untrained electricians think any ground is a safe ground. This is not true. Grounding was required starting in 1962.

From: Can I safely ground a light fixture if the ceiling box has two hot and one neutral wire (no ground wire)

What the heck is that suppose to mean.

Is he saying that you shouldn't ground a metallic box (i.e. run a EGC from the panel or another EGC at another box) that has MC cable

flexible metallic tubing (flex)

going to it if it didn't carry a EGC with it because if it was in contact with combustibles (e.g. studs, lathe & plaster, etc.) and the box became shorted it could start a fire.

  • What do you mean by "MC cable", and where are you on this planet? Can you post photos of your situation? Feb 13, 2017 at 2:25
  • I edited post and I posted the link to where the original commentator said that. There is no situation. I am at coordinates 34.0522° N, 118.2437° W i.e. USA.
    – Max R.
    Feb 13, 2017 at 4:59

1 Answer 1


Don't rely on MC as a ground

What he is saying is that it's not reliable to count on the metal jacket of MC (metal armored cable) as the ground.

In other words, for grounding purposes, treat MC exactly like it is NM. If there's a ground wire inside the MC, that is an acceptable grounding path. Otherwise treat the cable and boxes relying on it as ungrounded.

Retrofit your heart out

Go ahead and retrofit grounds if you want to. The post is saying since MC armor is not a valid grounding path, you will want to retrofit a ground in parallel. Keep in mind the ground retrofit rules have eased since that post.

When retrofitting grounds, think first about your biggest loads, because once you lay the thickest ground wire, it can also serve as a ground path for any other circuit(s) which require that size or smaller. For instance when running your oven's ground, stop in a couple other junction boxes and use them as a hub for other grounds you are retrofitting. That way you don't need to do a home run for every circuit. The concept is that ground doesn't normally flow current, and it's rather unlikely for 2 circuits to develop a bolted ground fault at the same time.

  • I'm still not sure if the answer that is being referred to by the OP here is referring to actual FMC/FMT/LFMC (flexible conduit), MC (which you nail for the most part -- the rare MCC/MCS and the newfangled MCI-A both are exceptions, but we haven't seen either here on DIY.SE AFAICT :P), or AC/BX (which is its own can of worms). We really need to start getting people to clarify what they mean instead of just throwing trade slang like "flex" or "BX" at the wall and assuming we get their drift... Feb 14, 2017 at 1:58
  • I agree with getting clarity on the type of wire system used but most people don't have a clue including my wife but she can identify smurf tube LOL. + for taking an unclear question and giving a good answer.
    – Ed Beal
    Feb 14, 2017 at 2:05
  • Perhaps we should have a canonical question about "what is this wire/cable coming into my junction box?" Feb 14, 2017 at 2:55
  • +threephase. FINE, my mistake. I " assumed '' that it was "clear" where I quoted from was speaking about Metal Clad or Flexible metal conduit. " I " was talking about Flexible metal conduit, like this (google.com/…).
    – Max R.
    Feb 14, 2017 at 3:39
  • When in doubt, ground it out :) Feb 14, 2017 at 4:09

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