MC Cable with NM boxes.

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curt swartz

Electrical Contractor - San Jose, CA
Location
San Jose, CA
Occupation
Electrical Contractor
Even though the cable is bonded on the supply end what happens if there is a ground fault to the connector at the load end? If the cable is fairly long the jacket my not allow enough current to open the overcurrent device before generating enough heat to start a fire. That is the reason that other flexible wring methods are not listed as the EGC in lengths over 6'. Without grounding bushing installed on the connector I would not accept the installation either.
 
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.

If there is no galvanic connection between the internal EGC and the MC armor, the armor still can get energized due to an insulation fault. In the case of NM box, nothing assures the galvanic continuity(bonding) of the MC armor betwwen the incoming and out-going MC cables. External bonding of the armor or the use of grounding bushings and internally bonding both bushings may be a solution. For large MC cables this is available, but it is questionable if household wiring cladded cables have such bushings available. (I have to admit that my exposure is more industrial than housing, but the same principles apply.)
 
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