Ground wire in MC cable

mbrooke

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Why is the ground wire in MC cable insulated? I feel like its more cost without anything in return so I'm guessing their must be a reason.
 

winnie

Senior Member
Tradition!

Presumably buried somewhere in the UL requirements.

Actually the ground wire in 'MC-AP' is non-insulated aluminium, and placed outside of the wrap that protects the other conductors, so that it is in contact with the spiral sheath. So you get a cheaper aluminium conductor that lets you use the sheath as the EGC. The 'full size' aluminium conductor is roughly the same size as the insulated copper conductors, so everything falls together pretty nicely.

-Jon
 

hbiss

EC, Westchester, New York NEC: 2014
Location
Hawthorne, New York NEC: 2014
Occupation
EC
We talked about redesigning NM to have an insulated ground. :happyyes:

Uninsulated EGCs in boxes are a never ending problem, especially now with AFCI and GFCI breakers that readily show up the problem of an EGC contacting the neutral if someone is not very careful installing devices.

Also, unlike NM, MC is likely to be used at higher voltages in commercial installations, so a bare EGC floating around in device boxes is asking for trouble.

So, really, an insulated EGC in NM equals higher quality and a safer, more trouble free installation IMO.

As far as MC-AP, as far as I'm concerned it should be considered type AC. MC was never allowed to use the armor as an EGC and unless the ground wire is required to be brought into the box and bonded to the box with a ground screw, I don't see the difference between it and AC. They both rely on just the box clamp to armor to provide the ground bond. I'm thinking the NEC caved to some manufacturer once again.

-Hal
 

mbrooke

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We talked about redesigning NM to have an insulated ground. :happyyes:
Missed it. :( Is there a thread?

Uninsulated EGCs in boxes are a never ending problem, especially now with AFCI and GFCI breakers that readily show up the problem of an EGC contacting the neutral if someone is not very careful installing devices.

Also, unlike NM, MC is likely to be used at higher voltages in commercial installations, so a bare EGC floating around in device boxes is asking for trouble.

So, really, an insulated EGC in NM equals higher quality and a safer, more trouble free installation IMO.



-Hal

Can I be honest? I always thought a bare EGC was cheesey, even thought that code should mandate insulation- though not without getting some vocal critiques.



As far as MC-AP, as far as I'm concerned it should be considered type AC. MC was never allowed to use the armor as an EGC and unless the ground wire is required to be brought into the box and bonded to the box with a ground screw, I don't see the difference between it and AC. They both rely on just the box clamp to armor to provide the ground bond. I'm thinking the NEC caved to some manufacturer once again.

Exactly! I am in full agreement here. To me MC without a dedicated ECG is simply AC cable.
 

kec

Member
Location
CT
We talked about redesigning NM to have an insulated ground. :happyyes:

Uninsulated EGCs in boxes are a never ending problem, especially now with AFCI and GFCI breakers that readily show up the problem of an EGC contacting the neutral if someone is not very careful installing devices.

Also, unlike NM, MC is likely to be used at higher voltages in commercial installations, so a bare EGC floating around in device boxes is asking for trouble.

So, really, an insulated EGC in NM equals higher quality and a safer, more trouble free installation IMO.

As far as MC-AP, as far as I'm concerned it should be considered type AC. MC was never allowed to use the armor as an EGC and unless the ground wire is required to be brought into the box and bonded to the box with a ground screw, I don't see the difference between it and AC. They both rely on just the box clamp to armor to provide the ground bond. I'm thinking the NEC caved to some manufacturer once again.

-Hal
Finally someone else has spoken up about the problems with with a uninsulated bond wire that I been thinking about for yes.

Especially now with trying to install a AFCI brk on a existing circuit you are trying to meet code with remodelling work amongst other things.

I couldn't agree more.
 

hbiss

EC, Westchester, New York NEC: 2014
Location
Hawthorne, New York NEC: 2014
Occupation
EC
Right. An insulated or bare EGC in itself doesn't make a difference as far as being able to carry a fault current and trip the breaker, sizes being equal.

The difference above with HCF vs MC is that HCF provides a redundant EGC using the armor plus a separate EGC run with the other conductors. MC is prohibited from using its armor as an EGC and has a separate EGC run with its other conductors.

Now they have MC-AP that has a full size bonding conductor under the armor allowing the armor to be used as an EGC. Like AC, there is no separate green grounding conductor. Only difference between it and AC is that there is no paper wrapping under the armor. Might as well call it MC-AC

-Hal
 
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winnie

Senior Member
One thing that makes MC-AP closer to normal MC than to AC is that that the full size ground conductor _may_ be used as a wire EGC in the same fashion as the green insulated EGC in normal MC.

MC-AP is _marketed_ and generally intended to be used where you cut the EGC wire short and use the connection to the sheath via to cable clamp as the EGC. Essentially exactly the way AC gets used. However you are _permitted_ to bring the wire into the enclosure and splice just like any other wire EGC...except that your splice needs to be suitable for aluminium conductors.....


-Jon
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
How's this for a guess, the cable was originally designed for pools where an insulated EGC was required and MC was easier to run than a metallic raceway. Of course this has changed.
 

hbiss

EC, Westchester, New York NEC: 2014
Location
Hawthorne, New York NEC: 2014
Occupation
EC
Looking for the history of when and why MC came about, I found this. From the Mike Holt archives, a post by Thomas Horne dated Feb. 25, 2002:

Thomas Horne said:
[FONT=Verdana, Arial][FONT=Verdana, Arial]The restrictions on wiring methods are based on the fire history of that type of occupancy. Since the jacket of type AC cable is the Equipment Grounding Conductor (EGC) if the jacket is damaged the EGC will be broken. In type MC cable any damage that will break the separately insulated EGC is likely to break the other circuit conductors and fault out the circuit. So in places were physical damage to the cable is more likely, such as a commercial garage or movie studio AC is forbidden. In places were large numbers of people are at risk such as movie theaters it is also forbidden. [/FONT][/FONT]
So here we have yet another opinion that MC came about because a separate insulated EGC offers a more reliable ground conductor.

-Hal
 

mbrooke

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Looking for the history of when and why MC came about, I found this. From the Mike Holt archives, a post by Thomas Horne dated Feb. 25, 2002:



So here we have yet another opinion that MC came about because a separate insulated EGC offers a more reliable ground conductor.

-Hal
Could be... but I think that involves a lot of "magical" thinking. Not saying thats how the code thinks/thought, but I personally on my own disagree. However, I thank you none the less for showing me this.
 

romex jockey

Senior Member
Location
Vermont
JMHO, but the jacket is 'wound' and (i'm told) 5-6 X's the length of interior conductors

they found this out with the old BX

if the shell was energized , it wouldn't trip

but not always because it was made off K&T, because of VD

we'd see this line of dots, in the old horsehair plaster...and we knew....:eek:

~RJ~
 
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