Flexible conduit used as ground?

1924 house with two individual conductors run through flexible conduit and the conduit is used as grounding. I know that rigid conduit is generally acceptable as a ground, but is flexible?


I think it was common practice at one time. But older BX shouldn’t be used as a ground. Newer Armored Cable has a ground strip in it and can be used as a ground.

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?

Thanks for the followup! The cable is probably an early (original to the house, 1924) BX cable. The conductors have the same type of insulation found on knob and tube wiring. While it’s certainly grandfathered in, the City of Dearborn is requesting the following on their occupancy cert: “Install GFI protected outlets in all wall outlets–exterior and interior.” I would assume they mean wherever normally needed like kitchen, bath, basement, garage, etc. But they are all already present. I’m concerned that if they are upgrading existing three prong outlets to GFI this conduit as ground may become a factor.

Are the outlets actually grounded. Did you remove a cover and look? Three prong outlets are not a guarantee of what you have. Never assume. Just because you see armored cable, does not mean the boxes are metallic. Two wire ungrounded was very common in 1924. Did you find any K&T in the home? It could just be a partial upgrade at the panel from years ago.

Based on the age of the home and the wiring methods, this is generally a system where the branch circuits are not grounded. It sounds to me as if the City is requiring an upgrade for compliance with current standards, which would mean that ALL branch circuit receptacles be protected by ground-fault circuit interrupters.

I didn’t assume anything with these. The outlets are actually grounded, I verified every outlet. I also opened a number of them and they were consistently lacking ground wires even though they had continuity to ground. There was no evidence of the house ever having had Knob and Tube, no remnants, no holes, etc. Parts of the system have been upgraded to newer NM, but several parts of the house still used the BX or FMC as ground.

Are you saying that proper grounds will need to be run to install the GFIs or will they simply need to install them and mark them as non-grounded? Thanks for the help guys.

Yes old AC cable will test good for ground continuity because it’s made of metal and is attached to a metal box. The question is will it have a low enough impedance under fault conditions to open the OCPD? Maybe, maybe not.

GFCI protection is not a substitute for grounding but is permitted for increasing safety in an ungrounded system. Only problem is that when used with old AC cable the metallic jacket of the cable will still be carrying the fault current during a ground fault condition so the GFCI protection will actually do nothing.

The NEC allows for replacement of non-grounding type receptacles with GFCI protected receptacles, without requiring that the GFCI be grounded.

Out of curiosity, what type of instrument did you use to determine that the existing receptacles were grounded? Did you happen to check the impedance of the grounding connection?

I used a simple tester at first and when I visually observed that the breaker panel had a number of incoming wires without grounds, I traced a couple of circuits to outlets that showed up as grounded. I then removed an outlet from its box and noted that no ground wire was present and that there was no continuity to ground when not connected to the box. I did not check impedance.

Any kind of basic tester would show this receptacle as grounded before you unscrewed it.

That’s why I asked. I would have been inclined to check impedance in an older system like this. The SureTest has that functionality.

Yes, unless you could test it with something more sophisticated than a standard tester you would only know that the ground was continuous. If it is suitable to carry the entire fault current in a ground fault condition would require other methods.

So how does the Suretest test for impedance?

I don’t know if all do it… I have the 165, You scroll via the arrows to that function. You can test the impedance of the Hot Neutral or Ground

I don’t test every outlet with it… I will sample upon finishing with the 3 bulb. Interesting results can be found in McMansions…AND old houses.



Interesting tester, but I’m skeptical that the impedance test is real world predictor of how the armored cable will react when an actual ground fault occurs. The reason being that when a ground fault is connected across a piece of old AC cable the fault current is potentially in the thousands of amps. Can this tester actually simulate that?


In real world applications, I’ve used to pick up a lot of info that isn’t available on a 3 bulb HI tester :slight_smile: Shared neutrals on AFCI’s… slow GFCI’s among others.

Not sure the results or basis of this tool vs a multimeter would be much different to measure impedance. Just a great way to analyze circuits in one shot… you pay a price for it as well, $300 +

I don’t think a “tool bag” class of tester that I can think of could simulate that much amperage beyond the mathematical computations in the microprocessor of this tool or a multimeter.

Wish I could be more help… the manu is Ideal and the product link is here http://www.idealindustries.com/prodDetail.do?prodId=61-165

The tester has the ability to apply a small load to the circuit for a fraction of a second, but most of the diagnostics are accomplished through an algorithm using line-voltage and line-impedance.

You can learn a bit more here SureTest

Obviously, the results of the SureTest are not definitive, but you can get much more accurate information when using this type of instrument over the three-light testers.

Thanks for the info, I’ll peruse the links when i get a chance.