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CT Sizing for the Meter

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    CT Sizing for the Meter

    Hey Guys,

    One of our project has a system amperage of 960A and the POCO says that there will not be a problem to install a 600A CT for the metering cabinet, is this true?

    #2
    It might be. I remember seeing that CT's are capable of measuring current significantly above their rating.

    Comment


      #3
      Originally posted by ggunn View Post
      It might be. I remember seeing that CT's are capable of measuring current significantly above their rating.
      Thank you, is this because the CT might actually have a higher rating or is there any other math behind this like changing the ratio?

      Comment


        #4
        Originally posted by TheElectrician View Post

        Thank you, is this because the CT might actually have a higher rating or is there any other math behind this like changing the ratio?
        Sorry, but I don't know. That's just what I was told and I don't recall who told me.

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          #5
          A CT's rating typicially represents a ratio between how many amps on the measured conductor produces how many amps on the CT leads. So 600A is not exactly a max rating like for most other equipment. I'm sure that if you measured too many amps with that CT then you could overload the CT leads or the equipment they connect to. But as long as those things are accounted for it could measure more than the ratio rating. For example if the CT leads and equipment can safely handle 10A then a 600/5 rated CT I presume could safely measure 1200A.

          I bet the 600A are cheaper than the next size up they would use, too.

          Comment


            #6
            Originally posted by jaggedben View Post
            A CT's rating typicially represents a ratio between how many amps on the measured conductor produces how many amps on the CT leads. So 600A is not exactly a max rating like for most other equipment. I'm sure that if you measured too many amps with that CT then you could overload the CT leads or the equipment they connect to. But as long as those things are accounted for it could measure more than the ratio rating. For example if the CT leads and equipment can safely handle 10A then a 600/5 rated CT I presume could safely measure 1200A.

            I bet the 600A are cheaper than the next size up they would use, too.
            Got it, Thank you very much!

            Comment


              #7
              If you get too high on the input amps the core saturates and it clips the output, usually around 10 to 20 times rated. For metering purposes generally you want to stay below “name plate”. For protection most relay taps can’t be set below around 0.5 so 2.5 A on a 5 A nominal CT and you can expect severe distortion at around 50-100 A output. It’s not very linear above rated (5 A) anyways but usually at 400% or less it’s not severe (4 A or 20 A). If you are on the low side and the cable and CT opening size is large enough you can run multiple turns of the primary but if you are on the high side the only trick I’ve ever seen done is putting a CT on a CT lead. Obviously in this case you have greater errors,

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                #8
                If the CT gets overloaded, will the meter record less than what you're actually using?? Just curious!

                Comment


                  #9
                  Originally posted by PaulMmn View Post
                  If the CT gets overloaded, will the meter record less than what you're actually using?? Just curious!
                  As the current through a CT increases, at some point it will become saturated and no longer be able to register an increase, so yes. That point is somewhat more than its published rating.

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                    #10
                    Many CTs have a rating factor (usually denoted on the nameplate as "rf"). The rating factor is a number by which you can multiply the CT primary rating to get the maximum current that can go through the CT before saturation occurs. If a manufacturer does not publish a rating factor, then it may be 1. For example, a CT with a nameplate of 600/5 and a rating factor of 2.5 would be able to measure up to 1,500A accurately. More than 1,500A, however, the CT enters saturation, where there is no degree of accuracy.

                    Is the POCO providing the CTs? If they are approving a 600A CT for a 960A system, then it is safe to assume that the POCO uses CTs with a rating factor of at least 1.6. If you are providing the CTs, be sure to review the specs to make sure it will work.

                    Comment


                      #11
                      Not a problem.. we do it all the time.
                      besides, it’s doubtful you’ll use 960 amps at once if your calcs are correct.

                      CTs are listed at some number to 5 ratio (600/5 for example) due to convention. CTs were rated by 5 because the meters used way back when would only handle 5 amps! So you had to choose the CTs carefully so as to not damage the revenue meter.

                      today it would be confusing to call a 100/5 CT a 400/20 if the rating factor was 4

                      Todays meters are 20 amp meters, and many CTs have a rating factor as mentioned of 2,3, or even 4 times rating. Loading these we need to stay below 20 amps, but I want to at least have 5 amps going to the meter with the lowest load if possible.

                      the new extended range CTs are some of the best out there now.
                      old CTs have no accuracy guarantee below 10%. The new extended range guarantee accuracy to 1%.

                      Comment


                        #12
                        Very rarely do you stay below "nameplate". It is quite normal to target 3X or so at full load. This helps ensure accuracy at low load also. Just make sure the rating factor is adequate AND at the right ambient temperature.

                        You may have R=3 at 30C but I like to target at least 55C so that adjusted factor may only be 2X at 55C. Usually I see 30C, 55C, 85C as the most common specs. So an R=2 at 85C would probably be ok at 3X at a 55C ambient.

                        I don't like to push the 4X boundary too close for metering.
                        BB+/BB=?

                        Comment


                          #13
                          Also keep in mind that the POCO is also considering realistic load vs the NEC calculated load. If the NEC calc is 960 amps then I highly doubt that will ever be what the POCO will see.

                          The POCO can usually expect the actual to be 60-80% of panel size calc or even less. Similar industry loads and diversities are used to estimate actual load expectations.

                          Combine that with some RF > 1.0 and it is not suprising they said 600:5 would work.
                          BB+/BB=?

                          Comment


                            #14
                            Originally posted by TheElectrician View Post
                            One of our project has a system amperage of 960A and the POCO says that there will not be a problem to install a 600A CT for the metering cabinet, is this true?
                            As mentioned by Mivey, the POCO likely doesn't believe your calculated NEC 220 load. You might come out to the service drop with 3 sets of conductors and they will meet you with 1 or 2 sets because they would have estimated the load to be less.
                            Ron

                            Comment


                              #15
                              Originally posted by ron View Post
                              As mentioned by Mivey, the POCO likely doesn't believe your calculated NEC 220 load. You might come out to the service drop with 3 sets of conductors and they will meet you with 1 or 2 sets because they would have estimated the load to be less.
                              Correct. POCO believes the calcs are per NEC but they realize the NEC is way too conservative. Recent example is a NEC load calc of over 240 amps. The reality is the load will probably never pass 160 amps at the maximum peak and will peak 100 amps or so routinely and average less than 30 amps. Real data from years of measuring similar loads.

                              So EC puts in a 400 amp service and POCO serves it with a 200 amp supply. See it all the time.
                              BB+/BB=?

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