While I am reluctant to post anything I will do so for my "friends" out there.
1) Type MC Cable is evaluated under UL 1569. There are specific test procedures that have to be met to be considered suitable for use in a wet location. Here is the language for that evaluation.
12.11 In the case of a cable that is intended for use in wet locations and has interlocked armor over
conductors insulated for use in only dry locations, a jacket resistant to moisture shall be applied under the
armor. That jacket shall comply with 12.8 – 12.10 and also have a mechanical absorption of water (MWA)
value that complies with 33.1. See 12.15 for the long-term evaluation of a jacket material not named in
this requirement or not complying with the short-term tests specified for the material.
Now since most Type MC Cable is produced with dual rated insulation (i.e THHN/THWN-2) the conductors are not necessarily "dry" location only rated so we have other provisions within the standard.
Now the option for a covering over the armor is based on this rule within UL 1569:
14.1 A covering is required over the interlocked, smooth, or corrugated armor of a cable that is marked
[see 40.1(j)] for direct burial. In the case of a cable that is not marked for direct burial, a covering is
acceptable over the armor but is not required. A covering is required over the interlocked armor of a cable
that is intended for use in wet locations if the cable does not include a jacket over the assembly under the
armor (see Table 9.1).
So as you can see it applies in many ways. However, here is the premise of why we (and all do) place a dual rated conductor under the insulation of Type MC Cable in general.
4.2 The insulation in each of the following cables shall be of a material that is acceptable for use in wet
locations if the cable has armor consisting of interlocked metal strip but does not include a jacket over the
assembly under the armor:
a) Cable that is marked for direct burial; and
b) Cable that is intended for use in wet locations.
Rather than create two different products it is easier to produce Type MC Cable with conductors within the armor that is already rated for a wet location....then choose to either jacket the inners of dry location only conductors or use wet location conductors and jacket the armor itself.
The jacket material is tested under the specifications found in UL 2556 and those in UL 1569 so it is considered impervious and in most all cases made of the same material that would be considered wet location rated for the very conductors within the cable.
Hope this helps explain it.....I am more than happy to elaborate at the request of the very friend that asked me to comment in the first place. Here is a GREAT chart from UL 1569 that kinda explains it all.
I think statement 14.1 of UL 1569 and the Table 9 kinda answers all of the questions about the uses in terms of wet or dry locations.
NOW....my only issue to which I need to drive a change is the intent of a "Damp" location in terms of the standards.
Damp does not appear at all in UL 1569 and only once within Article 330, except as stated below:
(10) In dry locations and embedded in plaster finish on brick
or other masonry except in damp or wet locations.
So the term "Damp" is left to the local AHJ. We know that A wet location rated cable is fine in a damp or dry location. We know that a dry location cable is only acceptable in a dry location installation. However, unlike Article 320 or 334 the prohibited use of Type MC Cable in a specific damp location (where not embedded in plaster finish on brick or masonry in damp or wet location) we seem to have an absence of guidance from both the standard and the NEC.
The closest we can get to not allowing Type MC Cable in a "damp" location, even with conductors inside that are rated for a wet location is to look at 110.11. Since Type MC Cable is not " identified" for a damp location and clearly in the standard not evaluated for it either we have to fall on the wet location "identified" products to be used in a lesser exposed environment.
Some will argue it is clear.....others will argue it is not.....the arguments can continue. However, a damp locations is very different than a wet location (if even by definition alone) and an opinion (my personal opinion) is that a damp location is not a wet location and is not subject to saturation but moderate degrees of moisture. The armor is bonded at each end with a listed fitting, the armor is not an effective ground fault current path or EGC...it offers a level of mechanical protection that I do not feel a damp location would degrade over time.
Now....every install is different.....every "damp" location is different (i.e near oceans and so on with heavy damp salt laden air) but again without some direction the AHJ will more than likely refuse the use of Type MC with wet location inners in a damp location because it is not actually evaluated or identified for a "damp" location.
Ok...enough.......gotta run..but those are my thoughts...you can clearly agree to disagree....it's all good.
