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Thread: AC and MC Cable

  1. #1
    Join Date
    Mar 2005
    Location
    Wyoming
    Posts
    51

    AC and MC Cable

    Hey all, I've got a dumb question that has probably been answered before, but here goes. I do strictly residential work so typically use NM and occasionally MC cable, but I have never used AC cable. Exactly why do they make AC and MC? I know the physical characteristics of the two, i.e bonding strip, paper wrapping, the requirement of redheads and so fourth, but what's the deal? Why is AC typically the only flexible metal cable allowed in hospitals and the like? Is there a worry that a conductor could come in contact with the armor of the cable and that since MC does not have a bonding strip it is possible that even with the correct fittings essentially bonding the armor to the EGC that it may not trip the OCPD due to lack of continuity throughout the interlocking armor itself? Sorry to ramble or sound ignorant, but I couldn't think of a better way to ask. Also, I like how the two cables almost look identical for the most part, but AC requires more stringent fastening requirements.

    Thanks for the help......

    [ September 29, 2005, 01:40 AM: Message edited by: torcho ]

  2. #2
    Join Date
    Jul 2003
    Location
    New Jersey
    Posts
    18,908

    Re: AC and MC Cable

    The basic difference is that Type AC cable has an outer jacket that is a listed Equipment Grounding Conductor, the outer jacket on the similar looking Type MC cable is not a listed EGC. With standard AC cable there will not be a green conductor within the cable, however armored cable type HCFC, which is Health Care Facility Cable there will be a green insulated EGC within the cable. This is the type of cable used in hospitals etc. where two redundant grounding paths are required. One path being the cable jacket the other the green EGC.
    Rob

    Chief Moderator

    All responses based on the 2011 NEC unless otherwise noted

  3. #3
    Join Date
    Feb 2003
    Location
    Massachusetts
    Posts
    48,200

    Re: AC and MC Cable

    Not a dumb question, I have worked with AC quite a bit and now work with mostly MC and still I am not sure about the answer.

    In Hospitals many circuits require redundant grounding conductors. Hospital AC cable can provide this redundant grounding as it has an insulated green grounding conductor (like MC) and it has the bonding strip so the armor is also a grounding conductor (unlike most MC).

    When we used to wire condos with AC it was convenient as there are almost no grounding connections to make. In areas that you can use AC it does save labor.

    Most of the job specs we work under require actual copper grounding conductors, that puts AC out of the picture even if allowed by the NEC.

    The specs often go onto require that we use steel armored MC as opposed to aluminum armor.

    There is supposed to be a Hospital type MC cable coming to market. Once it is out and the price settles out I doubt we will be using any AC cable.

  4. #4
    Join Date
    Mar 2005
    Location
    Wyoming
    Posts
    51

    Re: AC and MC Cable

    Thanks for the help guys. I was just trying to learn some more stuff.

  5. #5
    Join Date
    Apr 2004
    Location
    Minneapolis, Minnesota
    Posts
    1,710

    Re: AC and MC Cable

    I'm also glad you asked. I ran into a similar situation earlier this week and wound up using MC rather than AC. I use mostly NM and EMT, so I was a little unsure of choosing MC vs. AC.
    Jeff
    EIT (full-time)
    EC (part-time)

  6. #6
    Join Date
    May 2003
    Location
    Salt Lake City, Utah
    Posts
    5,319

    Re: AC and MC Cable

    I'd like to make a quick point, in that the requirement for a "redundant ground" applies to all patient care areas in any health care facility, not just hospitals (517.13). This would include dentists, doctors, etc.
    Ryan Jackson, Salt Lake City
    Inspector, Instructor

  7. #7
    Join Date
    Feb 2003
    Location
    Minnesota
    Posts
    4,929

    Re: AC and MC Cable

    I learned the hard way, back during the enforcement of the '93 - '96 NECs that AC is not permitted in damp locations.

    I got gigged for AC run on the surface of concrete masonry units (CMU) that was to be covered by a firred out wall surface. The CMUs above grade were fine, but the AC below the outside grade was in a damp location.

    The "below grade" language is no longer in the text, but the "dry location" limitation still applies.

    MC is much more versatile with the dry, damp, wet (with appropriate conductor insulation) permission.
    Another Al in Minnesota

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