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woodturner 08-04-2008 10:54 PM

ground wire in mc circuit
I am running a circuit of recepticals using mc cable. I am starting at an existing junction box in the ceiling, which has mc cable running to it, but has no ground wire. The cable I purchased is 12-2 with a green ground wire. My question is, what do I do with the green ground wire in my new circuit?

Ash 08-05-2008 04:43 PM

Nothing. if you have no ground dont connect the wire. Dont connect ground between the 2 new sockets either

There may be requirements like to use sockets without ground connection or the like (check that)

Speedy Petey 08-05-2008 05:05 PM

Uhh, sorry Ash, BAD advice!
PLEASE be careful giving advice like this on these forums. Be aware that this site is US based and the majority of the posters are either from the US or Canada.

YES, you certainly DO connect the ground wire, IF the existing cable is AC cable.
If there is no ground WIRE present then the old cable is NOT MC. It might be AC cable, which has the thin aluminum tracer strip. This strip is what gives the outer sheathing it's continuity and allows its use as a ground.
In this case the ground wire simply connects to the box using a ground screw in the small threaded hole in the box. If there is more than one MC cable then splice all the grounds together with a tail to the box.

IF this is not AC cable it cannot be used as a ground and you CANNOT extend this circuit.

Stubbie 08-05-2008 06:12 PM

If you can run a new circuit for your receptacles from the panel this is by far the best choice.
For the sake of accuracy and code acceptable methods you can extend this circuit even if the old armored cable does not have the bonding strip for the use of the metal of the cable for equipment ground.

NEC 250.130(C)1-5 provides options to allow the extension of this circuit for receptacles from the junction box.

Now having said that we will be glad to give you those instructions, but it is my opinion that complying with those options will be no easier than running a new branch circuit from your panel in almost all cases.

Generally I nor Speedy reccommend using 250.130 as it is rare that it becomes an advantage or easier over a new branch circuit.

Ash 08-06-2008 06:58 AM

Cant you just install sockets with 2 holes if you dont have ground ?

fw2007 08-06-2008 08:12 AM


Originally Posted by Ash (Post 146360)
Cant you just install sockets with 2 holes if you dont have ground ?

I don't think you'll find any outlets without ground any longer. Why don't you ohm out the ground in the existing boxes?
Check the resistance (with the circuit dead) between the box and a known good ground, such as a newer outlet's ground.

I have tried using the thin "tracer" wire in old BX cable, and I could not attach it to the green ground screw in the box without having it break. I ended up doing what the previous electrician did- just wrapped it around the outside of the BX, then attached the clamp.
I checked the grounds, and they are good.

For all new work, I always use NMB cable.


Stubbie 08-06-2008 03:49 PM


Originally Posted by Ash (Post 146360)
Cant you just install sockets with 2 holes if you dont have ground ?

Ash...we are not allowed to continue or extend an ungrounded branch circuit. We can extend it but the added outlets must be of the grounding type and have an equipment ground ran as laid out in 250.130.

We are allowed to replace existing ungrounded outlets with the same. Meaning another 2 prong receptacle.

Speedy Petey 08-06-2008 03:52 PM

Two prong receptacles certainly ARE available, and they are legal to use as replacements.

The tracer is NOT a ground and is NOT meant to be terminated anywhere. Wrapping it back on the sheath IS what you are supposed to do. In fact, just bending it back is OK too.
The tracer is just there to give linear continuity to the AC cable sheath. Without it fault current will want to follow the spiral of the sheath an can overheat.

Simply using an ohm meter to check continuity is NOT all that is required to check for ground. Some metallic wiring methods will show continuity to ground but are not acceptable methods of an equipment grounding conductor.

fw2007 08-06-2008 04:16 PM

I apologize for making it sound too simple. I have to admit that I wouldn't trust an ohmmeter, which doesn't subject the ground system to any current, and therefore cannot reliably test for a good ground.

Sounds like this problem is best solved by rewiring the branch circuit, as another post recommended.
I've been doing that all over my house now. Lots of old AC/BX, and I am replacing it with NMB, mostly #12.

I'm gonna have to think a little more about my posting before I hit the submit button :surrender:


woodturner 08-06-2008 09:16 PM

correction on posting
I realized after reading all the replies to my post, (and thank you all for your input), that I made an error describing the circuit. It is conduit coming from the main panel to the junction box in the cieling. From there I installed mc cable 12-2 with ground to the receptical circuit.

J. V. 08-07-2008 11:34 AM

Well now, that changes this thread completely. I wish you would have told us that from the start. Since you have conduit, you have a ground, provided it is unbroken all the way back to the panel. Not meaning it has to go straight to the panel, but must have a continuous unbroken path back to the panel.
Connect the green wire to the ceiling box and all receptacles.

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