MC Cable with NM boxes.

Status
Not open for further replies.
F

FrancisDoody

Senior Member
Location
Durham, CT
Arlington makes a great box for drop ceillings in the support of heavier loads. Can't see why we can't use this box in our commerical jobs. Inspector shot it down and called the state inspector and he backed him up saying it was illegal. I was looking at 314.3 and I believe that article supports my view. Any thoughts?

Thanks
 
FrancisDoody said:
Arlington makes a great box for drop ceillings in the support of heavier loads. Can't see why we can't use this box in our commerical jobs. Inspector shot it down and called the state inspector and he backed him up saying it was illegal. I was looking at 314.3 and I believe that article supports my view. Any thoughts?

Thanks
Click to expand...

It appears that exception #1 would cover this as long as you complied with that in your installation. Did he give you a code ref. as to why it is illegal?
 
FrancisDoody said:
It is the Cam light FL430 fixture. It is made in both plastic and steel.
Click to expand...
I noticed in the Arlington cut-sheet it stated "steel version for use where metal raceways are required". Did you install the steel version ?
 
nhfire77 said:
Is MC cable a 'raceway'? I vote no, but what do I know.
Click to expand...

You are correct. It is not listed as a "raceway".
My thinking was incomplete. When I notice "required metal", I thought of a plenum rated ceiling with a requirement for a metallic wiring system.
I would be hesitant to accept the non-metallic box in such a situation without further investigation.
 
Building type

Building type

Commerical Restaurant. Mixing plastic and MC never a great design idea. But going to an outlet box for an exit sign seems minor to me.
 
augie47 said:
You are correct. It is not listed as a "raceway".
My thinking was incomplete. When I notice "required metal", I thought of a plenum rated ceiling with a requirement for a metallic wiring system.
I would be hesitant to accept the non-metallic box in such a situation without further investigation.
Click to expand...

I think that the main intent of the code here is that the M part of the cable required to be grounded and the insertion of a NM box interrupts that continuity if the circuit is continued with an MC cable. The NM flex conduits require external bonding between the rigid conduit and the motor, for example. If the MC cable is externally bonded 'around' the NM box, it would provide the ground continuity and may be accepted by the AHJ, even though this is not spelled out in the NEC.
 
weressl said:
I think that the main intent of the code here is that the M part of the cable required to be grounded and the insertion of a NM box interrupts that continuity if the circuit is continued with an MC cable. The NM flex conduits require external bonding between the rigid conduit and the motor, for example. If the MC cable is externally bonded 'around' the NM box, it would provide the ground continuity and may be accepted by the AHJ, even though this is not spelled out in the NEC.
Click to expand...

Wouldn't the MC jacket be bonded on the opposite end, where it's fed from?
 
Looking at the box (both plastic and steel) it seems to have only one KO. I suppose if you use a duplex MC connector in the plastic box KO it would satisfy the sheath bonding requirement. I think we need to look at the box listing itself to see whether the manufacturer intends it to be used with MC. Nowhere do I get that impression from the instructions as they only mention NM cable.

-Hal
 
Doesn't the title answer your question?
NM Box wired with MC cable?
First, is the box listed for MC?
Second, ok, you brought a ground into the box. What about the outer shield? where is it connected? Do you have a grounding bushing? (being that least one end is not connected to a metallic part). A metallic connector should be connected to a metallic box (least on one side).
Personally when I install a metallic connector into a non metallic box (if I really have to do it), I install a grounding bushing.

FrancisDoody said:
Arlington makes a great box for drop ceillings in the support of heavier loads. Can't see why we can't use this box in our commerical jobs. Inspector shot it down and called the state inspector and he backed him up saying it was illegal. I was looking at 314.3 and I believe that article supports my view. Any thoughts?

Thanks
Click to expand...
 
My thinking was that the MC between the CFL-430 fixture and a junction box was bonded via a MC circuit that starts at the main panel. All junction boxes have a direct connection to an insulated #12 EGC. This offers an effective path for fault current to travel compared to travelling along the jacket of the MC. The bottom line is that I would perfer not to do it again. I believe the the phase "internal bonding means" is the key. Are they talking about a metal plate built into the NM box so that all threaded connector are bonded. Or are they describing the "contact point" inside of a NM box where the EGC is connected through a grouding screw.
 
paul said:
Wouldn't the MC jacket be bonded on the opposite end, where it's fed from?
Click to expand...

Of course, but what happens to the cable that continues FROM the box? If the box is metallic, the ground continuity is ensured by the use of listed connectors that bonds each cables jacket to the box and to each other, thus results in ground continuity. In the case of NM box there is no galvanic connection to it, so another means need to be provided. (You still connect all grounding wires together inside, but they maybe insulated/isolated from the MC.)
 
Status
Not open for further replies.
Top