Electrical – Suitability of flexible metallic conduit serving as ground

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EDIT: The attached photo is the most easily accessible example of the conduit used in the entire house. This shows the conduit attached to the garbage disposal with the previously mentioned 3/4 set screw connector. Not shown is the wire contained inside. However, the wire is just loose insulated conductors that have a fairly large amount of play and in some cases, slack where you can pull additional wire out of the conduit up to a point. It doesn't look to me like that would be possible with the conductors in an armored cable.

All of the outlets in our house test as being properly wired (i.e. include the presence of ground). The house is wired with 12ga THHN pulled through flexible metal conduit. However, ground conductors are not present anywhere. Thus, each jbox is grounded only by its connection to ground through the conduit.

My concern is, although there may be electrical continuity to ground via the flexible conduit, the electrical connection at each jbox must be rather poor since the conduit is physically connected only by a set screw. Said another way, I really have to question the ability of the conduit to carry any significant fault currents.

I suppose if all the circuits in the house were on GFCI breakers, a crappy ground wouldn't be much of a safety issue since the ground could certainly carry enough current for the breaker to detect the current mismatch and trip. Since that isn't the case, it seem to me that any path to ground needs to carry enough current to be able to trip a regular 20 amp breaker in a timely manner.

Are there any significant safety concerns with this method of grounding, and/or should I consider having ground conductors pulled or, I suppose, having the house rewired with new 12/2 (or 12/3 for the multi-wire branches)?

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Best Answer

Re-pulling through installed flex is a nightmare. If you do, keep in mind that new-style dimers require a neutral. -EMT fittings? I'm guessing there's no red bushing either.

Flexible conduit used as ground? - InterNACHI Inspection Forum:

"In general FMC or flexible metal conduit is limited to 6' in length when used as an EGC (equipment grounding conductor). Since this is an old installation it's likely grandfathered in, however by modern standards it's could be considered ungrounded even though all of the components of the system are metallic. Is the stuff in the photo actually FMC or is it old AC (BX) cable?"

250.118

(5) Listed flexible metal conduit meeting all the following conditions:

a. The conduit is terminated in listed fittings.

b. The circuit conductors contained in the conduit are protected by overcurrent devices rated at 20 amperes or less.

c. The combined length of flexible metal conduit and flexible metallic tubing and liquidtight flexible metal conduit in the same ground return path does not exceed 1.8 m (6 ft). (does not include AC)

d. Where used to connect equipment where flexibility is necessary after installation, an equipment grounding conductor shall be installed.

Non-Armored Cable flexible runs more than 6 feet need a ground wire.

said by nunya, at dslreports.com, +1'ed by Speedy Petey:

Most BX is neither safe or acceptable for use as an equipment grounding conductor. Only BX which includes a bonding wire or bonding strip may be used as ground. Most "BX" style AC cable installed in the 30's-60's does not have the required strip.

Using BX as a ground is extremely dangerous, because it is not a low impedance path to ground. Rather than clearing a fault, the spiral jacket (especially in places where it might have some corrosion) will heat up like a toaster element. I've witnessed this several times. The cable clamps in the original boxes also were not typically listed for use as a grounding electrode conductor.

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