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Sizing wires with ambient temperature correction factors.

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    Sizing wires with ambient temperature correction factors.

    We are pulling out an old compressor and replacing it with a new air cooled package compressor. Full load amps of the package compressor is 318 Amps. There is one 4" conduit running to the old compressor I would like to reuse. Here is what I need help under standing. In the summer the compressor room stays around 100 F. The wire will be THHN. I would like to make the run a parallel run in the one 4" conduit just to make it easier to work with. If I run it parallel in the same conduit will I need to correct for 6 current carrying conductor in one conduit? Please tell me if this math is right.

    80% of load.
    318 Amps X 1.25 = 397.5 Amps

    6 current carrying conductors 80%
    397.5 Amps X 1.25 = 496.87 Amps

    Ambient temperature correction factor THHN 90C column 100 F ambient 87%
    496.87 Amps X 1.15 = 571.40 Amps

    I need to size my wire for 571.40 Amps

    Is this math right or am I going the wrong route with this.


    #2
    I believe you are on the right track, and you correctly are multiplying by the inverses of the derating/adjustment factors. Check your 100F adjustment number; I might be reading the table wrong but I see 91%; has this changed?

    The only thing which I presume you know but don't state explicitly is that the 571.4A needs to be divided by two to get the pre-adjustment rating of each single conductor.

    -Jon

    Comment


      #3
      Originally posted by kda3310 View Post

      80% of load.
      318 Amps X 1.25 = 397.5 Amps
      I'm wondering about this step. Surely the packaged compressor has an MCA. Start from there.

      Comment


        #4
        Originally posted by david luchini View Post

        I'm wondering about this step. Surely the packaged compressor has an MCA. Start from there.
        I suspect this is an industrial air compressor. It will likely only specify FLA.

        Comment


          #5
          Originally posted by winnie View Post
          I believe you are on the right track, and you correctly are multiplying by the inverses of the derating/adjustment factors. Check your 100F adjustment number; I might be reading the table wrong but I see 91%; has this changed?

          The only thing which I presume you know but don't state explicitly is that the 571.4A needs to be divided by two to get the pre-adjustment rating of each single conductor.

          -Jon
          Yes, I will have to divide that number. But I was really hoping that I was doing something wrong. Running them in the same conduit looks like it's going to cost me more then ripping out that 4in and running it in two 2in conduits. By the time I derate for 6 current carrying conductors I have lost all I was trying to save by reusing the four inch conduit.

          Comment


            #6
            Originally posted by retirede View Post

            I suspect this is an industrial air compressor. It will likely only specify FLA.
            Yes, it is an industrial package air compressor.

            Comment


              #7
              Originally posted by winnie View Post
              I believe you are on the right track, and you correctly are multiplying by the inverses of the derating/adjustment factors. Check your 100F adjustment number; I might be reading the table wrong but I see 91%; has this changed?


              -Jon
              You are right. It is 91%. Thanks

              Comment

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