MC Cable in Cable Tray

[steve66]

Already answered in post #9.

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

I thought single conductors vs. MC cable might make a difference. But looking at your references, it does look like the tray could be used as the sole ground, in either case. (We still won't do this, just like we wouldn't normally use EMT as the only ground).

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.

I had actually wondered about that. It probably depends a lot on the size of the wire. It's hard to believe an extra 350KCM wire is going to be cheaper than a #1. But I might list that as an option.

I guess with the wire being 350KCM, there wouldn't be any problem with the color of the wire? The electricians would just tape the end with green tape?

Steve

 

[rbalex]

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.

It is indeed generally less expensive to order 4/C cable and use ??in no case shall they be required to be larger than the circuit conductors supplying the equipment? from 250.122(A). However, what is usually even more economical is to eliminate the EGC in the cables entirely and use the tray. This is one of the reasons cable tray is not defined as raceway in the Code.

 

[Smart $]

It is indeed generally less expensive to order 4/C cable and use ??in no case shall they be required to be larger than the circuit conductors supplying the equipment? from 250.122(A). However, what is usually even more economical is to eliminate the EGC in the cables entirely and use the tray. This is one of the reasons cable tray is not defined as raceway in the Code.

Hmmm... that seems to present a situation when using multiconductor cables...

250.122(F) Conductors in Parallel. Where conductors are run in
parallel in multiple raceways or cables as permitted in
310.4, the equipment grounding conductors, where used,
shall be run in parallel in each raceway or cable.
Each parallel equipment grounding conductor shall be
sized on the basis of the ampere rating of the overcurrent
device protecting the circuit conductors in the raceway or
cable in accordance with Table 250.122.

The way I'm currently reading the requirement, when running parallel multiconductor cables, the EGC's, where used, must be in the cable. Cable tray is an identified, but provisional type of EGC... so we have a "where used" EGC... but cable tray cannot be in each cable...

And what of single conductor cables? This requirement should become moot if using single conductor cable, but there is no exception. I can see it now... some green inspector coming along and trying to make a contractor run single-conductor cable with an EGC in each cable.

I'd say the wording needs to be improved. :roll:

 

[jdsmith]

We run parallel cables in cable tray frequently. We do not buy special cable with an upsized ground to meet 250.122. We purchase the standard 3/C w/ground cable and install a single conductor ground with the cables that is sized according the breaker protecting the parallel cables. The single conductor meets 250.122, and the smaller grounds in the cables are connected but serve no code required purpose.There is no reason the undersized grounds can't be connected if they are present in the cable, they just don't meet the code requirement for the circuit ground.

Do you need continuous armored or interlocked armored MC rated cable in this application? Ordinary type TC tray cable is much cheaper to purchase and faster to install.

 

[rbalex]

Hmmm... that seems to present a situation when using multiconductor cables...The way I'm currently reading the requirement, when running parallel multiconductor cables, the EGC's, where used, must be in the cable. Cable tray is an identified, but provisional type of EGC... so we have a "where used" EGC... but cable tray cannot be in each cable...

And what of single conductor cables? This requirement should become moot if using single conductor cable, but there is no exception. I can see it now... some green inspector coming along and trying to make a contractor run single-conductor cable with an EGC in each cable.

I'd say the wording needs to be improved. :roll:

The key is "where used." The most literal interpretation would require the cable tray,"where used" as the EGC to be inside the cable rather than the other way around. If there is no EGC inside the cable, there still has to be one somewhere and that somewhere is the tray itself, which may be used as a common EGC for any circuit in it as long as it otherwise meets the properly associated groundfault OCPD requirements.

Unfortunately, the miriad edits to the Code over the years has left it full of poorly worded rules - many with some failry bizzare unintended consequences if taken to the most literal wording.

 

[steve66]

Hmmm... that seems to present a situation when using multiconductor cables...The way I'm currently reading the requirement, when running parallel multiconductor cables, the EGC's, where used, must be in the cable. Cable tray is an identified, but provisional type of EGC... so we have a "where used" EGC... but cable tray cannot be in each cable...

Smart: I see what you are saying, but think the real problem is with the NEC's definition of "conductor". It seems like sometimes a "conductor" can be any conducting path - including either a raceway or a wire. In 250.120(A), it clearly includes some metal raceways.

In 250.120 (F), I think the definition is clearly limited to a "wire". It can't include raceways or cable trays because you can't size those per table 250.122.

So (F) is really saying if we use a "wire" in each cable or raceway, each "wire" has to be sized per table 250.122. If you don't use a wire, you don't have to comply with a EGC in each cable.

And what of single conductor cables? This requirement should become moot if using single conductor cable, but there is no exception. I can see it now... some green inspector coming along and trying to make a contractor run single-conductor cable with an EGC in each cable.

Smart$:

I think by definition, a "cable" means more than one conductor. So when running single "conductors", this doesn't apply there aren't any "cables".

 

[steve66]

It is indeed generally less expensive to order 4/C cable and use ??in no case shall they be required to be larger than the circuit conductors supplying the equipment? from 250.122(A). However, what is usually even more economical is to eliminate the EGC in the cables entirely and use the tray. This is one of the reasons cable tray is not defined as raceway in the Code.

If I just use the cable tray as a ground, do I just leave the ground EGC inside the MC cable unconnected?? Or do I have to order special cable without the EGC??

If I just use the cable tray as a ground, can I run a single #1 conductor along the tray and bond it to every section? Or does that violate 250.122(F)?

This seems to be the cheapest way to go while having a very reliable ground path.

 

[rbalex]

My general "Type MC" experience is with the continuously corrugated sheath construction and my responses reflect that.

If I just use the cable tray as a ground, do I just leave the ground EGC inside the MC cable unconnected?? Or do I have to order special cable without the EGC??
...

Not sure who your regular supplier is, but 3/c MC without ground is available.

...
If I just use the cable tray as a ground, can I run a single #1 conductor along the tray and bond it to every section? Or does that violate 250.122(F)?
...

This is fairly common practice - in refineries no less. Even more common is using Type TC.

 

[Smart $]

Smart: I see what you are saying, but think the real problem is with the NEC's definition of "conductor". It seems like sometimes a "conductor" can be any conducting path - including either a raceway or a wire. In 250.120(A), it clearly includes some metal raceways.

In the NEC, conductor is pretty much used in the context of "recognized" conducting paths.

In 250.120 (F), I think the definition is clearly limited to a "wire". It can't include raceways or cable trays because you can't size those per table 250.122.

So (F) is really saying if we use a "wire" in each cable or raceway, each "wire" has to be sized per table 250.122. If you don't use a wire, you don't have to comply with a EGC in each cable.

I agree it is limited to a wire-type conductor. But that's the point. It states: the EGC, where used... but it should say something like: where a wire-type EGC is used in parallel raceways or cables...

I think by definition, a "cable" means more than one conductor. So when running single "conductors", this doesn't apply there aren't any "cables".

Well the NEC has no explicit definition of cable... and it should not be construed to mean multiconductor cable, since there are numerous instances of multiconductor cable used throughout the code. Even though the majority of cables are multiconductor, there are sections and subsections in Article 392 Cable Trays which are dedicated to and use the term single-conductor cable.

All I'm emphasizing is that the current wording could be cleaned up a tad. When one gets into a situation such as the subject of this thread, it's difficult to know for certain whether an upsized, wire-type EGC in each cable is required or optional.

 

[steve66]

In the NEC, conductor is pretty much used in the context of "recognized" conducting paths.


I agree it is limited to a wire-type conductor. But that's the point. It states: the EGC, where used... but it should say something like: where a wire-type EGC is used in parallel raceways or cables...


Well the NEC has no explicit definition of cable... and it should not be construed to mean multiconductor cable, since there are numerous instances of multiconductor cable used throughout the code. Even though the majority of cables are multiconductor, there are sections and subsections in Article 392 Cable Trays which are dedicated to and use the term single-conductor cable.

All I'm emphasizing is that the current wording could be cleaned up a tad. When one gets into a situation such as the subject of this thread, it's difficult to know for certain whether an upsized, wire-type EGC in each cable is required or optional.

Yes, we both agree on the first two points.

Regarding the "single conductor cables", I thought the Code defined the difference somewhere. But I cant seem to find it now.

I thought conductors were like THHN or XHHW, and all the other types listed in 310. I thought "cables" had an extra outter covering that made them a "chapter 3 wiring method". So "cables" could be ran without a raceway, where conductors could not.

But again, I cant find that now, and like you said, that doesn't seem to fit with the paragraphs in Article 392 that keep listing "conductor cables". It leaves me confused. And like Bob mentioned, I think it makes for some odd interpertations if taken literally.

Steve