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750's in 3" conduit.

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    750's in 3" conduit.

    Two 750's and one 350 in 3" pvc/rigid. Two 90's(that I know of), 150 feet long, existing conduit under parking lot and concrete slab.

    Can I do it or am I doomed to fail? Conduit fill ratio works, but jamming is the concern. Only ever put 750's in 4 inch.

    I need more than a 400 amp single phase service according to a quick manual load calc. Thinking about getting to 475 amps with the 750's wire ampacity and then using the round up rule to allow a 500 amp main breaker or use a combination of service disconnects/panelboards. Haven't decided yet, just trying to figure out if the wire size in the conduit will work to get the ampacity I need.

    Thanks for the help.

    #2
    Have you considered parallel conductors like 300's? Two sets gets you to a full 512 amps. (320*2*80%=512 amps)
    Rob

    Moderator

    All responses based on the 2017 NEC unless otherwise noted

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      #3
      I hadn't. Probably should of slept on it first. May have a had a bit of tunnel vision when I posted since we don't typically parallel in the same conduit. This being a different situation than the usual, I'll run it through the way you suggested and see where we end up. Thank you.

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        #4
        I would stay away from that bad jam ratio, it will be nothing but a headache. I like the parallel's Idea. You could even do three sets of cheap 4/0 AL and and because its single phase still have a .8 derate and gets you 492 amps. Three sets might make terminating a hassle and not worth it however.
        Ethan Brush - East West Electric. NY, WA. MA

        "You can't generalize"

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          #5
          If it's PVC you'll need an equipment ground.
          Sometimes I don't know whether I'm the boxer or the bag.

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            #6
            Originally posted by qcroanoke View Post
            If it's PVC you'll need an equipment ground.
            He stated in the OP that is a service.
            Rob

            Moderator

            All responses based on the 2017 NEC unless otherwise noted

            Comment


              #7
              Originally posted by infinity View Post

              He stated in the OP that is a service.
              Missed that.
              Sometimes I don't know whether I'm the boxer or the bag.

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