MC cable and grounding

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goldstar

Senior Member
Location
New Jersey
Occupation
Electrical Contractor
Is the metal sheath of MC considered an effective ground path ? In looking at the make-up of AC cable the metallic strip installed inside the AC jacket (or as I like to call it a "shunt") is there to help form an effective ground path for that cable assembly. MC obviously does not have that strip. I'm just trying to find out what most EC's are doing. I'm getting conflicting answers from some of my fellow EC's and some in my contractors' association as to what their practices are. The following questions are in instances where an isolated ground is not required and for the purposes of this thread let's say were talking about 12/2 MC:

  1. If the MC jacket is considered an effective ground path do you still use green ground screws and bond green wires to the JB ?
  2. If the answer to #1 is yes are you just splicing the green wires together and leaving them tucked inside the JB ?
  3. Are you installing grounding pig-tails for the devices you are installing in the JB ?
  4. Are you using self-grounding devices ? If so, do you believe you still need grounding pig-tails ?
  5. If your run of MC leaves a breaker panel via the use of an approved MC connector can you terminate the load end into a plastic JB ?
  6. If the answer to #5 is yes, do you still need to use an approved MC connector or can you terminate the MC into say a 1-gang plastic nail-on JB ?

I know some of these questions seem kind of unusual for an experienced EC but these are some of the discussions I've had over the years. Many times I don't get straight forward "yes" or "no" answers. In many cases I get "I've always done it that way" or "that's the way I've been taught".

I'd appreciate any opinions any of you may have. If you have Code sections you can cite please include them but it's not required.

Thanks in advance.:thumbsup:
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Semi-Retired Electrician
I don't have time to look up code ref. so you can look in 330, 250, 404, & 406

In short.............
MC cables sheath is not considered an effective ground or allowed to be used as such
You do need to ground a metal box via green screw or clip
If you are putting a receptacle or switch in the box and using a raised cover with the flat corners, you don't need the pigtail to the devices. You can use the pigtail but it's not necessary
To my knowledge, there are no plastic boxes that are listed for connections of a metal connector, especially the nail-on with the punch out openings
 

kwired

Electron manager
Location
NE Nebraska
Though there is in more recent years "MC-AP" and other brands have similar products, that does have an aluminum bonding wire similar to what AC cable has and the sheath of that is rated for grounding just like it is for AC cable.
 

Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
Is the metal sheath of MC considered an effective ground path ? In looking at the make-up of AC cable the metallic strip installed inside the AC jacket (or as I like to call it a "shunt") is there to help form an effective ground path for that cable assembly. MC obviously does not have that strip. I'm just trying to find out what most EC's are doing. I'm getting conflicting answers from some of my fellow EC's and some in my contractors' association as to what their practices are. The following questions are in instances where an isolated ground is not required and for the purposes of this thread let's say were talking about 12/2 MC:

  1. If the MC jacket is considered an effective ground path do you still use green ground screws and bond green wires to the JB ?
  2. If the answer to #1 is yes are you just splicing the green wires together and leaving them tucked inside the JB ?
  3. Are you installing grounding pig-tails for the devices you are installing in the JB ?
  4. Are you using self-grounding devices ? If so, do you believe you still need grounding pig-tails ?
  5. If your run of MC leaves a breaker panel via the use of an approved MC connector can you terminate the load end into a plastic JB ?
  6. If the answer to #5 is yes, do you still need to use an approved MC connector or can you terminate the MC into say a 1-gang plastic nail-on JB ?

I know some of these questions seem kind of unusual for an experienced EC but these are some of the discussions I've had over the years. Many times I don't get straight forward "yes" or "no" answers. In many cases I get "I've always done it that way" or "that's the way I've been taught".

I'd appreciate any opinions any of you may have. If you have Code sections you can cite please include them but it's not required.

Thanks in advance.:thumbsup:

As kwired stated, there is MC that the jacket is listed as the ground and there is MC where the cable is not listed as ground. sot that is question #1
2. When listed, as a ground it won't have a green wire, unless it is hospital grade MC listed as hospital grade. In that case article 517 requires the green wire to be bonded to the metal box as a redundant ground.
3. and 4. Pigtail isn't require for self grounding devices unless it is a raised cover mounted receptacle. But I see more contractors installing the pigtail and attaching than not. This is similar to wheter you run a green wirre in conduit. Not required by code, but I have never seen a job with engineer's specifications that doesn't require one anyway.
5. I don't know the answer right now and I'm not going to look it up at the moment, but I doubt it. even if the cable is listed as a grounding means.
 

JFletcher

Senior Member
Location
Williamsburg, VA
To answer your questions number one, it is not an effective ground path, number 2 is not applicable, number 3 yes, or buying boxes with pigtails already installed, number 4 sometimes, if they are not self grounding they get pigtailed, number 5 no plastic boxes I know of that are designed to take MC cable, though those with knockouts could possibly be used with an MC connector. In no case do I know of is it legal to terminate MC anywhere without connectors, and number six, well you got me there... Looking at code section 330.40 for MC cable, it says fittings used must be listed and identified not that they must be used unlike Section 320.40 for AC cables which is substantially different. I would think most plastic single gang boxes are listed for type NM or UF cable only, which would preclude running MC into one.

There is also the last line of code section 314.17 c that states 'in all instances all permitted wiring methods shall be secured to the boxes".* even if you can jam a piece of MC cable through the one-way holding fingers or opening in a box, I'd be willing to bet that 110.3B, manufacturer's instructions, will preclude the use of MC Cable in a standard single gang nail on box.

As K wired wrote, MCAP has a bonding wire on the sheath like armor-clad does. I would have to investigate as to whether or not it qualifies completely as the EGC... I have the feeling that it does since the stuff is primarily used in hospitals.

*I haven't looked at this code section in awhile, however that means that busting out the little flip wing in those Carlon boxes or using fiberglass boxes without the internal clamps is a code violation for NM cable, one that I have never seen or heard of being cited.
 

nickelec

Senior Member
Location
US
Is the metal sheath of MC considered an effective ground path ? In looking at the make-up of AC cable the metallic strip installed inside the AC jacket (or as I like to call it a "shunt") is there to help form an effective ground path for that cable assembly. MC obviously does not have that strip. I'm just trying to find out what most EC's are doing. I'm getting conflicting answers from some of my fellow EC's and some in my contractors' association as to what their practices are. The following questions are in instances where an isolated ground is not required and for the purposes of this thread let's say were talking about 12/2 MC:

  1. If the MC jacket is considered an effective ground path do you still use green ground screws and bond green wires to the JB ?
  2. If the answer to #1 is yes are you just splicing the green wires together and leaving them tucked inside the JB ?
  3. Are you installing grounding pig-tails for the devices you are installing in the JB ?
  4. Are you using self-grounding devices ? If so, do you believe you still need grounding pig-tails ?
  5. If your run of MC leaves a breaker panel via the use of an approved MC connector can you terminate the load end into a plastic JB ?
  6. If the answer to #5 is yes, do you still need to use an approved MC connector or can you terminate the MC into say a 1-gang plastic nail-on JB ?

I know some of these questions seem kind of unusual for an experienced EC but these are some of the discussions I've had over the years. Many times I don't get straight forward "yes" or "no" answers. In many cases I get "I've always done it that way" or "that's the way I've been taught".

I'd appreciate any opinions any of you may have. If you have Code sections you can cite please include them but it's not required.

Thanks in advance.
Do you do work in NYC?. Most people here put a ground tail because they don't know any better

Sent from my SM-G950U using Tapatalk
 

Isaiah

Senior Member
Location
Baton Rouge
Occupation
Electrical Inspector
Is the metal sheath of MC considered an effective ground path ? In looking at the make-up of AC cable the metallic strip installed inside the AC jacket (or as I like to call it a "shunt") is there to help form an effective ground path for that cable assembly. MC obviously does not have that strip. I'm just trying to find out what most EC's are doing. I'm getting conflicting answers from some of my fellow EC's and some in my contractors' association as to what their practices are. The following questions are in instances where an isolated ground is not required and for the purposes of this thread let's say were talking about 12/2 MC:

  1. If the MC jacket is considered an effective ground path do you still use green ground screws and bond green wires to the JB ?
  2. If the answer to #1 is yes are you just splicing the green wires together and leaving them tucked inside the JB ?
  3. Are you installing grounding pig-tails for the devices you are installing in the JB ?
  4. Are you using self-grounding devices ? If so, do you believe you still need grounding pig-tails ?
  5. If your run of MC leaves a breaker panel via the use of an approved MC connector can you terminate the load end into a plastic JB ?
  6. If the answer to #5 is yes, do you still need to use an approved MC connector or can you terminate the MC into say a 1-gang plastic nail-on JB ?

I know some of these questions seem kind of unusual for an experienced EC but these are some of the discussions I've had over the years. Many times I don't get straight forward "yes" or "no" answers. In many cases I get "I've always done it that way" or "that's the way I've been taught".

I'd appreciate any opinions any of you may have. If you have Code sections you can cite please include them but it's not required.

Thanks in advance.:thumbsup:

I would suggest you look at section 250.118(10) in the 2017NEC
 

hbiss

EC, Westchester, New York NEC: 2014
Location
Hawthorne, New York NEC: 2014
Occupation
EC
General rules of thumb concerning MC- you install it as you would AC as far as boxes and connectors go, meaning NO plastic. Steel boxes and approved connectors only.

You treat the green ground the same as you would the ground wire with Romex unless you are using MC-AP that doesn't have a green ground but a bare ground under the armor. With that you don't wrap the ground wire back like you do with AC, just cut it off flush with the end of the armor and use it like AC.

-Hal
 
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