MC Cable Ground Size

[necnotevenclose]

I'm trying to put together a MC Feeder Schedule from 20A to 4000A the problem that I'm having is that the ground size for the 4000A is a 750kcmil when I believe it should be at least an 800 Kcmil.

Has anyone run into this issue if so what was the fix?

 

[iwire]

You can not use typically stocked cable assemblies as parallel feeder conductors due to the EGC size.

Off the top of my head I would guess the largest MC feeder you will be able to run will be approx 500 -600 amps.

 

[necnotevenclose]

Well thats not what I wanted to hear. So in order to properly use the size for 4000A it will have to be customized?! I'm wondering if the manufacturer is multiplying 750kcmil * (11) runs.

 

[iwire]

I am not sure I am following you.

Here is what is on my mind.

When you run parallel cables or raceways there must be a full sized ground in each. 250.122(F)

Example, you need to run an 800 amp feeder Table 250.122 requires a 1/0 CU EGC for this.

If you decide to run it in four separate raceways each with 3/0 circuit conductors the EGC in each conduit would need to be 1/0.

If you try this same run with 3/0 MC cable the EGC in each cable will likely only be 6 AWG, a violation of 250.122(F).

 

[cpal]

There is a new MC from southwire on the market that has its armor listed as a EGC if they Mfg in the conductor sizes your require it may resolve the issue of the paralleld EG

http://www.southwire.com/processGetPage.do?bp=featuredProduct.jsp

Charlie

 

[iwire]

There is a new MC from southwire on the market that has its armor listed as a EGC if they Mfg in the conductor sizes your require it may resolve the issue of the paralleled EG

That is a good point Charlie, right now that cable is only available in 14, 12 and 10 but the future may bring larger cables.

On a side note I don't see this new stuff taking over from regular MC to quickly.

The engineers in my area always want copper grounding conductors run.

 

[cpal]

That is a good point Charlie, right now that cable is only available in 14, 12 and 10 but the future may bring larger cables.

On a side note I don't see this new stuff taking over from regular MC to quickly.

The engineers in my area always want copper grounding conductors run.

Thanks I could not find a PDF on max conductor size.
But!! if you are considering a special Mfg then maybe it may work out.

I'd rather the armor listed as a EG than a 500kCMIL !!

Charlie

 

[iwire]

Thanks I could not find a PDF on max conductor size.

Here you go

 

[cpal]

thanks again Bob. I will query southwire tomorrow from work.

Charlie

 

[iwire]

necnotevenclose, I agree entirely with Don.

We don't get to pick and choose which code sections apply.

250.122(F) applies to the 11 MC feeders you describe.

You may want to find someone more qualified than 'Tech Support' to answer NEC questions.

 

[necnotevenclose]

They aren't larger than the circuit conductors unless they are larger than the total area of all of the runs that make up one phase. In any case the more specific rule in 250.122(F) applies over the general rule.

Don please clarify, "They aren't larger than the circuit conductors unless they are larger than the total area of all of the runs that make up one phase". My question to you is why is 250.122(A) part of the code if 250.122(F) overrides it? I believe if you utilize a EGC in MC that is equal to the circuit conductors than you are meeting code.

The size of the EGC is based on the rating of the upstream OCPD, not the size of the circuit conductors. The size of the EGC is a problem with parallel installations, for both conduit and cable. The easiest way out is to use a metallic conduit.
Don

I'm also curious you state that running seperate conduit and conductors would be easier. Please clarify. My understand of MC is that it is cheaper and easire to install.

Thanks to everyone for their replies.

 

[don_resqcapt19]

necnotevenclose,

Don please clarify, "They aren't larger than the circuit conductors unless they are larger than the total area of all of the runs that make up one phase".

You are installing parallel conductors. A circuit conductor is not the single phase conductor in each cable. It is the total area of all of the conductors that make up that phase...in your case a circuit conductor is 8250 kcmil.

My question to you is why is 250.122(A) part of the code if 250.122(F) overrides it?

The rule in 250.122(A) is a general rule, the rules (B) through (G) modify the general rule. Look at (B). If (A) always applies, why do we have (B)?

I believe if you utilize a EGC in MC that is equal to the circuit conductors than you are meeting code.

I don't agree in this case.

I'm also curious you state that running seperate conduit and conductors would be easier. Please clarify. My understand of MC is that it is cheaper and easire to install.

I said this only because if you use the raceway as the EGC, there is no change no matter how many raceways are run in parallel. This is not true if you install an EGC in the raceway. Actually, unless this is an underground feeder, I would be looking at bus duct for any circuit that requires more than 4 raceways or cables in parallel.
Don

 

[necnotevenclose]

I spoke with tech support and the person I spoke with stated that for some of the feeds such as the 750kcmil they utilize NEC 250.122(A) instead of 250.122(f) which states the grounding conductor does not need to be larger than the circuit conductors supplying the equipment. He also stated that they calculate the equipment ground size by taking the (11) runs of 750 and multiply it by it's amperage for a total of 4235A.

 

[don_resqcapt19]

I spoke with tech support and the person I spoke with stated that for some of the feeds such as the 750kcmil they utilize NEC 250.122(A) instead of 250.122(f) which states the grounding conductor does not need to be larger than the circuit conductors supplying the equipment.

They aren't larger than the circuit conductors unless they are larger than the total area of all of the runs that make up one phase. In any case the more specific rule in 250.122(F) applies over the general rule.

He also stated that they calculate the equipment ground size by taking the (11) runs of 750 and multiply it by it's amperage for a total of 4235A.

The size of the EGC is based on the rating of the upstream OCPD, not the size of the circuit conductors.
The size of the EGC is a problem with parallel installations, for both conduit and cable. The easiest way out is to use a metallic conduit.
Don

 

[jcook980]

There are a couple of other options...

There are a couple of other options...

One option is to use 250.122(F)(2) and use ground fault protection as indicated.

The second option has been discussed in this forum before and there are considerable differences of opinion about it. Note that the exact wording of 250.122(F) says "...the equipment grounding conductor, where used,...". The presence of an EGC within a cable does not require it's use as an EGC. If the EGC is not used in the cable, then an alternative EGC is required.

This could be, for example, a listed, correctly sized and properly installed cable tray as permitted in 250.118(12). It can also be "...one or more or a combination of..." of the types of EGC listed in 250.118. That implies that, for MC, 250.118(10) can be used in combination with the other types of EGC in 250.118. For the Cable Tray Institute's take on the EGC in paralleled cable issue see:

http://www.cabletrays.com/techbl11.htm

Note that the NEC references in this bulletin are to earlier revisions of the Code.

 

[don_resqcapt19]

jcook,

One option is to use 250.122(F)(2) and use ground fault protection as indicated.

The only problem is that there is no ground fault protection equipment listed for the purpose of protecting an EGC as required by the 250.122(F)(2)(3).

(3) The ground-fault protection is listed for the purpose of protecting the equipment grounding conductor.

Don

 

[jcook980]

250.122(f)(3)

250.122(f)(3)

Yet another questionable edit between Code revisions... There was nothing wrong with the way this was worded in the 2002 Code. All this edit does is to effectively eliminate the ability to use this section of the Code. Why not just delete the section and get it over with! Who is going to go to the effort and expense to get a listing specifically to project the equipment grounding conductor? Is there even a UL standard for such a use?

 

[iwire]

Yet another questionable edit between Code revisions... There was nothing wrong with the way this was worded in the 2002 Code.

The 2002 also had a listed for the purpose requirement.

 

[necnotevenclose]

I'm meeting with a rep this week to further discuss this issue. I'll keep everyone posted once I get a response.

 

[don_resqcapt19]

I'm meeting with a rep this week to further discuss this issue.

Remember that they are in the business of selling product and not in the business of enforcing the code.
Don