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Thread: exisiting 120/277v 2x4

  1. #1
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    exisiting 120/277v 2x4

    we are doing a project where they want to reuse exisitng 2x4 laying fixtures T8. The fixtures had been 277 now the new circuit for lighting is 120v and the fixtures are missing the small J box cover with green screw that is normally on fixture. I have a 2 part question

    first Can I supplement the J box cover with a fabricated cover and 1/2'' knockout and ground screw and still be cover under NEC and UL listing of fixture ? I was thinking of screwing a handy box cover with 1/2'' ko they self tapping a ground scew, what you think is this OK?

    Second Since the ballast was already tapped at 277 will the 120v ballast take to new voltage, I keeo hearing it wont work. I am going to try tomorrow and see if it does But I will give no guarantee they will stay on. Also these fixtures are in ware house with OS sensor. Is there a specific ballast i should be using for this application any way to work with OS, like a rapid start ? The fixtures are pretty old and not sure what is already mounted I will check morning

  2. #2
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    It will depend on the specific ballast being used.

    See this old thread:
    http://forums.mikeholt.com/showthread.php?t=137797

    Some ballasts adjust for the supply voltage, other apparently 'blow' (somehow?) the unused feed when used at one or the other voltage.

    -Jon

  3. #3
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    Quote Originally Posted by winnie View Post
    It will depend on the specific ballast being used.

    See this old thread:
    http://forums.mikeholt.com/showthread.php?t=137797

    Some ballasts adjust for the supply voltage, other apparently 'blow' (somehow?) the unused feed when used at one or the other voltage.

    -Jon
    If you put fuses in each tap, then in theory the higher current through the low voltage tap could blow that fuse. But that would imply that the various inductor "taps" would actually have to be separate coils without mutual inductance to avoid shorting out energized turns by paralleling them.

    Modern electronics could make the same choice reversibly.

    mobile

  4. #4
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    Quote Originally Posted by mannyb View Post
    they self tapping a ground scew, what you think is this OK?
    In general you can't use self tapping screws for EGCs.

  5. #5
    My experience has been newer multi-volt ballasts don't have a "memory", the ballast will adjust to the supply voltage within the range of the ballast, older ballasts, 10+ years, can have an issue sometimes.

  6. #6
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    Quote Originally Posted by iwire View Post
    In general you can't use self tapping screws for EGCs.
    You can use a self tapping screw that utilizes "machine screw" type of threads. I believe there must be a minimum number of threads engaged into the metal, which is why you sometimes must use the raised bump that has a hole for a grounding screw in it - the metal is thick enough there to comply.

    From recollection anything less then 32 threads per inch is not acceptable in 1/16" or less of metal, which would be two threads must be engaged in 1/16 of an inch. A nut on the back side however would be acceptable if not enough threads are engaged.

    What you generally can't use is "sheet metal" variety of screws

  7. #7
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    Quote Originally Posted by kwired View Post
    You can use a self tapping screw that utilizes "machine screw" type of threads. I believe there must be a minimum number of threads engaged into the metal, which is why you sometimes must use the raised bump that has a hole for a grounding screw in it - the metal is thick enough there to comply.
    Two threads.

    250.8(A)

    (5) Machine screw-type fasteners that engage not less than
    two threads or are secured with a nut
    (6) Thread-forming machine screws that engage not less
    than two threads in the enclosure
    "Electricity is really just organized lightning." George Carlin


    Derek

  8. #8
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    Note that you cannot use a nut instead of two threads with a self-tapping screw.

    mobile

  9. #9
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    Quote Originally Posted by GoldDigger View Post
    Note that you cannot use a nut instead of two threads with a self-tapping screw.

    mobile
    I will only say that there are machine thread screws that can tap their own threads and there are non machine thread screws that tap their own threads. There are also screws of both types that can drill their own hole before tapping threads.

    Most of the 10-32 "ground screws" I have purchased for several years need a previously drilled hole, but can tap threads in that hole if there is none.

  10. #10
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    Quote Originally Posted by kwired View Post
    You can use a self tapping screw that utilizes "machine screw" type of threads. I believe there must be a minimum number of threads engaged into the metal, which is why you sometimes must use the raised bump that has a hole for a grounding screw in it - the metal is thick enough there to comply.

    From recollection anything less then 32 threads per inch is not acceptable in 1/16" or less of metal, which would be two threads must be engaged in 1/16 of an inch. A nut on the back side however would be acceptable if not enough threads are engaged.

    What you generally can't use is "sheet metal" variety of screws
    I stand by what I said.

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