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What fuses for motors?

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    What fuses for motors?

    What class & type of fuse would work for the following:

    200HP / 300 amp fuses

    150HP / 225 amp fuses

    125HP / 200 amp fuses

    100HP / 150 amp fuses

    60HP / 90 amp fuses

    40HP / 60 amp fuses

    20HP / 30 amp fuses

    15HP / 25 amp fuses

    10HP / 20 amp fuses

    5HP / 10 amp fuses

    3HP / 6 amp fuses

    1HP / 3 amp fuses

    All motors are 460 volts 3 phase


    Do I size the wire to the fuse, or do to the motor's FLC?

    1.15 and over service factor means fuses are sized at 125%, but under 1.15 I use 115%, correct?




    #2
    Wire is sized to 125% of full-load current in Table 430.250

    I thought we went over using fuses for running overcurrent protection a thread or two back. The 300 amp fuses may not allow a 200 HP to start. In that case a 500 could be allowed. Do you really want to have 500 amp fuses providing running overcurrent protection for a motor that should draw 240 FLA?

    430.52(C) (1)(b)
    Tom
    TBLO

    Comment


      #3
      Originally posted by ptonsparky View Post
      Wire is sized to 125% of full-load current in Table 430.250
      Even when the fuse is acting as the overload and larger then the FLC?



      [QUOTE]
      I thought we went over using fuses for running overcurrent protection a thread or two back. The 300 amp fuses may not allow a 200 HP to start. In that case a 500 could be allowed. Do you really want to have 500 amp fuses providing running overcurrent protection for a motor that should draw 240 FLA?

      430.52(C) (1)(b)

      There has to be fuses out there that can handle inrush and also provide descent over current protection- I'd think.


      FWIW, I came across these:


      https://m.littelfuse.com/~/media/ele...tion-guide.pdf


      http://www1.cooperbussmann.com/pdf/6...65a95db302.pdf

      Comment


        #4
        My experience usually ended in single phased motors. Under 5 HP are disposable, depending on how critical they are to your process so the cost of a NEMA starter may not be warranted. IEC starters are a different story.

        Fifty horsepower motors are in the neighborhood of $3,000 and you will need at least ATL start. Why skip the overload relay?


        Tom
        TBLO

        Comment


          #5
          Originally posted by ptonsparky View Post
          My experience usually ended in single phased motors. Under 5 HP are disposable, depending on how critical they are to your process so the cost of a NEMA starter may not be warranted. IEC starters are a different story.

          Fifty horsepower motors are in the neighborhood of $3,000 and you will need at least ATL start. Why skip the overload relay?

          The way I figure it fuses are already required. So why use separate overloads?
          Attached Files

          Comment


            #6
            I can't read the details, not that it matters.

            You are the EE, if you are happy with fuses, go for it.

            The Bussman and Littelfuse reps will be glad to take your call.
            Tom
            TBLO

            Comment


              #7
              Originally posted by ptonsparky View Post
              I can't read the details, not that it matters.

              You are the EE, if you are happy with fuses, go for it.

              The Bussman and Littelfuse reps will be glad to take your call.
              But- I am embarrassed to admit that I don't know the inrush properties of 3 phase induction motors for various horse powers when switched on at various points in the sine wave, when there is a momentary "blink" and when there is residual magnetism still in the iron.

              I know transformer inrush varies based on those 3 factors, and, I'd imagine its the same for motors.

              Comment


                #8
                Originally posted by ptonsparky View Post
                I can't read the details, not that it matters.

                You are the EE, if you are happy with fuses, go for it.

                The Bussman and Littelfuse reps will be glad to take your call.
                FWIW, manufacturers frequently do it:


                https://youtu.be/Z7e4-KE-kWM?t=148


                Not sure how they get away with a compressor though- don't these stall when power is removed and re-applied?


                Comment


                  #9
                  Originally posted by mbrooke View Post

                  But- I am embarrassed to admit that I don't know the inrush properties of 3 phase induction motors for various horse powers when switched on at various points in the sine wave, when there is a momentary "blink" and when there is residual magnetism still in the iron.

                  I know transformer inrush varies based on those 3 factors, and, I'd imagine its the same for motors.
                  But, only worst-case numbers matter, right?
                  Master Electrician
                  Electrical Contractor
                  Richmond, VA

                  Comment


                    #10
                    Originally posted by LarryFine View Post

                    But, only worst-case numbers matter, right?
                    Correct- however- I don't know what those are.

                    Comment


                      #11
                      Impedance of source along with supply conductors (feeders and branch circuit conductors) can effect how much surge current may flow when first energizing a motor. More impedance will be current limiting, however will also result in less initial motor torque - soft starters intentionally do this.
                      I live for today, I'm just a day behind.

                      Comment


                        #12
                        Originally posted by mbrooke View Post

                        Correct- however- I don't know what those are.
                        You could look at Table 430.7(B).

                        I'm sure you know the code letter on each motor nameplate and they will always be the same.
                        Tom
                        TBLO

                        Comment


                          #13
                          Originally posted by kwired View Post
                          Impedance of source along with supply conductors (feeders and branch circuit conductors) can effect how much surge current may flow when first energizing a motor. More impedance will be current limiting, however will also result in less initial motor torque - soft starters intentionally do this.
                          Can we assume infinite bus?

                          Comment


                            #14
                            Originally posted by ptonsparky View Post

                            You could look at Table 430.7(B).

                            I'm sure you know the code letter on each motor nameplate and they will always be the same.
                            But is it the same as inrush? My understanding is the you first need to magnetize the iron before the current drops below the I2R impedance of the windings which is very low.

                            NEC tables link:

                            http://www.superb.org/power/nec%20tables.pdf

                            Comment


                              #15
                              Originally posted by mbrooke View Post

                              Can we assume infinite bus?
                              Certainly, but will result in larger numbers.
                              Master Electrician
                              Electrical Contractor
                              Richmond, VA

                              Comment

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