MC Cable in parallel

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steve66

steve66

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Can I parallel 2 sets of 350KCM armored cable if each cable has a #3 ground wire? (This is 3 conductor + ground AC or MC cable).

The reason I ask, is because if I ran 2 sets of 350KCM in conduit in parallel, a #3 ground in each conduit wouldn't be large enough.
 
Yes you can.....if you can find GFP listed for the purpose of protecting the EGC. 250.122(F)(2)(3)

I don't think it can be found.
 
steve66 said:
Can I parallel 2 sets of 350KCM armored cable if each cable has a #3 ground wire? (This is 3 conductor + ground AC or MC cable).

The reason I ask, is because if I ran 2 sets of 350KCM in conduit in parallel, a #3 ground in each conduit wouldn't be large enough.
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I'm curious what are you trying to connect? Last summer I made up a MC cable schedule based on ALCAN. It was pretty tricky because even though your phase conductors were large enough the equipment ground was not. So what I had to do is go to the next larger size that included the correct EGC. However for the larger feeds such as the 4000A ALCAN utilized NEC 2005 250.122(A) which says you can utilize an EGC that is the same size as the current carrying conductors.

As for secondary feeders from transformers that is a little more complicated because of the bonding jumper required or if you have a isolated or k-rated transformer. Hopefully the MC manufactures will offer feeders for such a purpose.
 
necnotevenclose said:
So what I had to do is go to the next larger size that included the correct EGC.
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That really may not make it NEC compliant.

You would likely end up with a 250.122(B) violation.

250 really puts a damper on parallel MC cables.
 
steve66 said:
Can I parallel 2 sets of 350KCM armored cable if each cable has a #3 ground wire? ...
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Steve -
Yes if you get the right cable. (2002 250.118.11). We use a lot of Okonite MC-HL. The sheath is listed as being part of the grounding conductor. The CU equivalent is pretty big. As I recall, you don't get into trouble until 3 - parallel 750kcmil. Okonite has most of the data you need on their website. There was one piece of data I needed (cross sectional area of the sheath) that wasn't, and the distributor in Portland, OR emailed it right up.

Southwire also makes a similar product and maybe Rockbestos as well - but i have not used either of these.

I don't know about the interlocked armor types either.

carl
 
iwire said:
Carl

Are you using the sheath as the EGC?

If so do you cut out the undersized grounding conductor?
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coulter said:
The sheath is listed as being part of the grounding conductor. The CU equivalent is pretty big.
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Sounds to me like he's using the EGC and sheath in parallel as the equivalent of an adequately-sized EGC.
 
LarryFine said:
Sounds to me like he's using the EGC and sheath in parallel as the equivalent of an adequately-sized EGC.
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That is what it sounds like to me as well.

And I have no trouble believing that combination will provide an adequate ground fault path.

What I do not see is anything in the NEC allowing this 'combining' to become an equivalent EGC.
 
I'm with Bob on this one. But don't worry. I promise not to make a habit of it. :grin:
 
So nobody makes a ground fault device listed for protection of equipment grounding conductors?

Why do I have the feeling conduit manufacturers got that put into the code?

I still think there is probably something that lets me out of this. Doesn't the ground on some types of AC or MC cable make contact with the sheath the entire length of the cable? That seems different than a grounding conductor in a conduit.

On the other hand, if the ground wire doesn't make contact with the sheath the entire length of the cable, I can just use the sheath for grounding, and not connect the cable grounding conductor.

Actually, now that I think about it, I can just use the cable tray for the ground path.

I notice in 250.118, some types of AC and MC cable have their combined sheath and ground conductor listed as a grounding path. Does that make a difference?

And MC cable says it specifically says it has to meet 250.122 (in 330.108). But this reference is absent from AC cable. It specifically refers to 250.4(A)(5) and (B)(4) (via 320.108).

Why can't everything be as simple as "Fixed electric space heating shall be considered continuous load".

Steve

And AC cable specifically says it has to meet
 
steve66 said:
Actually, now that I think about it, I can just use the cable tray for the ground path.
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Maybe not....300.3(B).

IMO 300.3(B) requires the EGC to be contained in the cable with the conductors it serves.
 
hardworkingstiff said:
I thought I remembered someone saying "if the code does not forbid it, it is allowed" (or something to that effect)?
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LOL, I may have said that once or a hundred times. :D

The problem is in this case the NEC has provided a list (250.118) of acceptable EGCs and the list does not include combinations of sheaths and wire EGCs.
 
iwire said:
LOL, I may have said that once or a hundred times. :D

The problem is in this case the NEC has provided a list (250.118) of acceptable EGCs and the list does not include combinations of sheaths and wire EGCs.
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I think the lesson being learned here is a very important lesson. That is the requirements for some installations may not all be located in one area/section of the NEC. Sometimes it will take a person with extensive knowledge of the NEC time to put together all of the different requirements of a particular installation. From my different classes, this is one of the biggest contributers to the frustration so many have in trying to apply the NEC to the installation. As most installers (the ones I see) are casual readers of the NEC, this is where they have the most problems with the NEC and more complex installations.
 
iwire said:
...Are you using the sheath as the EGC?

If so do you cut out the undersized grounding conductor?
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Yes. No.

The area of the sheath is large compared to the grounding conductor. I use the phrase "copper equivalent" cause it is the only way to add the sheath and grounding conductors together. Okonite uses that term in their tables showing the equavelent wire size of the Al sheath.

As for the copper GC, sometimes it is single conductor, and sometimes it is 3 conductor. A TMC connector grabs the sheath and the grounding connection is made to to a lug on the TMC bushing. The copper grounding conductors and the connection to the TMC are all terminated to the grounding connection.

One place to look is the UL spec on the cables. It has a pretty good illustration of how the sheath and copper are terminated for the grounding conductor tests. Both the sheath and the copper GC are used. Article 250.118(11)(b) (NEC 2002)specifically allows the use of the combined sheath and the grounding conductors where the cable is of the corrugated tube type MC.

So far, the AHJ is in agreement.

I wrote a paper on this a couple of years ago and can pm it is any one is interested. It is escrutiatingly, nauseously detailed.

carl
 
iwire said:
Maybe not....300.3(B).

IMO 300.3(B) requires the EGC to be contained in the cable with the conductors it serves.
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Yes, but if I don't use the ground wires in the cable, then the cable tray is a ground path, but not a conductor. It would be just like using metal conduit without a ground wire.

300.3(B) says: "and where used, the grounded conductor...."

And just to reiterate, I think the fact that the MC cable article specifically references 250.122 while AC cable only references 250.4 is an important difference.
 
iwire said:
The problem is in this case the NEC has provided a list (250.118) of acceptable EGCs and the list does not include combinations of sheaths and wire EGCs.
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I think the 2005 does include just that:

250.118 (10): a. The combined metallic sheath and grounding conductor of interlocked metal tape type MC cable.
b. The metallic sheath or combined metallic sheath and grounding conductors of the smooth or corugated tube type MC cable.

( I left out the part about being listed and identified for grounding :) Maybe nobody lists them like that. )

Steve
 
steve66 said:
Yes, but if I don't use the ground wires in the cable, then the cable tray is a ground path, but not a conductor. It would be just like using metal conduit without a ground wire.
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No it would not be like using metal conduit.

In your proposed installation the circuit conductors will be inside a metal sheathed cable and your ground fault path will be on the outside of that metal sheath.

During a ground fault this will likely raise the impedance of the ground fault path. This is the same problem with running a single GEC inside steal conduit and why the GEC has to be bonded to the raceway.

Look I like MC probably more than most but I think you will have problems.

I strongly suggest you talk to the AHJ before going to far.
 
steve66 said:
And just to reiterate, I think the fact that the MC cable article specifically references 250.122 while AC cable only references 250.4 is an important difference.
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Type AC does not generally come with an EGC that is why it is not mentioned in 250.122(F)
 
iwire said:
LOL, I may have said that once or a hundred times. :D

The problem is in this case the NEC has provided a list (250.118) of acceptable EGCs and the list does not include combinations of sheaths and wire EGCs.
Click to expand...
Actually 330.108 requires it through 250.118(10) for all MC cable used for equipment grounding.
330.108 Equipment Grounding Where Type MC Cable is used for equipment grounding, it shall comply with 250.118(10) and 250.122.​

But in the case of parallel runs, 250.118 does permit combinations, and it says so right up front:
250.118 Types of Equipment Grounding Conductors The equipment grounding conductor run with or enclosing the circuit conductors shall be one or more or a combination of the following: ...​
So where in the code does it say be it more than one or a combination of 250.118 EGC's, that such must be contained within the sheath of an MC cable?
 
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