Type MC cable

[mbrooke]

Wanna see that. Betcha it had nothing to do with the lack of a bonding strip.

-Hal

I'd upload the PDF but the forum won't let me:

https://imgur.com/uNN9Zob

 

[rbalex]

I wish UL were more generous with White Book information now. Type MC has three basic "flavors"(constructions). [Section 330.2] The following describes how each construction Type handles EGCs (bonding conductors).

METAL-CLAD CABLE (PJAZ)

GENERAL​

This category covers Type MC metal-clad cable. The cable is rated for use

up to 2000 V, and certified in sizes 18 AWG through 2000 kcmil for copper,

12 AWG through 2000 kcmil for aluminum or copper-clad aluminum, and

employs thermoset or thermoplastic insulated conductors. It is intended for

installation in accordance with Article 330 of ANSI/NFPA 70, ‘‘National

Electrical Code’’ (NEC).

The cable consists of one or more insulated circuit conductors, a grounding

path (grounding conductor, metal sheath, or combination thereof) as

described below, one or more optional optical fiber members, and an overall

metal sheath. The metal sheath is an interlocked metal tape, a corrugated

metal tube, or a smooth metal tube. The metal sheath of single-conductor

cable is nonferrous. A nonmetallic jacket may be provided under and/or

over the metal sheath. Cable with metal armor, rated 2400 to 35,000 V is

covered under Medium-voltage Power Cable (PITY) and is marked ‘‘Type

MV or MC.’’

Cable with interlocked armor that has been determined to be suitable for

use as a grounding means has interlocked aluminum or steel 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.

Bonding is a big issue in Hazardous Locations since circulating currents could cause a spark. (I know it isn't supposed to happen with proper bonding but hazardous locations often take a "belt and suspenders" approach. {Section 501.30(A)] All three construction types are fine in Class I. Division 2 [Section 501.10(B)(1)(5)] when installed per {Section 501.30(A)]. They would be fine in Division 1 also except interlocked armor has the potential for transmitting gases,

 

[mbrooke]

Hmmm... basically lots of options... But I take it manufacturers choose "A nonmetallic jacket may be provided under and/or

over the metal sheath" when the armor is without bond strip?

 

[mbrooke]

But I guess the inner wrap doesn't have to be dielectric in the paper can be used:

9:40 shows paper insulation with the bond strip, but not on normal MC.

I want to say manufacturers are "double insulating" MC when it doesn't have a bonding strip.

 

[winnie]

I guess since the sheath of normal MC cable is not considered an effective ground fault current path, the risk is some sort of damage to the cable which energizes the sheath but doesn't trip a breaker.

IMHO this falls under the 'NEC provides for practical safeguarding' rather than _perfect_ safeguarding. Sometimes it happens that a nail gets fired through NM cable in just such a way as to intercept the hot conductor and nothing else. Or a cable clamp is over-tightened and pinches through the insulation. I don't believe that this failure mode warrants a change in the requirements for MC cable.

Separate question: does anyone still manufacture 'smooth wall' MC cable?

-Jon

 

[mbrooke]

I don't believe that this failure mode warrants a change in the requirements for MC cable.

So your good with a panel full of GFCIs?

 

[winnie]

So your good with a panel full of GFCIs?

Oh, the transformer on my service is high resistance grounded, every breaker has GFPE protection, and every receptacle is GFCI. All use fiber optics to restrain the upstream ground fault device if a downstream device detects a ground fault, and each receptacle measures and reports voltage to the system so that changes in voltage drop can be detected and repaired. Instead of 5-15 receptacles all devices are connected using inductive coupling. If a problem is detected, the system schedules with an electrician to do the repair, and just before the EC arrives the automatic coffee dispenser by the panel turns on and provides a hot beverage on arrival.

 

[mbrooke]

Don't give them ideas

 

[Nuber]

Write a proposal for a ban due to the infinitesimal possibility that something bad can happen. Infinitesimal with no substantiation seems to dictate what they'll approve.

Post of the decade candidate right here. Unfortunately.

 

[romex jockey]

But I guess the inner wrap doesn't have to be dielectric in the paper can be used:

9:40 shows paper insulation with the bond strip, but not on normal MC.

I want to say manufacturers are "double insulating" MC when it doesn't have a bonding strip.

This UL 1569 may hold the answer to that .......~RJ~

 

[rbalex]

Hmmm... basically lots of options... But I take it manufacturers choose "A nonmetallic jacket may be provided under and/or

over the metal sheath" when the armor is without bond strip?

Possibly, but:

Cable with interlocked armor that has been determined to be suitable for
use as a grounding means has interlocked aluminum or steel 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.

If the paper were to interfere with the ground path, UL wouldn't permit marking it or list it under their Standard. Use any other way would violate Section 110.3(B).

 

[mbrooke]

Possibly, but:

If the paper were to interfere with the ground path, UL wouldn't mark or list it under their Standard. Use any other way would violate Section 110.3(B).

In no disagreement. But thats really throwing electrical theory underneath the bus.

 

[mbrooke]

This UL 1569 may hold the answer to that .......~RJ~

I know you're on to something- but I'm not making the connection

 

[rbalex]

In no disagreement. But thats really throwing electrical theory underneath the bus.

And what theory is that?

 

[mbrooke]

And what theory is that?

Any exposed metal needs an effective ground fault current path. Low Z to be precise.

 

[rbalex]

And how did it get "thrown under the bus" assuming it was otherwise installed correctly? BTW that's not "electrical theory" like a Maxwell equation; it's simply a Code requirement. Sections 250.4(A)(3), (4), & (5) are the easiest to cite. There are requirements for specific types of installations throughout Article 250 and the rest of the Code. (There's nothing theoretical about it)

 

[mbrooke]

And how did it get "thrown under the bus" assuming it was otherwise installed correctly?

Faults can and do happen. Nothing stops a nail through MC or damage during termination.

[/QBTW that's not "electrical theory" like a Maxwell equation; it's simply a Code requirement. Sections 250.4(A)(3), (4), & (5) are the easiest to cite. There are requirements for specific types of installations throughout Article 250 and the rest of the Code. (There's nothing theoretical about it

And thats the precisely the issue at hand. Effective ground fault current path is not defined. Ohms law is thrown under the bus.

 

[hbiss]

I'd upload the PDF but the forum won't let me:

https://imgur.com/uNN9Zob

Still waiting for that article. You can link to wherever you got that pdf. That magazine archive only goes back to 2014.

Also, MC "smart cable"? What's the difference between that and type AC?? For me, the advantage of MC is the integral EGC that provided a reliable ground, unlike AC that relied on the integrity of the connectors. So now, in the name of cutting labor costs they are back pushing what is actually AC cable and again relying on the connectors? I ain't buying it!! If they want to reinvent AC fine, but don't call it something it isn't and say how great it is just to sell product.

What if the middle of the run was damaged or drilled into?

I think the benefit of a separate EGC conductor far outweighs the very remote possibility that the EGC could be severed without damaging the current carrying conductors thereby tripping the breaker.

-Hal

 

[rbalex]

Faults can and do happen. Nothing stops a nail through MC or damage during termination.
...
And thats the precisely the issue at hand. Effective ground fault current path is not defined. Ohms law is thrown under the bus.

In that case, you are a perfect example of last sentence of Section 90.1(A) and have been for a long time.

 

[winnie]

Also, MC "smart cable"? What's the difference between that and type AC?? For me, the advantage of MC is the integral EGC that provided a reliable ground, unlike AC that relied on the integrity of the connectors. So now, in the name of cutting labor costs they are back pushing what is actually AC cable and again relying on the connectors? I ain't buying it!! If they want to reinvent AC fine, but don't call it something it isn't and say how great it is just to sell product.

Not that it gets used this way, but the full size egc in MC-AP _may_ be brought into the junction and spliced. If you use a splice suitable for Aluminium, that is. If you want a fully bonded armor _and_ a spliced wire egc. If you feel that the small safety benefit is worth the large cost.

-Jon