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    Wire Size

    I am back feeding a 45 kva transformer, which only feeds a 30 hp 460v air compressor. I spoke with Square D, they said the inrush on itial start up may trip the breaker and reccomend oversizing by at least 125% if not more. Can I legally use a # 8thhn on a 225 amp c.b., to feed this 45kva transformer (208 pri to 460v sec) which feeds a 40 amp time delay fused disconect on the secondary side to feed the 30hp 460v compressor, all three phase. If not why. Article 240-101 seems to indicate yes but not by more than 3 times.

    #2
    Re: Wire Size

    If the x-frmr is only feeding the motor refer to Table 430-152(1999 NEC), but make sure your x-frmr can handle the inrush current and starts per day(check with the manufacturer).

    Comment


      #3
      Re: Wire Size

      The Square D transformer (45T3H) nameplate shows a wiring diagram for a HV of 480V and an LV of 208Y/120V. This means you can "legally" use the LV as the primary and the HV as the secondary (often called backfeeding). The NEC will allow a primary OCPD of up to 250% when a secondary OCPD of 125%,is provided.

      In your case:
      The 208V winding Full Load current is 125A, and the 480V is 54A. So your maximum primary breaker could be 300A as long as your secondary fuses were not larger than 60A (well actually 312.5A and 67.5A, but you can't round up).

      The primary conductors will need to be sized based on the 125A FLC (such as #1 THHN), and the secondary sized based on your load and the secondary fuses. Your motor is 30HP at 480V so you should use #8 THHN and 60A fuses are recommended.
      Just because you can, doesn't mean you should.

      Comment


        #4
        Re: Wire Size

        Hmm. I put in a step up transformer and I used a SQ 120/208 x 480. It was a delta wye. When you back feed its wye delta. How does this "The Square D transformer (45T3H) nameplate shows a wiring diagram for a HV of 480V and an LV of 208Y/120V. This means you can "legally" use the LV as the primary and the HV as the secondary (often called backfeeding)." make it legal?
        Moderator-Washington State
        Ancora Imparo

        Comment


          #5
          Re: Wire Size

          Transformers are tested the direction they are intended to be used. The listing instructions will not include reversing the primary and secondary.

          The catalog description, from the manufacturer, will indicate the primary and secondary. This description can not be reversed.

          The high inrush current is due to the designed intended secondary not having current limiting inductors in the circuit. The designed intended primary will have limiters.

          Comment


            #6
            Re: Wire Size

            Duke
            Jims info is right on the mark. I wanted to point out that the protection for the transformer comes from table 450.3B. If the primary protection is 125% of the FLA then see note 1. If the primary protection is greater that 125% see note 3. Max protection is 250%. The transformer Secondary conductors are covered in section 240.21C.
            However since you are providing power to a motor
            240.21F comes into play.

            [ April 30, 2003, 07:52 PM: Message edited by: bob ]

            Comment


              #7
              Re: Wire Size

              Bennie,

              I am not aware of any manufacturer that puts limiters in their small to medium kva, dry type low voltage transformers.

              When I designed (back in the college) the only adjustment wemade was to include additional turns in the high voltage (typically the primary) to compensate for core (magnetizing) voltage losses. These additional turns acted like a built in "below normal" tap.

              Theoritically any transformer can be run backwards if you are willing to live with the very minor voltage ratio problem caused by having the taps on the secondary instead of the primary. However, transformers with tight voltage regulation (i.e. control power transformers) and small kva three phase units (which are usually built with a two winding T connection) should not be run backwards.

              Tom,

              If the transformer nameplate does not show a primary or a secondary designation, then how can you say it is not being used correctly? How do you explain why Buck-Boost transformers are always wired to HV and LV never to primary and secondary.
              Just because you can, doesn't mean you should.

              Comment


                #8
                Re: Wire Size

                jim: Do the added turns act as an inductor? Limiting inrush current?

                When reversed are the turns an impedance?

                Comment


                  #9
                  Re: Wire Size

                  Bennie,

                  Simplistic answer (i'm doing this off the top of my head, so I don't have the formulas available). The extra turns change the transformer ratio, just like any normal tap. They are there to compensate for the voltage drop caused by the magnetization of the core.
                  Just because you can, doesn't mean you should.

                  Comment


                    #10
                    Re: Wire Size

                    also try to remeber when change the transfomer from normal set up to reverse set up make sure you take the NX 0 banding off other wise it will have werid effect with grounding and just run the grounding wire as normal sans connection at transfomer per nec


                    if other question just drop a line

                    merci marc
                    Marc
                    master electrician
                    Wisconsin and Paris France

                    "Pas de problème, il marche n'est-ce pas?" (No problem, it works doesn't it?)

                    Comment


                      #11
                      Re: Wire Size

                      Marc: Thanks, this reminds me of another reason not to reverse a transformer. The UL listing instructions state "any special connections required shall be indicated on nameplate".

                      Comment


                        #12
                        Re: Wire Size

                        Jim, Sq D (and others)has a step up and step down transformer in their catalog. Only a few step up (much more expensive) and many many step down.
                        I maintain if you are stepping up you have to use a transformer that is designed (listed) for the application.
                        If they were not designed to be a step up or step down, then they would only have one universal type. I may be missing something, it could just be as simple as the step up is delta-wye for providing a grounded conductor, if the user needs.
                        If you use a wye delta as step up then you have a delta secondary...
                        Moderator-Washington State
                        Ancora Imparo

                        Comment


                          #13
                          Re: Wire Size

                          Tom,

                          All of the Square D step-up transformers provide a 480Y/277 4 wire output. I have not found any manufacturer's standard three phase 208V to 240V, 480V, or 600V trnasformer.

                          Bennie,

                          First let me apologize for mistyping the other day. Compensating windings are usually plced on the low voltage windings (not the high)and are most prominent on small transformer sizes.

                          The Square D nameplate no longer refers to primary or secondary, only HV and LV. In fact the wiring diagram has a "dotted line" to indicated the ability to ground the B phase of the HV delta windings.
                          I have looked at printed instructions for Square D, Olsun, and Hammond. They all say to connect the transformer per the nameplate. I do not see any indication that these manufacturers ship the transformers with the X0 terminal factory connected to the case (ground). In fact Olsun's literature says "...This ground point may be used to ground the neutral of the secondary, if desired...".
                          Just because you can, doesn't mean you should.

                          Comment


                            #14
                            Re: Wire Size

                            Aside from the labeling and listing of the transformer, let's do the calcs for the problem.
                            A 45 Kva, 3-phase,208V step up to 480 volts. Section 450.3(B)requires the transformer to be protected by OCP in the primary only or by OCP in both primary and secondary. Assuming OCP in pri. and sec. Primary I = 45,000/360 = 125A. @ 250% = 250 amperes. The conductors supplying the primary must have an ampacity based on the overcurrent protection. Secondary I = 45,000/830 = 54A @ 125% = 67.5A, Note 1 allows u to round up to next std. size which is 70A. The Code current for a 40HP motor is 40 amperes. The max size time-delay fuse allowed by 430.52 is 175 % of 40 = 70A max.
                            The minimum size of the motor circuit is 1.25 x 40A = 50A. Where the terminations are rated 75 degrees C, No. 8 cu. condutors are suitable when protected by a 70A fuse, since it is a motor branch-circuit and the fuse is the ground-fault short-circuit device.

                            Comment

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