Just an opinion poll---
Why would one specify MC cable over AC(BX) cable. Since the interlocking armor of MC is not a sufficient EGC in and of itself, you must utilize the internal ground wire. AC seems easier to install due the grounding capability of the armor in conjunction with the bonding strip. It also costs less. So, why spec. MC? (I have seen the armor come loose from the (box)connector in old residential applications, so this is one good reason. Any one else).

[This message has been edited by Redsy (edited 06-08-2001).]

I'm a big fan of equipment grounding conductors other than the raceway or cable armor. You have already pointed out the major drawback, the connector may be loose & you lose the equipment ground.

As for other advantages, all you have to do is compare "uses permitted" 333-3 & 334-3.

The only additional advantages I can think of are that MC doesn't require the use of bushings, & AC, to the best of my knowledge, is not available with an aluminum jacket. A roll of steel jacketed AC is really heavy.

Tom

[This message has been edited by Tom (edited 06-08-2001).]

If you unravel the outer jacket, it may be 5 times the length of the interior conductors. Is this longer return path in a fault, or leak to ground a consideration?

Tom,

AC cable is readily available in "Lite" versions. See:
http://www.afcweb.com/table2.html

For Places of Assembly I've only used MC cable but I notice that AC would be permitted if an insulated Grounding wire was included in the cable. Do they mean a Green wire? Or could you reidentify one wire (such as the red) with green tape?

Bill

Tom mentioned that MC cable doesn't have a requirement for bushings, but the 250' coils of MC I buy all come with a bag of anti-shorts. I had an inspector make me remove existing MC that was run without bushings from the connectors and install them. Isn't this covered in 110-3(b)?

Electure,

Your inspector dropped the ball. There is no requirement in Article 334 regarding the use of bushings. 110-3(b) does not apply because there are no written instructions attached to the roll of cable requireing installing the bushings and there is nothing in the listing of MC cable that requires their use either. You should consider sending that inspector a invoice for your time.

Bill, the insulated equipment ground you mentioned would have to meet the marking standard in 250-119, taping a conductor #6 & smaller would be prohibited. Unless, of course, the installation complied with 250-119(b), which would not apply in most cases. AC lite may be available, but I still like the insulated equipment ground in MC.


Tom

Tom is 100% right, my thoughts were 100% wrong. AFC is about a mile from my house, so I stopped by and asked why they include the bushings in MC. The tech said that they are included as an "added measure of protection", and although not required at all, (the mylar is considered adequate), their use in encouraged. The AC cable, on the other hand, has the paper more or less as a filler to keep the ground in contact with the armor, and the paper is not deemed as a proper insulator at the termination of the armor.

Tom, Electure,

Interesting,
I would always include a Bushing anyway, but it is good to know that in a pinch it can do without one.

Thanks,

Tom,

I've seen type AC cable used for Isolated grounding circuits where the red conductor was re-identified (taped green) for use as the Isolated grounding conductor. Is this kosher?

Bill

Bill,

I've seen this done plenty of times myself.
If it is #6 & smaller, it technically does not meet the marking requirements. I think this would be fairly low on the hazard scale, but you never know.

Tom

Tom,

Agreed, not a priority #1 hazard. I try to be as correct as possible to avoid as many infractions as I can.

Thanks,

Bill

This leads me to wonder about the original purpose of the anti-shorts. I assumed that they protected the insulation from being punctured by crudely cut armor, something the mylar doesnt seem capable of. However, if their purpose was to prevent ground faults to the armor due to someone damaging the insulation during cutting and removing the armor,(this was more common prior to the roto-split, when hacksaws, or dikes were used) the plastic(mylar) may be adequate.

MC is ok IMO, but i agree with you I like BX,AC cable, in fact I went to get some the other day and my supplier says, " They dont make it anymore", and I said, I will bet thats news to "AFC", so I had to take the MC instead. Later on he said he checked and said yeah they still make it. But the only kind my supplier carries is the HCF whhich is fine, but a little more expensive.

Tom is right, and dont ja just love AHJ, first they say your suppose to follow the manufactureers instructions, and they clearly state the 'red-head' is optional for MC because of the plastic that already exists in it.

>Is this longer return path in a fault, or leak to ground a consideration?

No.

here in both omaha and lincoln ne you have to get written special permission to use either mc or ac. too many so called maintence types electricians (ha ) would add a circuit by using the green wire sometimes they would put a piece of black tape on it but not always. after a few shocking and smoking incidents the AHJ said no more of that stuff. To get that permission is extreamly difficult.

And if they started using the bare wire in NM for a phase conductor, would the AHJ ban NM?

Why not instead require that cables have 3, 6, or 9 conductors plus the green?

Most of the EE's here that design hospital remodels/additions allow AC-ACH or ACHCI(?) to be used instead of MC with it's insulated ground. Both provide an insulated green ground as well as the jacket. They like it for the isolated ground it provides as well as the EGC (the jacket & bonding strip).

By the way, I noticed that someone attached the bonding strip from AC directly to the ground terminal bar. Here, that will get you a red tag. The bonding strip is supposed to be wrapped back against the jacket so that the clamp tightens it against the jacket. Most guys use it to also help hold the anti-short in place with a wrap across the bushing first.

AFC makes a special cable just for insulated/isolated called HCF-90, and already comes with a green grounding conductor, and with the metal sheath, there should not be any reason to remark another conductor, and I dont think remarking a red conducctor would be approved at least not in my area,but again that would be up to AHJ. The reasoning behind that, from what I have found out, is the same as conduit you cannot remark/re-identify a conducctor in conduit as you can in romex/NM, ie switch-leg. Remarking anything violates the no.6 and smaller rule.