2011 NEC, 250.118 (10) undersized ground conductor ???

Ranger86

Member
Location
Las Vegas, NV
This section talks about using the metallic sheath in combination with the ground conductor. Is this section allowing the ground conductor to be undersized and not meet table 250.122 requirements ???
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
If you read it again it says "The combined metallic sheath and uninsulated equipment grounding/bonding conductor". Sounds like the reference is actually for a cable like MC-ap where the sheath and the bare bonding conductor act in concert to form the EGC.
 

Ranger86

Member
Location
Las Vegas, NV
We have a MC type cable, and the ground conductor is smaller than Table 250.122. At this point, I do not know if it is listed as stated in 250.118 (10) c. But, if it is listed, does this mean that the ground conductor can be smaller than what it should be per T250.122 ?
 

Smart $

Esteemed Member
Location
Ohio
We have a MC type cable, and the ground conductor is smaller than Table 250.122. At this point, I do not know if it is listed as stated in 250.118 (10) c. But, if it is listed, does this mean that the ground conductor can be smaller than what it should be per T250.122 ?
Exactly what do you have?
Size and number of conductors.
Exact labeling (on packaging or reel; may be on binder wrap between armor and conductors).

Insulated EGC's in MC cable are sized for typical OCPD ratings. Other EGC sizes are available by special order.

There are several "flavors" of Type MC cable, and we don't know what you have until you tell us.

We don't read minds... at least not very well at times. :D
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
We have a MC type cable, and the ground conductor is smaller than Table 250.122. At this point, I do not know if it is listed as stated in 250.118 (10) c. But, if it is listed, does this mean that the ground conductor can be smaller than what it should be per T250.122 ?

If this ground conductor is actually an EGC then in cable sizes #14, #12 and #10 AWG the EGC must be the same size. Once you exceed #10 AWG then the EGC will be smaller as permitted in T250.122.
 

MasterTheNEC

Host of ElectricianLIVE.com
Location
McKinney, Texas
Occupation
Master Electrician & Director of Codes and Standards
If you read it again it says "The combined metallic sheath and uninsulated equipment grounding/bonding conductor". Sounds like the reference is actually for a cable like MC-ap where the sheath and the bare bonding conductor act in concert to form the EGC.
Thats exactly what that provision is for....Things like MC-SG and MC-AP type products.
 

Ranger86

Member
Location
Las Vegas, NV
Thats exactly what that provision is for....Things like MC-SG and MC-AP type products.
OK. Thank you. I wish that somewhere in 250.118 (10), it had "ground conductor can be smaller than the minimum required per T250.122". This would make it much easier to read. Thanks again !!!!
I will check on the MC cable UL listings.
 

Ranger86

Member
Location
Las Vegas, NV
Exactly what do you have?
Size and number of conductors.
Exact labeling (on packaging or reel; may be on binder wrap between armor and conductors).

Insulated EGC's in MC cable are sized for typical OCPD ratings. Other EGC sizes are available by special order.

There are several "flavors" of Type MC cable, and we don't know what you have until you tell us.

We don't read minds... at least not very well at times. :D
I have a couple of scenarios, here is one of them ....
A parallel 400A feeder, 400A OCPD, the conductor size also had to be increased for voltage drop. So, I ended up with parallel 4/0 phase & neutral with #2 ground. The 4/0 MC cable comes with #4 ground conductor.
 

Smart $

Esteemed Member
Location
Ohio
I have a couple of scenarios, here is one of them ....
A parallel 400A feeder, 400A OCPD, the conductor size also had to be increased for voltage drop. So, I ended up with parallel 4/0 phase & neutral with #2 ground. The 4/0 MC cable comes with #4 ground conductor.
You cannot use standard MC for that application. You will have to special order MC with the appropriate EGC size (note a few manufacturers offer non-standard-size EGC in there product listing, but your local supply house usually does not stock them).
 

Ranger86

Member
Location
Las Vegas, NV
You cannot use standard MC for that application. You will have to special order MC with the appropriate EGC size (note a few manufacturers offer non-standard-size EGC in there product listing, but your local supply house usually does not stock them).
This is the part that gets confusing to me..... So, 250.118 (10), does it or does it not allow the ground conductor to be smaller than what the T250.122 requires ?
 

Smart $

Esteemed Member
Location
Ohio
This is the part that gets confusing to me..... So, 250.118 (10), does it or does it not allow the ground conductor to be smaller than what the T250.122 requires ?
It does not. The general statement to (10) says effective ground-fault current path... and that effectiveness is determined by an EGC sized per 250.122. Where the combined sheath and conductor is listed as an EGC, the listing would have to indicate an effective size in order for 250.122 to be applied correctly. AFAIK, such are usually redundant EGC's, and compliance is met by the 'primary' EGC.
 

Ranger86

Member
Location
Las Vegas, NV
It does not. The general statement to (10) says effective ground-fault current path... and that effectiveness is determined by an EGC sized per 250.122. Where the combined sheath and conductor is listed as an EGC, the listing would have to indicate an effective size in order for 250.122 to be applied correctly. AFAIK, such are usually redundant EGC's, and compliance is met by the 'primary' EGC.
Thank you everybody, for the clarification on this.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
Ranger-- when we talk about T. 250.122 you should say equipment grounding conductor not ground conductor. A neutral can be looked at as a ground conductor because it is grounded. One normally says grounded conductor or neutral for the neutral-- although there is a technical difference and the grounding conductor is the equipment grounding conductor

This helps us understand what you are asking. At first I thought you wanted to know if the grounded conductor could be smaller than the equipment grounding conductor. Thanks
 

Ranger86

Member
Location
Las Vegas, NV
Ranger-- when we talk about T. 250.122 you should say equipment grounding conductor not ground conductor. A neutral can be looked at as a ground conductor because it is grounded. One normally says grounded conductor or neutral for the neutral-- although there is a technical difference and the grounding conductor is the equipment grounding conductor

This helps us understand what you are asking. At first I thought you wanted to know if the grounded conductor could be smaller than the equipment grounding conductor. Thanks
Sorry about that, thanks for letting me know. I am trying to get the manufacturer data to verify the listings of the MC cable... the manufacturer was bought by another one around 10 years ago so I am hoping they have that data.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Sorry about that, thanks for letting me know. I am trying to get the manufacturer data to verify the listings of the MC cable... the manufacturer was bought by another one around 10 years ago so I am hoping they have that data.
Just curious, exactly what data are you looking for?
 

MasterTheNEC

Host of ElectricianLIVE.com
Location
McKinney, Texas
Occupation
Master Electrician & Director of Codes and Standards
This is the part that gets confusing to me..... So, 250.118 (10), does it or does it not allow the ground conductor to be smaller than what the T250.122 requires ?
@Rander86-

I am a bit confused at your question. Where in Section 250.118(10) does it say anything regarding size of the EGC? It basically says that if you meet one of the following in (a) through (c) then the metal sheathing qualifies no different than it would for RMC, IMC, EMT and so on. Nothing in Section 250.118 has to do with sizing....but when you get to Section 250.122(A) you will read in the first sentence that it has to do with EGC's of the "Wire Type" when using T 250.122.

Hope that clears your confusion.
 

Ranger86

Member
Location
Las Vegas, NV
@Rander86-

I am a bit confused at your question. Where in Section 250.118(10) does it say anything regarding size of the EGC? It basically says that if you meet one of the following in (a) through (c) then the metal sheathing qualifies no different than it would for RMC, IMC, EMT and so on. Nothing in Section 250.118 has to do with sizing....but when you get to Section 250.122(A) you will read in the first sentence that it has to do with EGC's of the "Wire Type" when using T 250.122.

Hope that clears your confusion.
Here's where my confusion started ..... RMC, IMC, EMT can be used as equipment ground conductors per 250.118 (2), (3), and (4). So, the way I see this, if I have a feeder that would require a #2 EGC, and I am using EMT conduit, the EMT conduit would be equivalent to the #2 EGC and there is no need to install a EGC inside the conduit. Now, in 250.118 (10), talks about using the combination of the metallic sheath (or armor) with the EGC in the mc cable to act as one EGC. Neither RMC, IMC, nor EMT need to be in "combination" with a EGC to be used as an equipment ground conductor type. So, in my mind, IF the EGC inside the mc cable is already good in size and meets T250.122, why do we need the mc cable to be listed as a equipment ground type if the EGC inside the mc cable is good enough? That's pretty much it....
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
Suppose the cable shorts out in the middle of the run. If the mc cable is not capable of carrying the short back to the panel then the cable will just glow rather than short the breaker. This is why there is a steel or aluminum wire that runs the entire length of the cable. This wire insures that the impedance on the cable is not too great so that the breaker will trip. The equipment grounding conductor will tie all parts together
 

Ranger86

Member
Location
Las Vegas, NV
Suppose the cable shorts out in the middle of the run. If the mc cable is not capable of carrying the short back to the panel then the cable will just glow rather than short the breaker. This is why there is a steel or aluminum wire that runs the entire length of the cable. This wire insures that the impedance on the cable is not too great so that the breaker will trip. The equipment grounding conductor will tie all parts together
So, to summarize the forum responses, the EGC inside the mc cable needs to be size according the T250.122. It makes no difference if the mc cable armor is listed to be used in combination with the EGC. Do you all agree?
 
Top