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indoor-type electrical power wiring buried under lawn-- how long will it last?

Discussion in 'Electronic Design' started by Rob Lucas, Feb 16, 2007.

  1. Rob Lucas

    Rob Lucas Guest

    I've just discovered that the previous owner of my house installed a
    15 amp circuit feeding the detached garage by burying a standard 14
    gauge flexible-metal-conduit cable about 6 inches under the lawn. This
    is the stuff that has the two plastic coated conductors and an
    unshielded ground wrapped in a continous coil of glavanzied steel (or
    is it aluminum?).

    Anyway, how many years can I expect before this installation causes me
    problems? I figure worst case in 20 or 30 years the shielding and
    ground wire will have rotted away, but the first thing the cable does
    in the garage is go through a GFI outlet, so realistically it should
    still be safe even without the ground. How long before the standard
    plastic coating on the hot wire is deteriorated by the soil? (assuming
    someone doens't put a garden shovel through it first!)

    I really dont want to dig this whole thing and replace it. Cheers.
     
  2. Donald

    Donald Guest

    Don't let the city inspector find this, you'll be digging it up with a
    fine to boot.

    Underground AC cables need to be rubber coated, PVC does not count.

    So, choose you poison.

    donald
     
  3. I don't think the spiral armored cable is rated for direct
    ground contact. Not only will the galvanized steel shield
    rust off, giving you an unsafe ground path back to the
    panel, the wire insulation is not rated for the moisture of
    direct burial, and will eventually (if it hasn't already)
    develop current leaks that will spin your meter.

    I think I would replace it with type 12-3-UF with ground
    (rated for direct burial). This would allow a current
    rating of 20 amps, but lower voltage loss for a 15 amp
    circuit, but also would allow expansion to a full 240 volt
    circuit, if you ever decide to put something larger, like a
    compressor, in the garage. The cost of the wire is trivial
    compared to the cost of the slot to lay it in.
     
  4. I cannot believe this complies with your city codes.
     
  5. Rob Lucas

    Rob Lucas Guest

    Oh, I'm 100% sure it doesn't. But what I'm trying to do is satisfy my
    curiosity about why a PVC coated wire would ever develop a current
    leak just because its in contact with moist soil.

    Obviously, if/when I go to sell my house I would have to replace this,
    or fully disclose it. Unfortunately for me, I bought this house in an
    estate sale, so there was no living owner to provide me with any
    disclosures. live and learn...
     
  6. It won't. But BX isn't designed to be run without the sheath and that won't
    survive.

    We used to either run plastic conduit or we ran Romex 2 feet deep in sand
    and covered it with cement planks (approved where I was).

    The proper way over here is to use UF and install it according to code.


    --
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  7. Replace it with schedule 80 gray PVC electrical conduit at the depth
    required by your local building inspector. That mess you have now could
    develop leakage to the soil, and have a high voltage gradient that could
    kill someone.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  8. Andrew Edge

    Andrew Edge Guest

    Don't worry too much you can place cables in water and still
    comply with the code! A different story though.

    The metal protection is good as it gives mechanical protection and
    should satisfy your local regulations. I assume the metal shielding is
    around the 2 plastic wires.

    In case you don't have metal protection around the plastic.
    You need a metal slab or plate on top of the wires or a curved metal
    tile.

    The Plastic should be at least PVC R2 or a higher rating.
    If it is Rubber at least G7 rubber. (Older ones mostly had G5 rubber
    I noticed).

    6 inches under the lawn is wrong. Should be at least 2.5-3 feet for
    wires with metal shielding otherwise >=9feet.

    An unshielded ground wire is pretty normal and agrees totally with the
    code.

    Andy
     
  9. Fred Bloggs

    Fred Bloggs Guest

    It depends on soil conditions and the PH, it could dissolve the
    insulation within a year or so, and once the integrity of the insulation
    is breached, the conductor will corrode at an accelerated rate. The 6"
    depth is okay for low voltage but universally out of spec for line
    voltage which is usually 18". If all you have in the garage is lighting
    then a quick fix would be to convert to low-voltage and replace the BX
    with a direct burial UF type of bundle which can *possibly* be pulled
    through the existing conduit.
     
  10. mpm

    mpm Guest

    Obviously, there's the correct way to do this, and then the way it was
    actually done.

    If you're not going to correct this now, and you don't intend to
    disable the circuit, perhaps you could at least install a GFCI breaker
    on this branch circuit....?

    -mpm
     
  11. Rob Lucas

    Rob Lucas Guest

    Thats probably the best idea. If/when the insulation is ever breached,
    the GFI will kick out before someone gets hurt, and the problem won't
    go undetected spinning my electric meter
     
  12. Rich Grise

    Rich Grise Guest

    Of course, you know to put it at the entrance panel end, not the garage
    end, right?

    Cheers!
    Rich
     
  13. ehsjr

    ehsjr Guest

    Total rubbish.
     
  14. ehsjr

    ehsjr Guest

    The GFCI receptacle in the garage does absolutely
    nothing to ameliorate the problem. The receptacle
    can protect from a problem in a device plugged
    into it, or from a problem dowenstream of itself.
    It can do nothing to protect from a problem between
    itself and the power source.

    The only right answer is to replace with new, up to
    code wiring, or just disconnect (at the house end)
    and abandon or remove the old stuff. Per the US
    national code "up to code" with regard to burial
    depth, means, in general:
    Direct burial UF cable without a raceway must be
    buried at least 24" deep. If you use rigid metallic
    raceway listed for underground installation, it must be
    buried at least 6". If you use non-metalic raceway
    you have to go 18". You can reduce that to 12" if
    you GFCI protect the circuit, and if the circuit is
    no more than 20 amps.

    There are specific locations (for example under
    driveways) with different rules, per 300.5 in the
    National ELectrical Code

    Ed
     
  15. Jim Thompson

    Jim Thompson Guest

    At last, someone got out the code book.

    I was going to do it myself, but I've been trying to digest a
    nasty-worded contract this afternoon.

    So I sent it to my son-in-law ;-)

    ...Jim Thompson
     

  16. There are still local codes to worry about. Some soil conditions
    make the use of metalic conduit in violation of the code because the
    soil PH.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  17. Andrew Edge

    Andrew Edge Guest

    Very crappy


    Andy
     
  18. Klaus M.

    Klaus M. Guest

    Hey hey.

    Your answer Mr. Edge complies with the Central European directives
    which may seem crappy compared to the ones given by Mr. Eddy.

    Klaus
     
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