Thread: MC conduits

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Robert,

Hope this helps Pocket Installation Guide Page 5

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Originally Posted by RobertSmith

Can someone please provide a clear explanation as to why MC cables are not allowed for exterior use?

Tough question - it is permitted for "some" exterior uses, providing "specific conditions" are met.

In your photo, though, it is not only outdoors, it is direct burial.

Water entering the metal clad covering and corrosion are the two biggest reasons as the conductors inside are likely 'water proof' (rated for direct burial or underwater use).

330.12 Uses Not Permitted. Type MC cable shall not be used where exposed to the following destructive corrosive conditions, unless the metallic sheath is suitable for the conditions or is protected by material suitable for the conditions:
1) Where subject to physical damage
2) Direct burial in the earth
3) In concrete
4) Where subject to cinder fills, strong chlorides, caustic alkalis, or vapors of chlorine of hydrochloric acids

Other that than, it may be used for ...
330.10 Uses Permitted
(3) Indoors or outdoors
(5) To be direct buried where identified for such use
(11) In wet locations where any of the following conditions are met:
a. The metallic covering is impervious to moisture.
b. A lead sheath or moisture-impervious jacket is provided under the metal covering.
c. The insulated conductors under the metallic covering are listed for use in wet locations.

And there is more.

Originally Posted by RobertSmith

Can someone please provide a clear explanation as to why MC cables are not allowed for exterior use?

Robert:

Like JP says, the code is sometimes just a box of runes. I would hammer the physical damage aspect. Hypothetical scenario: extra happy homeowner after a couple of Saturday cold ones hits this thing with the mower and ZZAAPP! Actually, that could be an enlightening experience.

So, let's try this one: Juan Lawnmoerra, while working for happy Saturday Coors boy, hits this cable with his 500 h.p. double wide machine or his $700 wire weed whacker and ZZAAPPOLLERRO!

The next thing you know happy homeowner hears from Juan's abogado and shortly thereafter . . . Ka-Ching! Juan is retiring to Berumda.

Aaron

Type MC (metal-clad) cable is permitted only where it is not subject to physical damage, not directly buried in the earth, not in concrete, not where it might be subject to cinder fills, strong chlorides, caustic alkalis, or vapors of chlorine or of hydrochloric acids. Note that some MC cable is identified for direct burial. If so ID'd it is also suitable for installation in concrete.
Source: 2005 NEC Articles 330.10, 330.12

Of major significance in your particular situation, the picture that was included in your posting suggested that the MC cable you inspected is subject to physical damage. In addition, assuming that the cable was at grade level and at the perimeter of the house, this area is often subject to insecticide and other sprays which might contain ingredients that could deteriorate the sheath.

Lastly, the outer sheath of MC cable is not suitable as an effective ground-fault current path, that's why a seperate equipment grounding conductor (EGC) is included inside the cable. The interlocking sheath provides flexibility at the expense of ensuring water-tight integrity. Exceptions to use in wet locations can be found in Article 330.10(A)(11).