MC Cable in Cable Tray

[steve66]

I don't think I've ever seen anything quite as cryptic as the rules for running cables in cable tray. I have a question about cable spacing, and another question on the grounding.

I need a 600 amp feeder, so I'm assuming I'll specify two sets of 350KCM, 3 conductor, armored cable. Do I have to maintain a cable's diameter spacing between each cable? For example, if my cables are 1" in diameter, do I have to leave 1" of empty space between each cable per 392.11?? Or do I only need to do that if I'm calcualting ampacity per the engineering formula 310.15(C).??

If I don't leave the space between the cables, do I have to derate for 6 current carrying conductors? Or not since I only have 3 conductors per cable?

Finally, since I have a 600 amp breaker feeding these two parallel cables, do I need a #1 ground in each cable? 350KCM MC cable comes standard with a #3 ground. But doesn't the metal jacket count for something?

Steve

 

[chris kennedy]

Finally, since I have a 600 amp breaker feeding these two parallel cables, do I need a #1 ground in each cable? 350KCM MC cable comes standard with a #3 ground. But doesn't the metal jacket count for something?

Steve

I'll start with this one. The EGC has to be sized per 250.122(F). The metal clad means nothing here unless it meets the requirements of 250.118(10).

 

[markstg]

If you do the 1 cable diameter spacing within the cable tray, I would just go to table B.310.3.

Seems like a waste of good cable tray space, to try to save 1 cable size.

If you don't leave the space, then uses Table 310.16

 

[Smart $]

Spacing not required. However, it is permitted if you want to calculate a derating ampacity under 310.15(C). If not spaced, you only derate by the individual cable... 3 CCC's in your case, so no Table 310.15(B)(2) adjustment required.


Sounds like you already know the answer for the EGC size

 

[steve66]

Thanks guys. That's what I needed.

I guess they will have to special order the cable to get the larger ground sizes.

Steve

 

[chris kennedy]

I guess they will have to special order the cable to get the larger ground sizes.

Is there some reason conduit or EMT is out of the question for this install?

 

[markstg]

Thanks guys. That's what I needed.

I guess they will have to special order the cable to get the larger ground sizes.

Steve

There's no need to get a special cable for the ground conductor size. The #3 that comes with the 350Kcmil is fine.

 

[don_resqcapt19]

There's no need to get a special cable for the ground conductor size. The #3 that comes with the 350Kcmil is fine.

How does that comply with the rule in 250.122(F)?

 

[Smart $]

Is there some reason conduit or EMT is out of the question for this install?

If the tray is existing or just a tray-type environment (other reasons for tray besides this feeder), I'd consider running single conductor tray cable with only one EGC... which, depending on conditions of installation, the tray itself could be the EGC, and conduit the EGC for transitions.

If the transitions can be kept to less than the 10' or 10% rule, the derating ampacity for more than 3 CCC in raceway can be disregarded when all conductors are transitioned in one conduit.

 

[markstg]

How does that comply with the rule in 250.122(F)?

The continuous aluminum sheath exceeds the grounding conductor sizing requirements of Table 250-122. See product literature (Okonite, Southwire)for continuously corregated MC cable.

 

[steve66]

Is there some reason conduit or EMT is out of the question for this install?

The client just prefers to run feeders out of their substations in cable tray. I think they just like to be able to add more cables to the tray in the future. That's what brought up the spacing question: they have an existing cable tray that has some empty space in the middle.

 

[steve66]

The continuous aluminum sheath exceeds the grounding conductor sizing requirements of Table 250-122. See product literature (Okonite, Southwire)for continuously corregated MC cable.

I didn't have any luck navigating Okonite or General Cable's websites.

But the cutsheet for Southwire's Armorlite says oversized grounding conductors are available for parallel feeds by special order???

 

[don_resqcapt19]

The continuous aluminum sheath exceeds the grounding conductor sizing requirements of Table 250-122. See product literature (Okonite, Southwire)for continuously corregated MC cable.

That statement is true for the corregated MC armor, but not for the more common interlocking type of armor. I don't see anything in the thread to indicate that the corregated type of armor is being used for this installation, but it maybe be a more cost effective choice than a special order cable.

 

[steve66]

That statement is true for the corregated MC armor, but not for the more common interlocking type of armor. I don't see anything in the thread to indicate that the corregated type of armor is being used for this installation, but it maybe be a more cost effective choice than a special order cable.

I'm free to specifying whichever type of MC cable provides the best installation for the price. The only cutsheet I found on Southwire's website was interlocked armor.

Does anyone have a link to a cutsheet for the corregated type?

Steve

 

[markstg]

I didn't have any luck navigating Okonite or General Cable's websites.

But the cutsheet for Southwire's Armorlite says oversized grounding conductors are available for parallel feeds by special order???

Actually I'm going to retract my statement. I'm so use to working with High Resistance Ground at the 480V level, where ground currents are limeted to 10A or less.

It appears the letter of the NEC is to install in each MC cable a ground conductor sized per the OCP size, even if you have HRG.

This is not an industrial standard method, NEC or not.

 

[don_resqcapt19]

Actually I'm going to retract my statement. I'm so use to working with High Resistance Ground at the 480V level, where ground currents are limeted to 10A or less.

It appears the letter of the NEC is to install in each MC cable a ground conductor sized per the OCP size, even if you have HRG.

This is not an industrial standard method, NEC or not.

Yes, the code requires a full sized EGC even on HRG systems. The EGC provided in the cable will be the correct size as long as you don't install the cables in parallel.

If the sheath is qualified as an EGC, like the corregated type is, there is no code rule that requires an EGC of the wire type in the cable. Just like there is no rule that requires an EGC in a metal raceway. However if there is an EGC of the wire type in the cable and you use it, then the rule in 250.122(F) would apply.

 

[markstg]

I'm free to specifying whichever type of MC cable provides the best installation for the price. The only cutsheet I found on Southwire's website was interlocked armor.

Does anyone have a link to a cutsheet for the corregated type?

Steve

http://www.okonite.com/Product_Catalog/section3/sec3-pdf/3-9.pdf

 

[steve66]

http://www.okonite.com/Product_Catalog/section3/sec3-pdf/3-9.pdf

Sounds like if I use the ground wire at all, I have to use the larger size, even with the corregated cable.

I think we will specify the larger ground wire.

Not that I would do this, (I don't like this idea any better than using just the corregated cable as a ground), but would the NEC allow just using the cable tray as the ground??

I'm not sure how the rules apply when we have one wiring method installed inside another wiring method.

Steve

 

[Smart $]

...Not that I would do this, (I don't like this idea any better than using just the corregated cable as a ground), but would the NEC allow just using the cable tray as the ground??...

Already answered in post #9.

250.118(11)
392.7(B)
Table 392.7(B)

 

[don_resqcapt19]

Steve,
I have been told that sometimes it is cheaper to order a 4 conductor cable in place of the 3 conductor cable with the larger EGC. You would just not use the EGC in the 4 conductor cable.