Wiring questions

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Old 12-09-05, 10:31 PM
jimcpa
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Wiring questions

I am a homeowner and am rewiring a 55 year old home (mix of knob and tube and romex circuits). I've been using emt for exposed work (garage and attic) and romex for runs within the walls (all drywall). All of my boxes in the drywall are old work plastic. My questions are...

I'd like to use aluminum clad amored cable within the walls for the rest of my work (is this MC?). Can I terminate MC or other metal clad cable in a plastic box as long as the cable has a ground conductor? By ground conductor I mean the copper grounding conductor which I'll tie into the other grounding conductors in the junction box, not the bonding conductor in (old) bx cable. I haven't used armored cable for 20 years, so I'm not familiar with clamps, fixtures etc for this type of wiring.

The old work plastic boxes (I think they are Carleton boxes) have a round knock out which looks like it's marked "mc". Is it ok to drill a suitable sized hole if needed in plastic for bringing in mc cable if the knockout is not where I need it?

Finally, the plastic box indicates that mc cable must be "bonded". I'm not sure what this means - - what's the point of bonding the metal clad cable to a non-conductive box, especially if I'm using cable with a grounding conductor to tie into the other grounds in the box?

Thanks a lot.
 
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Old 12-10-05, 03:30 PM
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Originally Posted by jimcpa
I'd like to use aluminum clad amored cable within the walls for the rest of my work (is this MC?).
I am not an electrician, but I have a roll of 12/3 with ground MC in my basement. The armor is aluminum. The conductors are Black, Red, White and Green.

Originally Posted by jimcpa
Can I terminate MC or other metal clad cable in a plastic box as long as the cable has a ground conductor?
1999 NEC Sec. 370-3 allows you to use metal raceways and metal-jacketed cables with nonmetallic boxes with the condition you note below.

Originally Posted by jimcpa
The old work plastic boxes (I think they are Carleton boxes) have a round knock out which looks like it's marked "mc". Is it ok to drill a suitable sized hole if needed in plastic for bringing in mc cable if the knockout is not where I need it?
I've done this for ENT or "smurf" tube. A step bit works great. Make sure you get the actual size, not nominal.

Originally Posted by jimcpa
Finally, the plastic box indicates that mc cable must be "bonded". I'm not sure what this means - - what's the point of bonding the metal clad cable to a non-conductive box, especially if I'm using cable with a grounding conductor to tie into the other grounds in the box?
Maybe a pro has a better answer but ...
The NEC says if you have multiple metal-jacketed cables entering the box, the jackets of both must be bonded together to provide a continuous path.

For larger sizes such as 2", locknuts are available with a grounding screw. I've never looked for them in 1/2 or 3/4 nominal size, and in fact at that size they might impair your ability to turn the locknut inside the box, but you would want to bring your MC into a listed clamp and then thread the groundable locknut on there. Maybe somebody makes a washer with a lug that you could put under the regular locknut.

I am assuming you have a green grounding conductor inside your MC. You would then connect a pigtail from the locknut screw to the grounding wirenut, where you would also connect the device ground and the green conductor from the MC.

I think, but I do not know, that the idea is to make sure that anything metal anywhere in the electrical system is grounded so it does not become a shock hazard. If you only have one MC coming into the box, it might be acceptable to just have it grounded on the other end, for example at the load center. I have no idea if that is addressed specifically in the NEC.

Also maybe you've already figured this out but if you're bringing cable thru the top or bottom of the box, unless you really oversize the hole in the wall, you have to loosely assemble things first, then insert into the wall, then pull the connector down into the box and thread on the locknut.

Have you thought about using a metal box with clamps in the back for BX? It's a lot easier, although you will have to put a 10x32 ground screw in the back.

And don't forget the anti-short bushings.
 
  #3  
Old 12-10-05, 09:59 PM
jimcpa
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Thanks for your help - - you're right, anything conductive should be tied into the grounding system. I suppose that, without a bonding connection between the aluminum cable sheath and the ground, a short within the MC cable could make the sheathing "hot". That hadn't occurred to me. Does anyone know where to find a bushing or fitting with a ground lead to use in plastic boxes? I'll be using primarily 12-2 or 12-3 aluminum sheathed MC.

Thanks again.
 
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