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Thread: contactors

  1. #1
    Join Date
    May 2009
    Posts
    11

    contactors

    When I parallel contactors do they have to be the same in size or just have the ampacity of my conductor?

    I have a 350 AMP contactor and need 400 Amp the contactor I have is rated
    at 2,200 amps lock rotor curent.

    I think I'm going to have a big contactor on my shelf.

    Please help and thank's for the info.

    Mike

  2. #2
    Join Date
    Feb 2009
    Location
    Allentown, PA
    Posts
    390
    You want to parallel contactors to supply a higher ampacity then they are individually rated for?

  3. #3
    Join Date
    Nov 2007
    Location
    Columbus, Ohio
    Posts
    2,234
    I don't know offhand if there is a code complaint about that or not, but if done, the units will not likely switch at precisely the same time, which means the one that lags will share a larger potion of the burden of breaking current.

    If this were installed, I would want to cite 310.4(B)(5), and like to claim that they would need to be connected to identical devices to meet the letter of being "terminated in the same manner".

    Otherwise, for fractions of seconds, repeatedly, we may be violating 310.10.

  4. #4
    Join Date
    May 2009
    Posts
    11
    Yes I have the 350 amp contactor in service and two sets of 4/0 train cables coming off
    of it and would like to just add one size four contactor to my control cabinet and increase my feeders from a 250 amp to a 400 amp service.

    Thank you!

  5. #5
    Join Date
    May 2005
    Location
    Richmond, Virginia
    Posts
    21,838
    Mike, I suggest keeping the exsiting load on the existing contactor, and feeding the new load through the new contactor, if at all possible.

    Otherwise, I wouldn't try paralleling dissimilar contactors. You can't control the division of current, and will likely overload the smaller one.

    Added: Unless it's going to open under full load, I think the 350a unit will survive. What realistic loading do you think it will be subjected to?
    Last edited by LarryFine; 05-20-10 at 02:57 PM.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

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