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Having recently bought an older house (c1943) I am doing a little work on the place. While rewiring a light switch I noted that there is newer Romex wiring from the distribution panel to the switch, but older BX (2-conductors) wiring from the switch to the light fixture.

I now need to put in a junction box nearby to join the BX and a new length of Romex (which runs to a different appliance). This gives me a chance to connect the sheathing of the BX to the Romex ground wire (which is grounded at the distribution panel). Is there any reason not to do so?

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TOTALLY DIFFERENT ANSWER FROM THE ORIGINAL:

The original answer was not complete and had some misunderstandings. I hope to clarify here.

First off, there are many types of armored cable, most of which are colloquially called "BX". By "older BX" I assume you mean something like the wire at the bottom of this image:

enter image description here

Two conductors, no ground wire, no bonding strip. This kind of wire is an older design predating the grounding requirement. If the wire is original to the house, it was not designed to have the armor grounded (because it's 20 years older than the NEC grounding requirement itself). It's still available (hence the newer PVC insulation and overall new condition of the wire in the picture) because there are still situations in which it can be used safely (and jurisdictions that don't care); however, your situation and location probably falls under neither of those.

It can be dangerous to use the armor of older, "non-bonded" BX cable as a ground connection because:

  • The armor of this older cable was never rated to transmit current, much less 15+ amps of it; if it is too thin, or the run too long, it can present high resistance to current along its length, meaning less current will flow through it in a failure situation, and so the breaker may not trip when it is supposed to.
  • The armor may be aluminum (in fact it is likely to be), which is highly "anodic"; where aluminum touches copper (such as your proposed join between the copper Romex ground and the armor), or iron/steel (nails/screws/pipes), or really any metal besides other aluminum, zinc or magnesium, the aluminum will corrode preferentially when given any excuse at all (humidity, salt, high temperature, electrical flux) and eventually your ground path will fail.
  • Continuity of the armor becomes a primary concern; if the armor is damaged, its continuity can be reduced or destroyed. This can be problematic when the primary purpose of the armor as designed was to take a beating.
  • Aluminum, when an electrical current passes through it, heats up more than copper. In a sustained failure situation that doesn't pop a breaker/fuse, this could end up being a fire hazard, especially if there is increased resistance due to damage at any point in the armor.

It's possible, and it may even be likely, that you'll never have a problem, but electrical codes are extremely pessimistic for a reason. When "possible" takes another job on Murphy's side, and "likely" don't want to come in to work that day, that's the day you have a problem.

Nowadays, type AC cable (modern "BX") looks more like the following:

enter image description here

Notice the bare wire. This type of cable is designed specifically to have its armor and this "bonding strip" form the ground path in combination. You do still have to be careful about what you connect it to:

  • The bonding strip may still be aluminum, as in the picture (though copper-bonded AC cable is available); if so, it should not directly touch any copper or steel. You should instead ground the J-box (typically galvanized or zinc-plated steel; the zinc coating in either case protects both the steel and aluminum from galvanic corrosion) through the wiring clamp by bending the bonding strip backwards so it's also held by the clamp, then attach the copper wire somewhere else in the box (there are usually other attachment points in the J-box for just this purpose).
  • The bonding strip is 16AWG, because it's designed to complement the armor; if the armor is fully compromised at any point along its length, however, the bonding strip is all that's left, and it's 2AWG smaller (and aluminum provides more resistance) than the minimum gauge of copper wire rated for 15A.

This all just means you should be careful when installing it, to make sure that you don't abuse the armor or create a galvanic cell in your J-box. You also must not install this wire in any location where water could be an issue; instead, you should use a Type MC cable with a PVC outer jacket. However, if installed properly, this type of wire is perfectly safe.

Looking at the first image again, the middle cable is an example of non-bonded, grounded BX, known in the NEC as type MC (though newer MC has an extra PVC jacket between the conductors and armor; it's basically armored Romex). Because it doesn't have a bonding strip, the armor of this cable should not be used as ground. Instead, the green wire (in the image it's a very blue-green) should be used as the ground connection. MC severs these two responsibilities; the armor is there to take abuse, the ground is there to add electrical safety to the circuit.

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-1 Advising not to ground a BX cable is not a responsible answer! NEVER, EVER is it okay to leave a BX cable sheathing ungrounded! If that cable sheathing ever becomes energized, and is touching ductwork, plumbing, gas pipes, etc. as you say, a grounded system will trip the circuit breaker and protect the homeowner. That is how the system is designed to protect. If you left it ungrounded anything touching the cable will become live! Possibly going unnoticed until a victim comes along! The Romex gets bonded to the metal box with a bonding screw, the BX cable is bonded through the box connector. –  SteveR Mar 30 '12 at 23:37
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I have to agree. A HUGE point of the sheathing is to BE THE GROUND. The potential energizing of ductwork, gas lines, etc. is a risk if the bx is NOT GROUNDED. If the BX IS grounded, then the other lines won't energize because the charge will go to the ground, as designed. –  The Evil Greebo Mar 31 '12 at 0:39
    
"The cable may contact ventilation ducts, plumbing pipes, even gas pipes" - All of those things are supposed to be grounded anyways... –  BlueRaja - Danny Pflughoeft Sep 9 '13 at 20:12
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@wallyk: No, no, and no. First, fuses/breakers are "slow-blow" for a reason; you don't want a fast trip for a simple overload. It can take up to a second for a circuit breaker to break at its rating. Second, as I said, energizing grounded armor may not ever cause the breaker to trip, if there's enough resistance between the short and the bus strip that less than 15A is flowing. It's the same reason house plumbing is no longer an acceptable ground; it may well be grounded, but real ground can only absorb so much current. –  KeithS Sep 10 '13 at 0:17
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Lastly, think; what would the cable be dissipating that heat into? It's rare, but failures energizing older BX can make the stuff glow red (without popping a fuse, as we've covered). The BX would be clamped to wooden structural members, not far from paper-backed insulation. Expecting older BX to do the job of newer AC/MC cable is asking for a house fire. –  KeithS Sep 10 '13 at 0:26

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