Prohibiting MC Cable Use, Why?

[don_resqcapt19]

When you did not use the anti-short bushings?

I have never installed an anti short bushing...but also have never installed that type of cable.

 

[jeremysterling]

Back in the ninety's, the City of Houston prohibited MC cable. So we ran miles of AC cable, everywhere. I'm not sure when or why, but I believe the MC ban has been lifted.

Currently, the City of San Antonio prohibits the installation of MC without holding City-issued installer certification and a current rotosplit tool calibration affidavit.

Here in Austin, the MC rules are also onerous. There are numerous local amendments to Article 330. One of the head scratchers is the prohibition against more than four MC cables entering a box. ANY box. If you have a 12" X 12" X 6" box with five 12/2 MC cables and nothing else, that is a violation.

As far as the craftsman thing, I thinks its interesting that in Europe where most "craftsman" ideals stem from, they have abandoned conduit in favor of cabling almost exclusively.

 

[jim dungar]

Are any of the code rules that ban MC left over from the original BX days, where there was no fault clearing path and sometimes it would glow red from the fault current flowing on the armor?

Isn't BX of the AC family and not MC? Or, am I backwards again?

 

[PetrosA]

Isn't BX of the AC family and not MC? Or, am I backwards again?

I'm never really sure what people mean when they say BX. When I started, we used a steel clad cable with an aluminum tape in it that got bent back over the antishort and twisted around the cable and we called it BX. Others here seem to think of BX as the old wire that had only the CCCs in it with a steel jacket (that doesn't do well as a ground conductor). I mostly use MC-lite (AL clad) but it somehow feels flimsy to me, and most of what's available around these parts only pulls in one direction without opening up. Junk.

 

[iwire]

It's not that I don't trust you and I have seen this other places.

Why do they include a bag of antishort bushings when you buy a roll of "MC" cable? I'm pretty sure they come from the manufacturer that way.

So I use them because it's better to be safe than sorry.

If you look in the NEC article for type AC cable you will find a requirement for anti shorts for AC. you will not find the same requirement in the MC article.

That said, like many I use them anyway. Here is a link to the NEMA bulletin http://www.nema.org/Technical/Documents/Bulletin-90.pdf

 

[jtinge]

At my facility, we don't permit the raceway to be used as an EGC and require a properly sized insulated EGC be installed with the other circult conductors.

In re-reading the NEC 330.108 and the UL White Book desciption for MC Cable, the EGC requirement can be achieved by using a insulted EGC integral with the cable, several insulated EGC's whose total cross sectional area complies with Table 250.122, or the combination of insulated EGC with the metal sheath.

From the UL White Book, MC Cable:
"Cable with interlocked armor that has been determined to be suitable for use as a grounding means has interlocked aluminum armor in direct contact with a single, full-sized, bare aluminum grounding/bonding conductor. This cable is marked to indicate that the armor/grounding conductor combination is suitable for ground. The equipment grounding conductor required within all other cable with interlocked armor may be insulated or bare, may be sectioned, and is located in the cable core but not in contact
with the armor. Any additional grounding conductors of either design have green insulation. One insulated grounding conductor may be unmarked, one other may have only a yellow stripe and the balance have surface markings that indicate they are additional equipment grounding conductors or isolated grounding conductors.

The sheath of the smooth or corrugated tube Type MC cable or a combination of the sheath and a supplemental bare or unstriped green insulated conductor is suitable for use as the ground path required for equipment grounding. The supplemental grounding conductor may be sectioned. When sectioned, all sections are identical. Each additional green insulated grounding conductor has either a yellow stripe or a surface marking or both to indicate that it is an additional equipment or isolated grounding conductor. Additional grounding conductors, however marked, are not smaller than the required grounding conductor."

An MC cable with an integral insulated EGC could be considered to comply with the requirement not use the raceway as an EGC and to provide a dedicated EGC with the circuit conductors.

Knowing that MC Cable is available in several configurations to achieve the required EGC can make it difficult to guarantee that an MC Cable, specified with an integral insulated EGC, was actually provided. This would be an increased burden on an inspector to check this.

I could see this being a reason for someone not permitting MC Cable use due the increased burden of verifying the EGC configuration and cost and schedule impacts if it having to be replaced if detected after installation.

 

[iceworm]

jt -
You lost me. As I understand your post, the NEC and UL, say its okay to use the sheath as the EGC (if rated).

Some owner specs say to not use the raceway as the EGC. So MC must have internal EGC.

Okay?

If by 'inspector", you mean "AHJ", I don't see any increased burden. The AHJ is only concerned with NEC minimum. The owner specs are not her problem.

As for the owner enforcing the material and work meet their specs - yes, they have to do that - even with a good, reliable, honest contractor.

"Honest" defined as a fair day's work for a fair day's pay; material provided as speced with agreed markup; installation per design; snafu's in design plans RFIed before installation.
ice

 

[jtinge]

Point taken. Sorry for the confusion. Here at our facility (gov't installation) our inspectors are checking both compliance with the specified requirements and minimum code compliance as an agent of our AHJ. Forgot this is not the same arrangement outside of our fence.

What I was trying to convey was a possible technical reason that a locality might use to prohibit the use of MC Cable, which is a code permitted wiring method. The reasons described previously for prohibiting its use seeme more emotional to me and generally based on the perception unsing MC Cable encroaches on the use of skilled workers, resulting in inferior workmanship and sloppy installations. I'm not sure I've seen any evidence to support this perception and was simply trying to offer another perspective.

 

[don_resqcapt19]

Isn't BX of the AC family and not MC? Or, am I backwards again?

Jim, I think you are correct, but when local codes are changed the two are often treated the same...I was just suggesting that the problems with the old BX could have led to the local bans on the use of metal jacketed cable wiring methods.

 

[jim dungar]

Jim, I think you are correct, but when local codes are changed the two are often treated the same...I was just suggesting that the problems with the old BX could have led to the local bans on the use of metal jacketed cable wiring methods.

I believe you are correct.

 

[wirebender]

...Knowing that MC Cable is available in several configurations to achieve the required EGC can make it difficult to guarantee that an MC Cable, specified with an integral insulated EGC, was actually provided. This would be an increased burden on an inspector to check this.

I could see this being a reason for someone not permitting MC Cable use due the increased burden of verifying the EGC configuration and cost and schedule impacts if it having to be replaced if detected after installation.

I have only used two brands of MC without the wire grounding conductor.

One has a yellow marking on the sheath and the other has a purple marking.

This makes it readily identifiable from regular MC.

 

[martinpgrant]

Where I work (State University) we require all above ground wiring in either EMT or Rigid with flex allowed at lighting whips and moving equipment only. No cable assemblies. The logic is to allow future modification, pulling additional circuits, etc.

 

[patbet]

It all comes down to workmanship. A slob can make any install look bad, and a professional vice-versa.

 

[hbendillo]

A "neat and workmanlike manner" were a part of the NEC back when people took pride in their work, it seems as though things have trended the other way although I would not agree for the better of the trade and it all seems to spur around mc cable as I rarely here anyone debating over any other raceway the way that they do mc cable. I do agree that it has a purposeful use and I do use it at times and I do allow it use but there are times when I specify other raceways to the exclusion of mc cable, I was just saying that my personal experiences are what I described and they seem to come up negative more times than I care for.

Workmanship is sometimes an issue depending on the level of competence of your local work force. MC cable has been used extensively in some parts of the country and the work force has experienced installers. Whee I have practiced engineering the workmanship issue is a crap shoot. I think it is tempting to not strictly follow support and fastening requirements. However, I do like the option. I frankly specify my jobs conduit only and leave to the contractor to request it. I go over the values and issues using MC with the owner and let them decide. That way everyone is aware the installation will be scrutinized.

 

[hbendillo]

Wow - interesting responses.
Following responses are based on my work is 99.99% industrial

I can absolutely guarentee that MC-HL (specifically) is a god-send here is the great north - far superior to any conduit system.

As for having the cables "blow up" when moving them in a cabinet - I don't know what to suggest.
Maybe, "Any wiring method, improperly installed, is a danger".

ice

In industrial work MC is usually installed in cable trays in open ceiling areas. It is difficult to hide poor workmanship in that kind of environment.