5 Parallel runs of 350kcm MC Cable?

[charlie b]

Bob (iwire) has asked me to read through this thread and offer an opinion. First, Bob, I thank you for your good opinion of my opinions, even though we have disagreed on several questions in the past.

I "sort of" disagree on this one as well, but not because I think anything you said was wrong. Rather, I think you didn't say enough. You had all the right ideas, but you didn't take them far enough. By the way, I also disagree with jcook980, and for the same reason.

This is not a subject I know well. I had to read through the various articles several times, before I saw the crux of the situation. Here's my take: You can, as jcook980 (and the engineer whose article he quoted) has said, use both the cable tray and the EGC within the MC cable as EGCs. In that, I am disagreeing with Bob. But I am also saying that the engineer's article did not tell the whole story.

The rest of the story is that if you chose to do use more than one of the fourteen elements listed as EGCs in 250.118, then each element you use must be fully rated for the job, as though no other element were there.

What I mean is that you cannot say, for example, that the EGC within the MC cable is rated for 25% of what an EGC needs to be (i.e., good for a 400 amp OCPD instead of 1600), and then say that the tray is good for 75% of what it needs to be (e.g., 1.125 sq. in. instead of 1.5), add the two, and say that you have 100% EGC coverage.

Specifically, as Bob correctly pointed out, 250.122(F) says that if you use the EGC in each parallel cable, then each EGC must be rated for the OCPD (i.e., for 1600), or you need to use GFP in the manner Bob described. Furthermore, as jcook980 correctly pointed out, 250.122(A) says "equipment grounding conductors of the wire type shall not be smaller than shown in Table 250-122. . . ." But what jcook980 is missing is that the facility MikeW has described is already using the wire type. Perhaps they will take credit for the tray as well. But if the wire type is used, then the wire type has to be "full-sized."

So what is my solution for the EGC problem? Presuming the cable tray is adequately sized, just cut the EGC wires in each MC cable short, and don't connect them. Use the tray, and nothing but the tray, as your EGC. Would that be "just plain silly," a waste of an available resource? I think not. The one thing you don't want to happen is for some later fool to think that he can disturb the tray sections and not worry about the EGC, because there is a "backup EGC" in place (i.e., the MC cable's EGC is a backup to the tray being an EGC). Don't give anyone that option. Deny him the use of the MC cable's EGC, and there will be no uncertainty of what is what.

And while we are at it, what is my solution for the ampacity problem? Not enough information was given. If the calculated load is under 1550, and if the breaker can be set to trip at 1550, then 5 sets of 350 MCM MC XHHW will suffice. If the load is between 1551 and 1600 amps, or if the load was not calculated, or if the breaker cannot be set to trip below 1600, then you need to hire an engineer, or you need to use more copper.

 

[jcook980]

Don,

250.122(F) does not require the use of the EGC inside the cable. In fact, it specifically states "...equipment grounding conductors, where used, shall be run in parallel in each raceway or cable."

The phrase "where used" means the use of the internal "wire type" (250.122 (A)) equipment grounding conductor is optional. When it is used as the EGC, it has to meet the sizing requirements of Table 250.122 or it must be protected as described in 250.122(F)(2). As we all agree that #3 is not large enough for 1600A as in this example, it can't be used as the equipment grounding conductor (unless using GFP).

Using the Type MC equipment grounding conductor is optional. NEC 330.108 states "Where Type MC cable is used for equipment grounding, it shall comply with 250.118(10) and 250.122" Therefore, where Type MC cable is NOT used for equipment grounding, it does NOT have to comply with 250.118(10) and 250.122. And when the EGC in the Type MC is too small to be used as the equipment grounding conductor it can't be used as the EGC.

The Code references in the link below are old (1996, I think) but little has changed in the specific Code wording since this article was written by the Cable Tray Institute.

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

I'll quote a section here. Note that Section 250-95 is now 250-122. Boldface here is from the original document:

"A significant change was made in NEC Section 250-95. Size of Equipment Grounding Conductors for the 1993 and 1996 NECs which impacts on the paralleling of standard multiconductor cables in cable trays. This change requires an increase in the size of the EGCs in three conductor cables when the phase conductors are paralleled and the EGCs are paralleled or a separate EGC of the proper size must be installed in the cable tray.

The proposals that were accepted to revise NEC Section 250-95 didn't contain any documented safety problems. The submitter's substantiation was that conductors of cables are permitted to be paralleled so the single sized EGC as applied to raceway systems should be applied to multiconductor cables. As a result "or cable" was placed after the word "raceway" throughout NEC Section 250-95.

There haven't been any public facts presented on any safety or technical problems due to operating standard three conductor cables with standard sized EGCs in parallel. This has been a common industrial practice for several decades. In many Chemical, Plastics and Textiles Manufacturing Facilities, the 480 volt feeders (Type TC Cables) from the substations to the motor control centers have been paralleled standard three conductor cables with the standard sized EGCs paralleled since the early 1960s.

For paralleled three conductor cables installed in cable tray to be in compliance with the 1996 NEC, one of the following options must be selected:

Order special three conductor cables which contain larger sized EGCs. The size of the EGCs will depend on the rating or setting of the circuit's protective device as per NEC Table 250-95. This means that the size of the EGCs is dependent on the number of three conductor cables that are paralleled to obtain the circuit capacity desired.

Use three conductor cables without EGCs and install a single conductor EGC in the cable tray or use the cable tray as the EGC in qualifying installations as per Section 318-3(c).

Use standard three conductor cables with standard size EGCs and parallel the EGCs that are in the cable assemblies with the single conductor EGC (Sized as per Table 250-95) in the cable tray or with the cable tray if it is used as the EGC. This meets the NEC Section 250-95 requirements."

I have many years in industrial electrical distribution design. Papermills throughout the Northwest have used paralleled TC, MC and C-L-X cables for years prior to the Code changes relying only on the internal cable EGC. Little actually changed after the words "or cable" were added to what is now NEC 250.122. A properly sized cable tray by itself or with the addition of a Table 250.122 sized paralleled EGC installed with the cable tray meet NEC requirements.

James Cook, PE

 

[Pierre C Belarge]

James
Is the cable tray electrically connected to the equipment in question?

 

[don_resqcapt19]

James,

I have many years in industrial electrical distribution design. Papermills throughout the Northwest have used paralleled TC, MC and C-L-X cables for years prior to the Code changes relying only on the internal cable EGC. Little actually changed after the words "or cable" were added to what is now NEC 250.122. A properly sized cable tray by itself or with the addition of a Table 250.122 sized paralleled EGC installed with the cable tray meet NEC requirements.

I have no doubt that it works fine and is safe, I just don't agree that it meets the code rule.
Don

 

[iwire]

250.122(F) does not require the use of the EGC inside the cable. In fact, it specifically states "...equipment grounding conductors, where used, shall be run in parallel in each raceway or cable."

Correct so if you connect the EGCs in those MCs they are now 'used' and must be full sized as 250.122(F) requires.

Charlie B. had a thought of simply not using the EGCs in the cable at all, cut them out and ignore them.

That is also a violation, 300.3(B).

James I mean no disrespect and I also welcome you and your knowledge to this forum.

I just can not agree with the interpretation you have presented here.

The fact that you have been doing this for years without problem is not any indication of it's compliance with the NEC.

Can I ask you why you think 250.122(F) goes on about GFP protection and so forth for cable applications if we can simply ignore that section all together?

 

[iwire]

First, Bob, I thank you for your good opinion of my opinions, even though we have disagreed on several questions in the past.

My Momma did not raise a fool. :wink:

I "sort of" disagree on this one as well, but not because I think anything you said was wrong. Rather, I think you didn't say enough. You had all the right ideas, but you didn't take them far enough.

I again agree, I was in a hurry this morning and thought you could take a better run at this.

What I mean is that you cannot say, for example, that the EGC within the MC cable is rated for 25% of what an EGC needs to be (i.e., good for a 400 amp OCPD instead of 1600), and then say that the tray is good for 75% of what it needs to be (e.g., 1.125 sq. in. instead of 1.5), add the two, and say that you have 100% EGC coverage.

This is exactly what I was trying to state apparently without much luck.

So what is my solution for the EGC problem? Presuming the cable tray is adequately sized, just cut the EGC wires in each MC cable short, and don't connect them. Use the tray, and nothing but the tray, as your EGC.

Here we disagree as I see that solution as a violation of 300.3(B)

IMO simply having all the EGCs in the same cable tray (or using the cable tray itself is not the same as having them in the same metal armored cable with their respective circuit conductors

I believe this requirement has to do with impedance issues under fault conditions but admittedly I am now getting in over my head.

 

[iwire]

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.

I go back to this as I see it as an important distinction.

Cable tray is not a raceway.

It is a support system.

392.2 Definition.
Cable Tray System. A unit or assembly of units or sections and associated fittings forming a structural system used to securely fasten or support cables and raceways.

IMO (In My Opinion) you must forget the thought that you have installed the parallel conductors in a common raceway.

You have 5 distinct cables basically contained in their own raceways.

 

[jcook980]

Charlie,

I don't think I or James Daly suggested that you could mix and match conductors to produce an EGC with an impedance low enough to safely carry the ground fault current. Whatever equipment grounding conductor is used, it must, by itself, be large enough to carry the total ground fault current. Either the EGC in each individual cable OR the cable tray itself OR the EGC added to the tray MUST meet either Table 250.122 (if it is a "wire type" EGC) OR Table 392.7(B) if the tray is the EGC.

How big does this EGC need to be? If using the cable EGC OR the EGC added to the tray then, according to Table 250.122, it has to be 4/0 copper or 350kcmil Aluminum. If the tray is the EGC, then according to Table 392.7(B) the tray has to be aluminium and have a cross-sectional area at least 1.5 square inches. Cooper B-Line has a great cable tray information guide that you can download at:

http://www.b-line.com/product/PDFLibrary/CTManual.asp

Note this quote on page 28 of the 2005 NEC installation guide:

"If the maximum ampere rating of the cable tray is not sufficient for the protective device to be used, the cable tray cannot be used as the EGC and a separate EGC must be included within each cable assembly or a separate EGC has to be installed in or attached to the cable tray."

James Cook, PE

 

[charlie b]

So what is my solution for the EGC problem? Presuming the cable tray is adequately sized, just cut the EGC wires in each MC cable short, and don't connect them. Use the tray, and nothing but the tray, as your EGC.

Here we disagree as I see that solution as a violation of 300.3(B)

I think not. The fact that an MC cable has a bare (or is it green insulated??) conductor does not require you to use that conductor as an EGC. What I am suggesting is that we disregard the existence of those conductors, in each of the five MC cables. In other words, they are not EGCs, so no rule pertaining to EGCs will apply to them.

You are left with

". . . all equipment grounding conductors . . . shall be contained within the same . . . cable tray. . . ."

The bare wires within the MC cables are not EGCs. So what EGCs do you have, and are they all within the same cable tray? Perhaps the tray itself is the only EGC you need.

You'll have to help me here. I just don't see the violation. I think the following are all acceptable:

• 1. Cut and abandon the bare wires in the MC cables, and use the tray as an EGC.
• 2. Cut and abandon the bare wires in the MC cables, and run a separate conductor (sized for 1600 amp OCPD) to serve as an EGC.
• 3. Do both #1 and #2 above.
• 4. Use the EGCs in the MC cables, and use a GFP set at or below 400 amps (as you had suggested earlier).

Which of these do you see as a violation?

 

[iwire]

You'll have to help me here. I just don't see the violation. I think the following are all acceptable:

• 1. Cut and abandon the bare wires in the MC cables, and use the tray as an EGC.
• 2. Cut and abandon the bare wires in the MC cables, and run a separate conductor (sized for 1600 amp OCPD) to serve as an EGC.
• 3. Do both #1 and #2 above.
• 4. Use the EGCs in the MC cables, and use a GFP set at or below 400 amps (as you had suggested earlier).

Which of these do you see as a violation?

1) The EGC is not contained in the same cable as the circuit conductors.

2)The EGC is not contained in the same cable as the circuit conductors

4) This IMO would be compliant if a listed breaker can be obtained.

IMO once you have used a cable assembly running the EGC outside of the cable is a violation.

 

[fc]

I have to agree with iwire on this one. The only one I would do is # 4

I also see that # 1 & 2 is in violation.

 

[charlie b]

1) The EGC is not contained in the same cable as the circuit conductors.

IMO once you have used a cable assembly running the EGC outside of the cable is a violation.

OK. Now I understand what you are saying. I had missed it before.

But before I concede the point, let me ask this. 300.3(B) uses the word "or." All the circuit conductors, the neutral (if used), and the EGC have to be in the same cable OR they have to be in the same cable tray. There are a few more "or" opportunities listed. So if the circuit conductors are in an MC cable, and if the EGC is a separate conductor from the 5 MC cables, and all are in the same tray, have we not met the requirements of 300.3(B)?

 

[jcook980]

Bob,

In my poor effort to explain this I did say that incorrectly. A cable tray is not a raceway. But it is an acceptable equipment grounding conductor, just like a raceway. In fact listed metallic cable tray, is the only non-raceway, non-wire (or busbar) acceptable for use as an equipment grounding conductor.

The words "where used" are clearly included in 250.122(F). If they don't mean the use of the internal cable EGC is optional, then what do they mean? I also note that in Mike Holt's 2002 Bonding and Grounding book, when he paraphrases 250.122(F), he omits those two words.

James Cook, PE

 

[iwire]

300.3(B) uses the word "or." All the circuit conductors, the neutral (if used), and the EGC have to be in the same cable OR they have to be in the same cable tray. There are a few more "or" opportunities listed. So if the circuit conductors are in an MC cable, and if the EGC is a separate conductor from the 5 MC cables, and all are in the same tray, have we not met the requirements of 300.3(B)?

It depends on who is doing the reading. :lol:

IMO what it says is not what it means.

I know,.... I know that is weak and I would jump all over anyone posting that.

However I am honest and that is how I feel.

Could an inspector see it your way Charlie?

Yes I think they could.

Is it what the CMP intended, I do not think so.

As I said I believe that the reason for this requirement was to reduce impedance during fault conditions.

Using only the cable tray as the EGC and having the circuit conductors isolated from the tray by a continuous metal (likely steel) cable armor seems to my mind to be a bad design.

Would this not create the same problem (often referred to as a 'choke') as running an GEC in a metal raceway without bonding it at each end?

 

[fc]

"Where used" But 300.3 (B) tells us we have to use it with the circuit conductors doesn't it? It's says all equipment grounding conductors and bonding conductors shall be contained within the same raceway, auxiliary gutter,cable tray,cablebus assembly, trench, cable or cord.

So my question is whitch one of the above is it?

 

[Gmack]

Ok Ok Ok,

I choose to defend jcook980.

There is nothing wrong with his reasoning [electrical] wise.

Do we not run "down sized neutrals" in service entrance conductors "sized
in accordance with the NEC?

They too handle "fault/short circuit" current.

Just because these "cables" are placed elsewhere in a tray does not change the fact that fault/short circuit currents will "decide " to reference the NEC and behave differently.

If they are joined together at each end they will act/behave way better as a group than any single EGC conductor full sized, individually during a fault down stream.

The NEC allows "non" full sized EGC in larger cables.

Why?

 

[iwire]

Here is a good image of why we need full size grounds in each raceway as it is possible to have a fault that exposes only one EGC to the full fault current of the OCPD.

 

[iwire]

The NEC allows "non" full sized EGC in larger cables.

Why?

Gmack I think you may have missed what we where saying.

The MC cable in discussion is a 350 kcmil CU, XHHW-2, 3-AWG ground.

The 3 awg EGC is perfectly fine when this cable is not used in parallel.

Lets say we used this 350 kcmil CU, XHHW-2, 3-AWG ground MC cable for a 300 amp feeder. In this case the 3 AWG EGC is fine.

Now lets say we bump the feeder up to 600 amps so we add another 350 kcmil CU, XHHW-2, 3-AWG ground MC cable.

We have now have 600 amps of feeder which Table 250.122 says we need a 1 AWG EGC for this 600 amp feeder.

250.122(F) goes on to tell us we need a the 1 AWG in each cable or we must use ground fault protection.

 

[Gmack]

Yes Iwire, I might have. I should have stated that I have not reviewed this matter to "code" articles.

Having said that, I dont see anything as yet, wrong/unsafe/hazardous in the installation described. For reasons stated in my former post

Also, jcook sited code, and, on the surface I found that he might/could be correct.

The example photo you provided showed a shunt/fault to the conduit.
I think.

A down stream fault on these MC's/feeder EGC would look/behave otherwise.

 

[jcook980]

Bob,

I've seen that example in many copies of the Handbook and I agree completely that the EGC for each paralleled non-metallic raceway (as clearly indicated in the example) needs to be large enough to handle the ground fault current. But if metallic raceways are properly installed and large enough to handle the ground fault current, the internal EGC is not required by the NEC because the conduit is the EGC. Again, 250.122(F) requires that an individual EGC be sized per Table 250.122, where used.

Note that I personally never design using the conduit as the EGC. I always run a full size EGC in each paralleled conduit whether metallic or not. I've seen too many conduits in industrial settings accidently becoming electrically discontinuous and being left that way for years. At least with an internal EGC, there is at least one other ground-fault return path.

James Cook, PE