Thanks Smart$ and all. I appreciate your thoughts and feedback.
Smart$, are you suggesting that a bonding bushing would not be required if the connector is listed for use?
I would say that even though a particular connector is listed (and identified for use), that the armor on MC is not considered part of the EGC unless you're using a product like MCap or is it HCF MC (I think that these might be one of or to of the products which has armor suitable for use as EGC). I have actually never seen this type of MC used, I believe the interlocking armor is somehow connected more so than standard armor. I remember AC cable having a more significant armor that would meet the intent of 250.118(10)(b). What is NEC 250.118(10)(c) referring to?
And what do mfg's mean hen they state (per their spec's) that the armor is not an equipment ground, but supplements the internal grounding conductor equaling one grounding path? Is this all MC products?
I thought I was clear on this when I started, but I seem to have gotten more tangled as things move along? I questioned things on behalf of the owner in trying to save 500 or 600 bonding bushings for larger MC feeders. It seemed the material and labor savings was worth while. No good deed goes unpunished.
Thanks
There is bonding like what we do at the service, bond the grounded/neutral to the the earth and all equipment grounds and make sure the service raceways have an extra level of connectedness by using a bond bushing or a bonding lock nut....
But then there is bonding meaning, make sure metal parts are continuous and bonding meaning making sure there is an effective equipment ground fault path which is where Smart gets his reference-
250.4 General Requirements for Grounding
and Bonding
(3) Bonding of Electrical Equipment. Normally non?
current-carrying conductive materials enclosing electrical
conductors or equipment, or forming part of such equip-
ment, shall be connected together and to the electrical sup-
ply source in a manner that establishes an effective
ground-fault current path.
(4) Bonding of Electrically Conductive Materials and
Other Equipment. Normally non?current-carrying electri-
cally conductive materials that are likely to become ener-
gized shall be connected together and to the electrical supply
source in a manner that establishes an effective ground-fault
current path.
(5) Effective Ground-Fault Current Path. Electrical
equipment and wiring and other electrically conductive ma-
terial likely to become energized shall be installed in a man-
ner that creates a low-impedance circuit facilitating the
operation of the overcurrent 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. The
earth shall not be considered as an effective ground-fault
current path.
The armour on regular MC is bonded by the connector and that is all that is required since it is only there to protect the conductors. There is a separate EGC to make sure there is an effective fault clearing path. That is the way MC works.
Sure if there is a fault the armour is going to be part of the fault path but so is the building steel, metal studs, ceiling joists, HVAC ducts, sprinkler pipes, ceiling grids, trouffer lights, drywall screws, metal door frames....none of which are part of the effective fault clearing path. That is what the EGC inside the armour is for.
If one were to rely on the spiral armour to clear a fault trouble would follow. Those spirals act like a coil or a winding and make a high impedance fault path instead of a low one. That was the problem with the old BX cable. It was the armour that started fires when there was a fault so a shorting strip was added.
MCAP mimics BX with the fat chunk of uninsulated aluminium ran with the circuit conductors. Hospital grade MC has the shorting strip and an insulated EGC so there is a redundant EGC/low impedance path.
If the contractor was using FMC instead of MC he would need an EGC pulled in but he would not need anything more than a listed fitting. Same thing with MC.
It is a good fight and one worth fighting.
