Is BX cable sufficient ground in itself?

yazkAugust 25, 2010

Hi,

We have moved into a new house that is a bit of a fixer-upper. We're remodelling the kitchen and need to add a few ceiling lights in bedrooms and the living area.

Most of the existing outlets are 2 prong and looking at the wiring we have BX / armoured cabling with no separate ground wire.

I would like to change out a number of the outlets to 3 prong. Is the ground from electrical box -> metal cable -> house ground, considered 'good enough'?

I'm half thinking I may as well get the electrician to replace everything while he's doing the kitchen electrics and the celiling lights, before we decoreate the house.

Any thoughts??

Thanks.

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brickeyee

You are actually pretty unlike;y to have actual BX cable.

Look at the end of a cable in a junction box for a thin metal bonding strip.

The bonding strip allows the use of the armor on AC cable to be used as ground.

Real BX does not have a bonding strip.

    Bookmark   August 25, 2010 at 3:20PM
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btharmy

If it is in fact BX cable without a bonding strip, then no, it is not "good enough" to use the jacket as an equipment ground. My home, built in 1928 and many in the area built pre 1940 or so had 2 prong outlets with bx and knob and tube. I can only assume if the house was trimmed out with 2 prong devices that there was no equipment ground available to begin with.

    Bookmark   August 27, 2010 at 10:15PM
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DavidR

I don't know what the code called for in, say, the late 50s and early 60s - but I've encountered several houses from that era in which grounded circuits were run to the kitchen and garage, but 2-pin receptacles were fitted.

    Bookmark   August 29, 2010 at 10:24AM
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baymee

I believe the code allows up to 6 feet of BX in length to act as a ground without a bonding strip.

    Bookmark   August 31, 2010 at 5:53AM
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mike_kaiser_gw

I don't know what the code called for in, say, the late 50s and early 60s - but I've encountered several houses from that era in which grounded circuits were run to the kitchen and garage, but 2-pin receptacles were fitted.

My Mom's house is wired like that. She has non-metallic cable and the ground wire is just wrapped around the screw of the cable clamp. My guess is that at the time grounded receptacles were more expensive and the code was changing requirements for grounding so the electrician just went with the cheaper option.

    Bookmark   September 7, 2010 at 7:04AM
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brickeyee

Initial versions of NM cable did not have a grounding conductor.

Later a grounding conductor was added, but is was smaller then the current carrying conductors (around #16-18).

BX was discovered to have problems with ground continuity using just the spiral armor.

It was de-listed and not re-introduced until years later as AC cable with a bonding strip.

While it is possible for the armor on BX to heat up (it is a common demonstration with a Variac to control the applied voltage and make the armor glow red hot) it is still a rare event.

    Bookmark   September 7, 2010 at 9:05AM
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mike_kaiser_gw

Later a grounding conductor was added, but is was smaller then the current carrying conductors (around #16-18).

I thought the ground wire looked small but with aging eyes I thought it was my imagination....perhaps not.

    Bookmark   September 8, 2010 at 10:30PM
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DavidR

IIRC, the original ground in NM was allowed to be 2 sizes (4 numbers) smaller than the current carrying conductors. In #14 cable, a #18 ground was permitted; #12 cable used a #16 ground.

The method Mike mentions of "bonding" the ground wire to the box is one I've seen many times in work done around 1960. Kind of makes you wonder, eh?

    Bookmark   September 8, 2010 at 11:48PM
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