Selection of Specifications (CT) - Current Transformer


Selection of Specifications (CT) - Current Transformer - Current Transformer - Errors in determining the specifications of the Current Transformer to be used in an electrical installation system, either as metering - monitoring, protection or transactions will affect the expected measurement results.

To determine the Current Transformer specifications to suit its purpose and get the measurement results as expected, the specifications that must be of concern are as follows:

1. Current Transformer Ratio

2. Current Transformer Current Rating

3. Current Transformer Power Rating

4. Class Current Transformer (Precision Calss)

The explanation of the 3 criteria above is as follows:

1. Current Transformer Ratio

Current Transformer Ratio (Current Transformer) is the ratio between the primary current to the output current of the CT (secondary current). So the writing is like X / 5 A or X / 1 A where X is the primary current where the value is a certain multiple and adjusted to the needs of the amount of current to be measured, while the secondary currents only have 2 choices of values, 5A and 1A. And this ratio is written on the name plate on the CT (Current Transformer) itself

The more widely used CT (Current Transformer) secondary current is 5A. This is because the majority of high-precision metering devices use the current input from the CT (Current Transformer) is 5A.

The secondary current CT (Current Transformer) with a value of 1A was chosen more for technical and economic considerations, because the secondary current CT (Current Transformer) 1A is recommended to be used if the distance between the CT (Current Transformer) and the measuring instrument (metering) is too far But in accuracy, lower than the 5A.

2. Current Transformer Current Rating

The CT current rating or the CT nominal current is the amount of the CT primary current and the CT secondary current which is also indicated on the CT name plate.

In addition to the 0.2S and 0.5S classes, the current rating on the primary side of the CT is: 10A - 12.5A - 15A - 20A - 25A - 30A - 40A - 50A - 60A - 75A and their multiples. While on the secondary side are 1A and 5A. Generally, the most frequently used are those that are due to the factors mentioned above.

Whereas for 0.2S and 0.5S classes, the current rating on the primary side is 25A - 50A - 100A and its multiples. Whereas on the secondary side there is only a value of 5A.

The correct selection of the current rating of a Current Transformer, both on the primary and secondary sides, greatly affects the accuracy of current measurements. The recommendation for selection is that the current on the primary side of the CT should be slightly higher than the maximum load current (In max) to be measured. As for the secondary side, it is recommended to use a 5A rating.


The maximum In (Nominal Current) of a load is: 1170 A.

From the CT current rating criteria on the primary side, the primary current rating is 1250 A. And if a higher accuracy is desired, the CT current rating on the secondary side is selected which is 5A. So that the current rating for the CT to be used is: 1250 / 5A.

If the Current Transformer (CT) current rating is higher than the In max current, the measurement accuracy is reduced. For this reason, it is necessary to pay attention to the maximum nominal current of the load to be measured.

At the beginning of planning an installation system, the nominal current value can be determined easily, which is to adjust to any load that will be installed in the installation. However, when dealing with existing installations, and don't know what loads are already installed on the installation, a practical way is for us to determine the CT current rating by paying attention to the following criteria:

1. If the Current Transformer is installed right after or before the Power Transformer or Distribution Transformer, then the current Transformer is 1.1 times the amount of the Power Transformer, and is selected that is closest to the CT primary current rating as we discussed above.

2. If the Current Transformer is installed right near the Fuse protection device, then the current Transformer magnitude is slightly above the fuse current rating, and is chosen which is closest to the CT primary current rating as we discussed above.

However, the above also needs to be supported by manual measurements using a current multimeter or Tang Amperes for more accurate results.

3. Current Transformer Power Rating

This is the maximum value of power that can be output by CT on the secondary side in VA units. In order for the performance and accuracy of the CT to be fulfilled, the power output (VA) transmitted to the secondary side of the CT must not exceed its own capacity. The VA value of the CT is listed on the name plate. The standard VA values on CT are 2.5 - 5 - 10 - 15 - 30 VA.

To ensure that the VA value does not exceed the CT rating, what must be considered is what equipment uses the output of the CT secondary. So that the total VA of all equipment connected to the CT is lower than the rated (VA) CT. The illustration is as explained below:

In the picture above, you can see a CT that is used to measure the current passing through the conductor (Red Power Cable). The Ct is used for digital metering equipment and Analog Ampermeters with a VA of 2.5 VA and 1.5 VA, respectively.

For measuring needs, a 4 mm2 cable with a length of 10 M and 5 M is used so that the total length of the cable is 15 M.

From the circuit above, the rated power (VA) that must be selected for the Ct are:

Total VA of the meter = 2.5 + 1.5 VA = 4 VA

The VA for metering cabl is:

  P = Is2 x R x 2 x L,

Where: Is2 = CT secondary current

R: Value of cable resistance (m)

L: Cable length

2: Because of 2 pulls

So that the VA value of the cable is obtained:

P = 12 x (4.61 / 1000) x 2 x 15

P = 0.1383 VA

Total VA = 2.5 + 1.5 + 0.1383 VA = 4.1383 VA

So that the VA CT must be above the value of 4.1383 VA. For this case the approximate value is 5 VA. So that for metering installations above CT 100/1 A, it must have an rated output of 5VA.

The fourth criterion regarding CT accuracy, we have discussed in the previous post.

So, hope it helps