Announcement

Collapse
No announcement yet.

neutral conductor sizing for 3 phase panelboard

Collapse
This topic is closed.
X
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

    neutral conductor sizing for 3 phase panelboard

    Where can I find the neutral conductor sizing for a 3 phase panelboard 208/120? I would like to run 1 neutral for each 3, 120v. circuits. I would imagine it would only carry the imbalance of the 3 loads? I am looking in 250 of the NEC, but not finding anything. I must be way off base

    [ February 03, 2004, 10:14 AM: Message edited by: billete ]

    #2
    Re: neutral conductor sizing for 3 phase panelboard

    220.22

    Roger

    [ February 03, 2004, 09:15 AM: Message edited by: roger ]
    Moderator

    Comment


      #3
      Re: neutral conductor sizing for 3 phase panelboard

      See 250.24(B) as well.

      Notice, however, that this section is limitted in its application to service equipment.

      [ February 03, 2004, 09:26 AM: Message edited by: ryan_618 ]
      Ryan Jackson, Salt Lake City
      Inspector, Instructor

      Comment


        #4
        Re: neutral conductor sizing for 3 phase panelboard

        Please see 215.2 Minimum Rating and Size.
        [COLOR=DarkSlateBlue]Dave Nix[/COLOR]

        Comment


          #5
          Re: neutral conductor sizing for 3 phase panelboard

          This may not be in the Code, but my opinions. If you have anything in the way of harmonic-producing equipment on these circuits, computers, etc., I would oversize the neutral to handle possible triplen harmonics which could exceed the current in any one hot. You will find this in office cubicle wiring harnesses, usually 3-12 ga. hots with a 10 ga neutral. I prefer (data center) to run separate neutrals and grounds for every circuit, minimizes noise between those big computers!

          Comment


            #6
            Re: neutral conductor sizing for 3 phase panelboard

            Daveinca: I certainly don't disagree with your post, but I want to add that I find it very interesting that when talking about non-linear loads in an office environment, the flourescant lighting is seldom disussed. It is my understanding that these lights are just as big of a problem as computers for creating harmonics from their non-linear nature.
            Ryan Jackson, Salt Lake City
            Inspector, Instructor

            Comment


              #7
              Re: neutral conductor sizing for 3 phase panelboard

              I have heard the gloom and doom about harmonic current for 50 years. I always understood the appearance of the (in phase neutral) current points out the need for using full size neutral conductors instead of reducing the size.

              I never considered using a larger size neutral than the active line conductors.

              Does anyone have the calculations for the highest possible current on a system with a high THD?

              Comment


                #8
                Re: neutral conductor sizing for 3 phase panelboard

                AFC markets a line of "Super Neutral" cables, with both an oversized common neutral version, and one with a neutral for each phase.

                AFC cables

                Ed

                Comment


                  #9
                  Re: neutral conductor sizing for 3 phase panelboard

                  I have had students in my classes from a school district that have had neutrals burn up from harmonic currents.
                  Moderator-Washington State
                  Ancora Imparo

                  Comment


                    #10
                    Re: neutral conductor sizing for 3 phase panelboard

                    One of the things that I find interesting on the plans that I review is that the EE will often spec a 200% nuetral on feeder circuits to panelboards, but not on the branch circuits that supply the non-linear loads. I wonder why that is?
                    Ryan Jackson, Salt Lake City
                    Inspector, Instructor

                    Comment


                      #11
                      Re: neutral conductor sizing for 3 phase panelboard

                      Ryan I have three replies for that.

                      1)Oversizeing the neutral would not do anything for a standard two wire circuit.

                      2)At least in the circuits I deal with that feed non-linear loads, the ciruit is very lightly loaded. 4 or 5 amps.

                      3)Take a multiwire branch circuit fed with 12/4 MC, each of the three legs even if loaded with 10 amps of non linear loads will not overload the 12 THHN neutral with a rating of 25 amps.

                      JMO, I would like to here Derecks take on this.

                      Comment


                        #12
                        Re: neutral conductor sizing for 3 phase panelboard

                        Would reducing the load on each branch circuit and feeder be a more positive method to reduce current?

                        Comment


                          #13

                          Comment


                            #14
                            Re: neutral conductor sizing for 3 phase panelboard

                            Dereck thanks for the info, I have a question.

                            I thought I read that the harmonic currents could only add up to 1.73 of the phase current.

                            Is that a fact, or is there no limit to the amount of harmonic current.

                            Bob

                            Comment


                              #15
                              Re: neutral conductor sizing for 3 phase panelboard

                              Theoretical limit is 173%, but I have never witnessed it.

                              Comment

                              Working...
                              X