5 Parallel runs of 350kcm MC Cable?

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MikeW

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At work I have noticed a feeder circuit that was recently installed. This feeder is protected by a 1600 amp breaker. The feeder consists of five parallel runs of MC Cable. On the outer jacket of each MC Cable is the following information:

Type MC, 3-conductor, 350 kcmil CU, XHHW-2, 3-AWG ground, Sun Resistant, 600V insulation.

2002 NEC 392.11(A) (3), in an open cable tray, for multiconductor cables, 2000V or less, with not more than three conductors per cable (ampacity to be determined from Table B310.3 in Annex B).

I believe the five parallel 350 kcmil XHHW-2 MC Cables are large enough for the 1600 amp feeder. What do you think?

How about the size of the ground (3AWG CU) in each MC Cable? Is this size ground large enough? Should the size be based on 250.122 which calls for 4/0 CU?

Please respond with your thoughts and questions. Thank you.
 
Unless I am missing something it is 50 amps short of 1600 amps and you can not roll up to the next standard size breaker at that level.

As for the ground it is a violation unless the 1600 amp breaker has GFP with a setting matching the 3 AWG grounding conductor.

250.122(2)(F) Where ground-fault protection of equipment is installed, each parallel equipment grounding conductor in a multiconductor cable shall be permitted to be sized in accordance with Table 250.122 on the basis of the trip rating of the ground-fault protection where the following conditions are met:

(1)Conditions of maintenance and supervision ensure that only qualified persons will service the installation.

(2)The ground-fault protection equipment is set to trip at not more than the ampacity of a single ungrounded conductor of one of the cables in parallel.

(3)The ground-fault protection is listed for the purpose.
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This is a double whammy!!!

The 3 AWG is rated for up to a 400 amp OCPD. That is the problem with using cable types for parallel installations.

Also the installer is not aware of 240.4(C)/310.16 and the close relationship both sections have with each other.
 
That's the problem with using 3 wire cable for parallel feeders. You have to use 4 wire to get a full size ground. Lots cheaper than a GF breaker.
 
Look at 392.11(A) (3) NEC 2002.
The feeder cables meet this criteria.
FPN says look at Table B.310.3.

The 1600 amp breaker is not GFP.

Table B.310.3. says the ampacity of 350kcm cable is 337amps (in the 75deg C column). 337a x 5cond.s = 1685a.

I am thinking these 5 runs of MC cable are adequately sized for 1600 amps.

I am not sure about the grounds size in each MC cable?
 
MikeW said:
Look at 392.11(A) (3) NEC 2002.
The feeder cables meet this criteria.
FPN says look at Table B.310.3.
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Mike forget the FPN look at the main rule.

392.11 Ampacity of Cables, Rated 2000 Volts or Less, in Cable Trays.
(A) Multiconductor Cables. The allowable ampacity of multiconductor cables, nominally rated 2000 volts or less, installed according to the requirements of 392.9 shall be as given in Table 310.16 and Table 310.18, subject to the provisions of (1), (2), (3), and 310.15(A)(2).
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You can not use the Annex without an engineer.

B.310.15(B)(1) Formula Application Information.
This annex provides application information for ampacities calculated under engineering supervision.
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I think I see what you are saying Bob. So if I am a small contractor and I do not have or consult an engineer I have to use the larger wire sizes in Table 310.16 and Table 310.18, subject to the provisions of (1), (2), (3), and 310.15(A)(2).

These multiconductor cables are installed in a single layer in an uncovered tray, with maintained spacing of not less than one cable diameter between cables.

So if this contractor has the approval of an engineer this installation is code compliant but if he didn?t get approval he is in violation?

How about the size of the ground wires of each MC Cable? Would you have to use the size from T250.122 or is there some other code section to look at? :?: :?: :?:
 
mpd, I looked at 240.4 (C) and 250.122 (F) and they tell me that the conductors and the ground are undersized. I am not a contractor and do not have to make these calculations but I am interested in learning as much as I can.

I am wondering if I have missed something in the code book that would allow the size conductors used? If anyone can point me in the direction of some NEC article that would support the use of above mentioned cable sizes please chime in. Thanks - MJW
 
Thank you Pierre C Belarge, iwire, bdarnell, and mpd for your input. My final conclusion on this installation is it does not meet the minimum standards of the 2002 NEC.
 
Under engineering supervision, 5 runs of 350kcmil may well be acceptable.

As far as the EGC is concerned, the #3 EGC in each cable is too small if it were the sole means of providing the EGC for the circuit. NEC 2005 - 250.118 states that the ECG "...shall be one or more or a combination of the following:...
(10) Type MC cable... sheath...
(11) Cable trays as permitted in 392.3(C) and 392.7".

Note the wording of 250.122 (A): "...equipment grounding conductors of the wire type shall not be smaller than shown in Table 250-122..."

The key requirement of a raceway, cable armor or sheath used as an EGC is stated in 250.4(A): "...shall be capable of safely carrying the maximum ground-fault current likely to be imposed..."

"When paralleled Type MC cables are installed in cable tray, the required equipment grounding conductor can be provided by a combination of any of the following:

(1) the equipment grounding conductor(s) within each cable,
(2) the continuous aluminum sheath on each cable,
(3) the cable tray if identified for grounding purposes and,
(4) a bar equipment grounding conductor within the tray."

NEC Grounding Requirements for Parallel Circuits
James M. Daly
IEEE Industry Applications Magazine - March/April 1998
 
Thanks jcook980 for information. I'm going to go over everything that has been posted on this topic and see if I can learn something. :lol:
 
jcook980 said:
"When paralleled Type MC cables are installed in cable tray, the required equipment grounding conductor can be provided by a combination of any of the following:

(1) the equipment grounding conductor(s) within each cable,
(2) the continuous aluminum sheath on each cable,
(3) the cable tray if identified for grounding purposes and,
(4) a bar equipment grounding conductor within the tray."
".
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jcook980 I strongly disagree with your interpretation.

If one choses to run a copper EGC it must be full sized you can not combine it with the other methods.

If that was the case there would be no point at all to 250.122(F)
 
I am with Bob on this. It is such an important part of the installation. Jcook, I do not see where the NEC permits what you posted, maybe you can direct us as to how you derived this thought from.


The fault current will follow all of the paths available to it, but in calculating the size of the equipment ground conductor, you will need to reference 250.122(F).
 
I knew this would be controversial...

Gentlemen, you need to read the IEEE article I cited. That quote is not mine, it is the opinion expressed by an experienced engineer whose paper was submitted for critical review by other electrical engineers prior to publication.

Why would the NEC include the wording in 250.118 that the EGC shall be one OR more OR a combination of eleven different means of providing the EGC? Is the word "combination" a typo by the NEC?

Using parallel runs of MC is a common practice in heavy industry and a properly installed cable tray (or any metallic raceway) is an acceptable equipment grounding conductor. Provided, of course, that some engineering of the impedance of the raceway is performed to ensure that the fault current can be adequately handled. If it can't handle the fault current (we weren't given the size or construction of the cable tray in this example) then a bare conductor can also be run in the same raceway with the MC cables to decrease the impedance.

Think of it this way, we are really talking about 5 sets of parallel conductors properly installed in a common raceway. Not 5 sets of individual conductors in 5 separate raceways nor 5 cables in 5 separate raceways.
 
jcook980 said:
I knew this would be controversial...

Gentlemen, you need to read the IEEE article I cited. That quote is not mine, it is the opinion expressed by an experienced engineer whose paper was submitted for critical review by other electrical engineers prior to publication..
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I understand that, and he is mistaken.

Nothing against engineers but they generally do not have any formally NEC training.

jcook980 said:
Why would the NEC include the wording in 250.118 that the EGC shall be one OR more OR a combination of eleven different means of providing the EGC? Is the word "combination" a typo by the NEC?
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No it's not a typo.

Without that wording I could not voluntarily add a copper EGC to a metal raceway.

Think of it this way, we are really talking about 5 sets of parallel conductors properly installed in a common raceway
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No that is not the case, each MC is an individual cable regardless of the fact they are run in a cable tray and they fall under the rules of 250.122(F).

I will get back to this latter but in the meantime can you tell me where the section is that releases us from 250.122(F)?
 
(we weren't given the size or construction of the cable tray in this example)
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Hey jcook980 and anyone else interested I will try to get the size and construction of the cable tray sometime today or tomorrow. The cable tray was nothing special. It was just open cable tray wide enough to accommodate the five parallel MC Cables. There was no additional ground wires run in the tray. It was nut and bolted together and hung of threaded rods and unistrut.[/quote]
 
250.122(F) is very clear...a full sized EGC is "shall be run in parallel in each raceway or cable". You can either use a cable with a full size EGC or you can use a ground fault protection system that " is listed for the purpose of protecting the equipment grounding conductor". As far as I know there are none listed for this specific purpose.
Don
 
Don's last post is what some seem to forget. "a full size equipment ground installed with each parallel set".

One cannot assume that the cable tray and a conductor installed with it, is sufficient in size to carry the fault current.


I will see if I can find that paper your colleague wrote, and take a look at it, as that kind of work always interests me.


Jcook
It is good to have your opinion and your engineering skills here at the forum. Just because we do not agree with your posts, I hope you will continue to add your wisdom here.
 
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