Today, I spotted a rep for a cable company (As in saleman for wire manufacturer) who was showing off his wears. One iten in particular that he was trying so hard to pump up was an MC cable with #12 insulated circiut conductors (Copper), and a #12bare AL ground conductor. Like AC it would fold back to the connector, and become the ground path, with an "approved for ground" fitting. Anyway, I have always understood that the major difference between MC, and AC to be the equipment ground conductor. I questioned this salesman on this, and he inssted that it was listed as MC, not AC. And that this was "the way of the future", of course. So I looked more closely, and yep, it said MC on the tape.. So my question is, what are the differences between MC and AC cable?
Mark Heller "Well - I oughta....." -Jackie Gleason
There is nothing in article 330 that says it has to have an insulated EGC and the White Book says it could be insulated or bare. The only question is whether #12 aluminum meets the requirement of 250.122 <table> for a 20a circuit (the 12ga copper circuit conductors and I think not. Sure it wasn't a #10 al?
gfretwell, the size is one of the key things I noticed... #12 Al bare against the sheath, maybe they consider them as a combined conductive path? As I really wasn't paying too much attention past the sales pitch, I can't remember who made it, but it was listed.
This is from the NFPA commentary in the Handbook....
The armor of interlocking Type MC cable is not recognized by UL as the sole means of providing an equipment grounding circuit but may be used to supplement the internal grounding conductor. If an individual equipment grounding conductor is installed with a multiconductor Type MC cable installation, the separate equipment grounding conductor must be an integral part of the multiconductor Type MC cable. The size of an equipment grounding conductor is determined according to 250.122. The following explanation applies only to installations of single-conductor Type MC cable. If single Type MC conductors are installed as open runs or as messenger-supported wiring, the smooth or corrugated metallic sheath on each conductor must have sufficient cross-sectional area to comply with Table 250.122 or concentric conductors may be provided over the conductor under the metallic sheath. Either the metallic sheath or the combination of the concentric conductors in parallel with the metallic sheath may be used to provide the required equipment grounding path. For single Type MC conductors with an interlocking metal tape armor installed as open runs or as messenger-supported wiring, concentric conductors must be provided over the conductor under the metallic sheath. The total cross-sectional area of the concentric conductors must comply with Table 250.122. If single Type MC conductors are installed in cable tray, the cable tray, an equipment grounding conductor(s) within the cable tray, and/or the equipment grounding provided with the single conductor may be used individually or in any parallel combination to provide the equipment grounding path required by 392.3(C) and 250.118.
So maybe they are considering the conductor and sheath as "the cross-sectional area to comply with Table 250.122"
I did debate with the guy about it's supposed ease of instalation, and box fill benifit. As I don't see it as any easier than making up the grounds, and if you're worried about box fill of grounds, you already have too many in the box IMO. Also mentioned to him that many people don't even like EMT as a ground path, this would be a hard sell for those types. As the whole path would be compromised by a loose lock-nut. Regular MC also has a benifit, of providing enough GEC to have a pig-tail for a device if you play your cards right. And on jobs where I has supervised the install, I know which MC in a box is the path to the panel as the feed to that box, as it is the one that grounds the box. With this stuff I wouldn't know....
Mark Heller "Well - I oughta....." -Jackie Gleason
I believe this is one of the newer MC cables designed for Article 517 applications. One of the manufactures has been advertising a HCF MC cable but I don’t remember which one. If so the bare conductor is only used to provide low resistance of the interlocking armor and is not designed to be connected to anything.
Besides the difference in how grounding is accomplished, AC has "an overall moisture-resistant and fire-retardant fibrous covering" (to wit, paper) over the conductors, whereas MC does not.
I believe that both types can use either steel or aluminum for the armor. Or more correctly, the material isn't specified, except that it be "metallic"; presumably stainless steel or brass would be legitimate as well.
In IAEI magazine's "UL Question Corner"(May-June 2006 issue), there's a 1/2 page Q&A describing the new MC Cable. (For some reason the page is marked January-February). Southwire, the manufacturer of the new type cable,has a full page ad for their new cable in the same issue. I'm sure it's purely coincidence
MC has an insulated EGC whereas AC doesn't, and MC is made from AL whereas AC is steel.
Both AC and MC Cable are available with aluminum or steel armor, and both can have an insulated grounding conductor.