Grounding Question

[phiftyseven]

Not a licensed electrician, but pretty handy (and safe) with electrical. Helping a friend with a old home that has 2 wire receptacles. They are in metal boxes, and fed by 2 conductor metal clad cables. The metal boxes are grounded by the metal sheathing, as I get 120 volts when measuring between the box and one receptacle slot with my voltmeter. He wants to upgrade to grounded 3 prong receptacles. Not sure if this would be in code or not, but can you use those push on grounding clips on the box, and run a short ground wire to receptacle ? Trying to have a work around to avoid opening walls and rewiring.
Thanks in advance for any replies.

 

[Meursault74]

I'm not an electrician and won't speak to code.

I have the same set up in my home and have changed out some 2 prongs to 3. The metal boxes were in fact ground despite having only the two prong receptacles on them.

I checked with a DMM as well as a receptacle tester when I was done.

I believe the receptacles can be grounded to the grounded metal box through a tight connection with the mounting screws and the metal yoke/tabs. I'd make sure there's not paint/plaster in the way of the connection.


There are many electricians on here, that can give you answers on the code and advice on those mounting clips if you wait a little bit.

 

[Innovate1]

I can't answer that for sure but I do know that you can install 3 prong receptacles without a ground connection if they are GFCI protected.

 

[wyliesdiesels]

If the box is grounded and u use self grounding 3 prong receptacles it will be fine

 

[TRWham]

If the box is grounded and u use self grounding 3 prong receptacles it will be fine

Just because there is a path to ground via the armor, does not mean the cable is approved for use with the armor as an EGC. Modern AC is fine because it provides a separate metal ribbon as an EGC, but old BX is not listed for such a use. Other types of modern flexible metal cable and conduit can be used but have limitations.

They need to confirm what type of cable it is, but I imagine it is BX.

 

[mm08822]

The safest way is to ground the receptacle to the grounded box and add the gfci protection.

Even if the bx jacket path is unbroken today, it could change tomoorow due to breakage or water damage (rusted connections). There have been many instances reported of bx jacket glowing in the dark from passing a low current through the jacket due to high resistance upstream.

There could also be nm cable w/o a grd conductor or plastic boxes used in unseen locations that make grounding impossible w/o demo.

 

[nadogail]

The safest way is to ground the receptacle to the grounded box and add the gfci protection.

Even if the bx jacket path is unbroken today, it could change tomoorow due to breakage or water damage (rusted connections). There have been many instances reported of bx jacket glowing in the dark from passing a low current through the jacket due to high resistance upstream.

There could also be nm cable w/o a grd conductor or plastic boxes used in unseen locations that make grounding impossible w/o demo.

I would install GFI receptacles in each box.

 

[Innovate1]

I would install GFI receptacles in each box.

Why? One as the first on each circuit provides the same protection.

 

[acer66]

Why? One as the first on each circuit provides the same protection.

Sometimes done so the stuff plugged into the receptacles downstream stays on.

 

[Metal-Marc]

I would install GFI receptacles in each box.

Or install a GFCI breaker?

 

[mm08822]

I would install GFI receptacles in each box.

may be difficult to fit in the older boxes with multiple cables incoming. Also have to worry about sharp bends cracking the insulation as you're pounding a 5# gfci recept into a 6# box.

 

[Meursault74]

may be difficult to fit in the older boxes with multiple cables incoming. Also have to worry about sharp bends cracking the insulation as you're pounding a 5# gfci recept into a 6# box.

I had that situation when I needed to pigtail and install a GFCI in an old small box with multiple wires from other circuits coming in. I was given great advice by @sparky 1971 here on this site to use a wiremold starter box kit on the outside of the old box. That extra room made a big difference and it looks good as well.

I wouldn't install a GFCI and a wiremold box on every receptacle in the house though. That would get costly. But in needed location it's an easy and relatively hassle-free option. The backing plate did need some slight modification to allow the GFCI to fit though.

 

[The Cobbler]

If the box is grounded and u use self grounding 3 prong receptacles it will be fine

this seems to be the most logical approach ...

 

[wyliesdiesels]

Just because there is a path to ground via the armor, does not mean the cable is approved for use with the armor as an EGC. Modern AC is fine because it provides a separate metal ribbon as an EGC, but old BX is not listed for such a use. Other types of modern flexible metal cable and conduit can be used but have limitations.

They need to confirm what type of cable it is, but I imagine it is BX.

the OP said metal clad cables. those are different than BX. it could also be FMC/greenfield.

also, code at the time may have indeed allowed grounding thru the armor. i seem to recall that it used to and the code changed more recently. if indeed it did, then the current work is grandfathered in.

 

[alfredeneuman]

Sometimes done so the stuff plugged into the receptacles downstream stays on.

If the downstream receptacles still work with GFI tripped it's hooked up to the wrong terminals. (Line instead of Load)

 

[TRWham]

the OP said metal clad cables. those are different than BX. it could also be FMC/greenfield.

also, code at the time may have indeed allowed grounding thru the armor. i seem to recall that it used to and the code changed more recently. if indeed it did, then the current work is grandfathered in.

If the existing receptacles are 2 prong, you could run a hundred wires and they would still be ungrounded. If the code in force at the time it was built did not require grounding, which became a thing about 1964, then installing 3 wire receptacles would require a listed grounding conductor. Actual BX cable (not to say any flexible metal that people refer to generally as BX) did not provide a useful ground. AC is the only armored cable even now that can be used as a ground without limit. FMC is limited to 20 amps and 6’ of length.

 

[wyliesdiesels]

If the existing receptacles are 2 prong, you could run a hundred wires and they would still be ungrounded. If the code in force at the time it was built did not require grounding, which became a thing about 1964, then installing 3 wire receptacles would require a listed grounding conductor. Actual BX cable (not to say any flexible metal that people refer to generally as BX) did not provide a useful ground. AC is the only armored cable even now that can be used as a ground without limit. FMC is limited to 20 amps and 6’ of length.

You keep citing current code. But what was the code when the house was built? You dont know because we dont know the year the house was built

 

[TRWham]

You keep citing current code. But what was the code when the house was built? You dont know because we dont know the year the house was built

He is updating to 3 prong grounded receptacles. Why do that without providing either a ground or GFCI?

 

[acer66]

If the downstream receptacles still work with GFI tripped it's hooked up to the wrong terminals. (Line instead of Load)

I understand and maybe I phrased it wrong.
All the receptacles are gfci’s and are wired to only trip themselves.

 

[Jim greengo]

I would install GFI receptacles in each box.

Why in each box?

 

[Jim greengo]

Many older single gang metal boxes aren't big enough for a gfi.

 

[Meursault74]

Why in each box?

He's a stock-holder in Leviton Corp.

 

[phiftyseven]

Just because there is a path to ground via the armor, does not mean the cable is approved for use with the armor as an EGC. Modern AC is fine because it provides a separate metal ribbon as an EGC, but old BX is not listed for such a use. Other types of modern flexible metal cable and conduit can be used but have limitations.

They need to confirm what type of cable it is, but I imagine it is BX.

After reading all the responses (much appreciated) I do need to clear up my error of calling it metal clad. It is definitely BX, as it has just the 2 conductors, and no separate ground wire or ribbon as you and others have described.

 

[phiftyseven]

the OP said metal clad cables. those are different than BX. it could also be FMC/greenfield.

also, code at the time may have indeed allowed grounding thru the armor. i seem to recall that it used to and the code changed more recently. if indeed it did, then the current work is grandfathered in.

After reading all the responses (much appreciated) I do need to clear up my error of calling it metal clad. It is definitely BX, as it has just the 2 conductors, and no separate ground wire or ribbon as you and others have described.

 

[Norcal]

After reading all the responses (much appreciated) I do need to clear up my error of calling it metal clad. It is definitely BX, as it has just the 2 conductors, and no separate ground wire or ribbon as you and others have described.

If AC cable "BX" does not have the bond wire and that would be common in old AC cable the armor cannot be used as a grounding conductor, it's important to not mix up AC, & MC, as they have different NEC articles covering them & different installation requirements.