Confused (Two-wired ungrounded receptacles)

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Old 01-07-08, 05:00 PM
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Confused (Two-wired ungrounded receptacles)

I see many posts about two wire ungrounded receptacles recently. I have seen this with regards to 50yr old wire and 60yr old wire. Now I know a lot of wire back then was BX. Now I was under the assumption that the BX wire and the metal and bonding strip was the actual ground, therefore making the metal junction box grounded. Furthermore I also thought that if you replaced the two prong receptacle with a 3 prong grounded outlet as long as the receptacle was secured to the metal outlet box then it would be grounded. Throughout my house I had some of this old wiring and replaced the two prong outlets with 3 prong grounded outlets. I have always tested positive for the grounded outlet, however if I let the outlet hang outside the box and tested it when it was not secured it came up negative for a ground. Even after all that tested positive I went ahead and secured it more by adding a ground wire to the ground terminal on the receptacle and bonded it to the metal junction box. Now the reason I bring this up is if this is true what I am stating then there are a lot of people that keep putting posts on this forum about adding three grounded receptacles on two wire. Many of these posts talk nothing about bx wire and I think that some of these people think they need to rewire their whole house to get a ground when in fact maybe they don't. Any thoughts on this from the experts?
 
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Old 01-07-08, 05:19 PM
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I'm not an "expert" but I'll post my thoughts.

First off is that as far as the NEC (National Electrical Code) is concerned there is no such cable as "BX". That is a term that was given to a certain brand of armored cable a long time ago and has stuck just as there is no such cable as "Romex" in code terms.

Yes, the armor of this old cable (and the bonding strap that was added later) is often grounded but it is not usually grounded in a manner that is consistent with code requirements. It is not considered good practice to rely on this inadequate "ground" under current code requirements.
 
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Old 01-07-08, 06:43 PM
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AC - Armor Clad is still used today and is acceptable under the NEC and UL as a grounding means with listed fittings. Every place where we work (MN and WI) we can use it. We have NEVER had any problems using it and no inspector has told us not to use it. If you have BX (AC) in your home and you have ungrounded receptacles you can change then to grounding receptacles under 406.3(D)(1-3). You need to add a bonding wire from your receptacle to your steel box or use self grounding receptacles.
 
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Old 01-07-08, 06:49 PM
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BUT just because you see BX doesn't mean it's OK. I always check with a meter because alot of times the metal sheath is broken from handymen "fixing" things on their own without using the proper fittings or even just cutting the sheathing and taping the splices together and not using a box at all !!!
 
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Old 01-08-08, 01:50 PM
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when you say you check with a meter, how do you do that? Do you check the hot and touch the metal part of the cable? I would assume if was not grounded you would not get any results
 
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Old 01-08-08, 02:04 PM
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You measure voltage between the hot wire and the spiral sheath. If you get 115V +/- 10V, then it's okay. If you get any other reading, the sheath is not adequate for use as a ground.

The sheath on old AC is often deteriorated, broken and rusted or the cable is not terminated properly with fittings so it's not a great ground like a copper wire is, but it can suffice when installation of new conductors is not practical.
 
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Old 01-08-08, 03:50 PM
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Without the bond wire the short circuit path thru the spiral jacket is many times longer than the circuit conductors. High resistance due to this length and/or rust can cause the spiral jacket to heat up enough to start a fire and still not trip the breaker or fuse.
 
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Old 01-08-08, 05:50 PM
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We once tested that theory, pcboss, because it seemed so ridiculous to us that the current would follow the longer length of the spiral rather than jumping across the spirals, thus taking the shortest path to ground.

We measured ohms across a length of used buffalo pipe* and the meter showed several ohms. We then pulled it apart and measured it across the length of its now-separated spirals. Virtually the same reading. A new piece of the same length measured close to zero ohms. We pulled the new one apart and it measured oh-so-slightly higher, which told us that the current was jumping across the spirals.

Conclusion: As the material ages, the resistance increases and becomes less suitable for use as a ground.

* Electricians outside of Buffalo, NY jokingly refer to it as "buffalo pipe" because it's found everywhere inside the city limits.
 
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Old 01-08-08, 06:39 PM
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two questions. My first question is I noticed that in my house where there is this older wiring from 1952, there was added receptacles probably 10 yrs ago. They ran it off one of the older receptacle boxes. They wired the ground right off the the metal box. All of the new receptacles test positive for a ground. My question is, is this acceptable?

My second question is you talk about the metal sheathing or spiral being the ground and that it deters over the years making it less of a ground, how about the bonding strip, if you have a bonding strip and it is installed correctly doesn't that work as the ground? Or are you talking about older metal that did not have bonding strips?
 
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Old 01-09-08, 08:12 AM
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Originally Posted by brianhunter01 View Post
They wired the ground right off the the metal box. All of the new receptacles test positive for a ground. My question is, is this acceptable?
Yes, this is acceptacle. The AC sheath is bonded to the metal box at the termination fitting. The device (receptacle or switch) is then grounded to the metal box. Extensions of the circuit would also be grounded to the metal box. This is really my main problem with using old AC sheath as a ground. The bonding strip (if present) is terminated outside the box, so it cannot be inspected for proper termination.

if you have a bonding strip and it is installed correctly doesn't that work as the ground? Or are you talking about older metal that did not have bonding strips?
The correctly installed bonding strip vastly improves the quality of the ground. Very old AC does not have a bonding strip, and sometimes even in the newer stuff the bonding strip has not been terminated correctly.
 
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Old 01-09-08, 03:34 PM
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Now as far as checking the ground to see if you have a good ground with a multimeter you talked about touching the metal or spiral and hot and if you got around 120 +/- 10 then you had a good ground. Now can I check a receptacle rather than pulling out the receptacle?. Can I put the hot lead on the right side (hot side) of the receptacle and the negative in the third equipment ground prong? If I do this will this be the same results as using the actual hot wire and the metal spiral? I would assume yes but I want to make sure
 
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Old 01-09-08, 04:09 PM
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Your use of negative is incorrect. There is no negative. When your multi-meter is in an AC setting you can interchange the probes.

Measuring for 120 volts to test the ground is not a good test. But it;s better than nothing. Using the ground hole on a receptacle is only a good test if the receptacle is self grounding or if there is the proper pigtail installed between the metal box and the receptacle AND if the cable is properly connected to the metal box.
 
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Old 01-09-08, 04:30 PM
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What is the best way to check for a good ground. I have all my older receptacles with pigtails and secured to the metal box so I know for a fact that the receptacle to the box is good, I just want to make sure the whole ground circuit is good.
 
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Old 01-09-08, 04:38 PM
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The best way is to measure the resistance between the ground and the neutral. However, doing so requires that nothing be plugged in to the entire circuit. It also requires that you have a thorough understanding of what kind of reading to expect. For this reason I do not recommend it for novices.
 
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Old 01-10-08, 07:10 AM
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I like to try the things the easy way first.

I had a 50 year old house, two prong outlets with only one outlet in the kitchen being grounded. I had an electrcian in to fix one bad outlet in the basement and as I was watching him (and he didnt' even charge me extra!) I saw that he realized that they actually ran 14/2 WITH GROUND and nipped off all of the grounds (except in the kitchen). He just grabbed enough of the ground and pigtailed a new ground to the outlet.

After that, I chagned all the accessible outlets and every time we moved furniture, I changed those. Eventually, I had the entire house grounded.

You might want to take a close look at your wiring inside one of the boxes...maybe there already IS a ground.

Good luck,
Tom
 
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