Go Back  DoItYourself.com Community Forums > Electrical, AC & DC. Electronic Equipment and Computers > Electrical - AC & DC
Reload this Page >

Correcting open equipment ground on Receptacles fed from BX with no EGC?

Correcting open equipment ground on Receptacles fed from BX with no EGC?

Reply

  #1  
Old 12-11-12, 10:40 PM
Member
Thread Starter
Join Date: Dec 2012
Location: USA
Posts: 3
Correcting open equipment ground on Receptacles fed from BX with no EGC?

I just purchased a home an need to correct some open grounds that were flagged as not to code by an inspector. The receptacles are fed by OLD BX cable (cloth and rubber looking insulation) and there is no equipment grounding conductor and are housed in metal outlet boxes. Can I just connect a ground connection on the receptacle to the outlet box and assume the flexible metal shielding of the BX as the equipment ground conductor? Everything I read in the NEC 2011 shows that I can, but I read in a lot of other forums that it is not acceptable. What am I missing?

NEC 250.118 states Armor of Type AC cable as provided in 320.108
- here I assume BX is considered AC (armored Cable)

NEC 320.108 states Type AC cable should provide an adequate path for fault current as required by 250.4(A)(5) or B(4) to act as an equipment grounding conductor
- Everything still looks good

NEC 250.4 (A)(5) States "Effective Ground-Fault Current Path": Electrical equipment and other electrically conductive material likely to become energized shall be installed in a manner that creates a low-impedance circuit facilitating the operation of the over-current device or ground detector for high impedance grounded systems. It shall be capable of safely carrying the maximum ground fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source. Earth is not considered an effective ground force.
-
- WOW, ok well how does anyone verify that? I don't think it can be done.
But from what I can tell, there is nothing that states that the old style BX cable can not be used.

As a last resort, I would add GFCI outlets to comply with code per, 406(D)(2)(b) and labeling it "GFCI protected" and "No equipment Ground", but still connect a ground to the BX casing.
 
Sponsored Links
  #2  
Old 12-11-12, 11:48 PM
Nashkat1's Avatar
Member
Join Date: Mar 2012
Location: USA
Posts: 8,470
Can I just connect a ground connection on the receptacle to the outlet box and assume the flexible metal shielding of the BX as the equipment ground conductor? Everything I read in the NEC 2011 shows that I can, but I read in a lot of other forums that it is not acceptable. What am I missing?
What you're missing is that while you can, and probably should, bond the receptacles to their metal boxes, that will do nothing to connect the receptacles to ground unless the boxes themselves are connected to ground.

You cannot assume anything about the condition of any electrical device, system or installation. You must test and verify the condition. That is what the NEC requires.

You should buy two tools. One is an analog multimeter. The other is Wiring Simplified: Based on the 2011 National Electrical Code, which explains both how and why electrical systems are connected and assembled the way they are. You may be able to find both of these tools in the electrical aisle at your local Big Box HI center.
 
  #3  
Old 12-12-12, 07:28 AM
Member
Thread Starter
Join Date: Dec 2012
Location: USA
Posts: 3
Thanks for the reply Nashkat,

The fasteners at the main service panel and outlet box are securely fixed to the BX. The main service panel is grounded. I don't see what a multimeter will do, because ultimately I have to be able to know the impedance of the BX shield to know if it has a low enough impedance to operate the overcurrent breaker at the panel. Should I do an ohm meter test from the box back to the service panel to determine the Impedance of the shield? Then determine the available fault current to the circuit?
 
  #4  
Old 12-12-12, 08:56 AM
ray2047's Avatar
Group Moderator
Join Date: Mar 2006
Location: USA
Posts: 33,041
My totally unsupported but piece of mind for me only testing method is to connect a pigtail lamp holder with a 60 watt bulb to the box and the hot. Flip the breaker on for a few seconds and see if the bulb fully lights (not dim). If it does the cable is probably capable of carrying a fault current long enough to trip the breaker.

Warn every one not to touch metal appliances while your testing.
 
  #5  
Old 12-12-12, 09:16 AM
Member
Join Date: Jul 2009
Location: NYC
Posts: 302
I find this thread interesting and hope to see more pro's chime in. Out here in BX land, just to be clear BX/AC (bonding wire) - we have traditionally always treated the bond wire/jacket as the ground + just the screw to the box. Never even added a pigtail from the box to the ground screw.
 
  #6  
Old 12-12-12, 04:46 PM
Member
Join Date: Aug 2009
Location: USA
Posts: 4,941
with the breaker off, you can measure the resistance between the ground and neutral at the outlet. It's certainly farther than a short would have to travel, but could be helpful in analyzing the ground.
 
  #7  
Old 12-12-12, 04:49 PM
pcboss's Avatar
Forum Topic Moderator
Join Date: Jul 2002
Location: Maryland
Posts: 14,587
The AC cable needs to be listed as a grounding means. Early AC cable did not have the bond strip or wire under the sheath that provides the low impedance path to trip the breaker. Without the bond strip the sheath can glow red hot from a short and not trip the breaker. The resistance is just too high.

Look near your panel at the cable connections for a thin wire backwrapped in the spirals outside the cable. This is the bond strip.
 
  #8  
Old 12-12-12, 05:03 PM
Member
Join Date: Jul 2009
Location: NYC
Posts: 302
Sadly....at the local store, they stock AC and MC. And I'm pretty sure the average HO will just see the MC being a cheaper price...and install that thinking it is BX...
 

Last edited by pcboss; 12-13-12 at 06:03 AM.
  #9  
Old 12-12-12, 06:17 PM
pcboss's Avatar
Forum Topic Moderator
Join Date: Jul 2002
Location: Maryland
Posts: 14,587
Pingable, I do not understand. TYpe MC cable has a grounding conductor built in. The difference is that the sheath is not used as part of the grounding like Type AC cable which uses the sheath and the bond wire.
 
  #10  
Old 12-12-12, 09:23 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 46,064
In my opinion......and this is just MY opinion and thoughts.

I run into problems constantly in old houses where the BX jacket is not tight in the clamp, has pulled out of the clamp or is corroded from age causing no or poor ground.

I can agree with the pigtail to the device because many times I see the box recessed an inch below the finished surface due to renovations or as in the case of tile locations and long screws were used to hold the device and it was not a tight "device to box" fit.

Mod Note: The code requires the box to be set back no more than 1/4" from a non-combustible surface, or be flush with a combustible surface. Box extenders are available to fix too deep of a setback.

I personally think MC cable is great. I like the added ground wire. A true 100% positive ground and the jacket is strictly there as the conduit.

I can see the same problem happening with AC cable that BX was prone to..... grounding issues. You have the bond wire with AC but there is still the clamp issue.

I DO NOT like the cable clamp system that's used in gem and 1900 boxes.
 

Last edited by pcboss; 12-13-12 at 06:06 AM.
  #11  
Old 12-12-12, 09:39 PM
Nashkat1's Avatar
Member
Join Date: Mar 2012
Location: USA
Posts: 8,470
The fasteners at the main service panel and outlet box are securely fixed to the BX.
I don't understand. Are you talking about the cable connectors?

The main service panel is grounded.
That's good, but it isn't necessarily related to your question.

I don't see what a multimeter will do, because ultimately I have to be able to know the impedance of the BX shield to know if it has a low enough impedance to operate the overcurrent breaker at the panel. Should I do an ohm meter test from the box back to the service panel to determine the Impedance of the shield? Then determine the available fault current to the circuit?
You could. Or you could do the test and the visual check that Ray and PCboss suggested.
 
  #12  
Old 12-13-12, 06:02 AM
pcboss's Avatar
Forum Topic Moderator
Join Date: Jul 2002
Location: Maryland
Posts: 14,587
The grounding path circuit length on AC cable without the bond strip is many times the actual length of the cable due to the spiral wrap.

A meter may show the AC as grounded, but this does not guarantee an effective or low impedance path back to the panel.
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes