Old armoured cable without bond wire

[powerplay]

In an old garage, I ran into old steel sheath wiring with 2 conductors inside that didn't have a bare bond. They appeared to rely on the connectors for bonding, and although the sheath was difficult to cut, the rusty connectors made for a poor bond. Any feedback on this sort of wiring? I am running a new circuit to feed new receptacles and lighting to ensure a good bond... I assume adding to this system, with a new cable incorporating a bond would be almost as misleading as adding to knob and tube wiring?

 

[augie47]

That type system (old AC cable) can be dangerous. The metal sheath often forms a long and high resistance path and in the event of a ground-fault it can actually become a heater.

 

[G._S._Ohm]

Ditto.
Steel has 6x to 60x the resistance of copper and the length of sheath unwrapped is probably at least 3x longer than the actual cable length. Add in rusty connections at each end and it makes a pretty lousy ground.

 

[drive1968]

I run into that type of wiring quite often. You're talking about the 1/2" or 3/4" variety correct? I see no problem using the existing conduit as long as a ground wire is pulled and attached to the junction boxes with a ground screw. The existing connectors are suitable for bonding, just not grounding. If the connectors are rusty, then they can be replaced with new ones and the bond is fine. I rarely see a connector that is rusty and needs replacement, even ones that are 40-50 years old. It's my understanding that if the conduit is less than six feet, it doesn't even require a ground wire, but I pull one anyway. Many of the houses in my area have this type of conduit snaked throughout the house and actually make pulling a new circuit much easier than if romex or mc had been used instead.

 

[pete m.]

That type system (old AC cable) can be dangerous. The metal sheath often forms a long and high resistance path and in the event of a ground-fault it can actually become a heater.

100% true! Have witnessed (after the fact) house fires resulting from what Gus describes.

Pete

 

[powerplay]

...an interesting question.... would you add a circuit to an existing old system such as this with a bond in the new cable tied to the metal box? hmm...

 

[kwired]

I run into that type of wiring quite often. You're talking about the 1/2" or 3/4" variety correct? I see no problem using the existing conduit as long as a ground wire is pulled and attached to the junction boxes with a ground screw. The existing connectors are suitable for bonding, just not grounding. If the connectors are rusty, then they can be replaced with new ones and the bond is fine. I rarely see a connector that is rusty and needs replacement, even ones that are 40-50 years old. It's my understanding that if the conduit is less than six feet, it doesn't even require a ground wire, but I pull one anyway. Many of the houses in my area have this type of conduit snaked throughout the house and actually make pulling a new circuit much easier than if romex or mc had been used instead.

The OP was talking about AC cable not flexible metal conduit.

 

[LarryFine]

...an interesting question.... would you add a circuit to an existing old system such as this with a bond in the new cable tied to the metal box? hmm...

The bond need not be landed; the sheath is the EGC. The strip bonds each wrap of the spiral to the next, so the sheath does not act as an inductor during a high-current fault to it, which causes heat and possibly enough impedance to keep the OCPD from opening.

To answer the question, I might for one small load and where it doesn't break other rules, but just about every existing BX/AC circuit should probably not have any new loads added anyway. I always recommend a new circuit, or at least tying into a newer existing one.