CT Ratio Calculator & CT Ratio Calculation Formula

In this article, we will discuss the CT ratio calculator and CT ratio calculation formula. The CT ratio is very important as a design factor of CT for a particular application.

What is CT Ratio?

A current transformer(CT) has primary and secondary winding. CT reduces the primary current in a certain proportion, and the secondary current is always less than the primary current. The lower current is required for energy metering and circuit protections. The reason for use of a lower current for metering and protection is that the energy meters and the protection relays can not function at higher voltage and higher current.

We can define the CT ratio as the ratio of primary current to secondary current. In the mathematical expression, we can write it as;

CT Ratio Formula

CT ratio formula

Ip = Primary Current in amperes
Is = Secondary Current in amperes

What does it mean if the CTR(CT Ratio) of the CT is 1000/5? It means when the primary of the CT carries 1000 amperes current, then the secondary of the CT will carry 5 amperes.

Ct ratio Calculator

Here, note that practically the CT does not output the secondary current in the same ratio as mentioned on the nameplate of the CT. This is because of the ratio error in the CT. However, we can consider the secondary current as per the CTR of the CT.

The CTs are available in -/5 and -/1 secondary current. The selection of the secondary current of the CT depends on the applications. The lower secondary current CT-like -/1 secondary is suitable for switchyard applications.

The CT ratio has a direct relationship with the number of turns of the primary and secondary winding. The relationship between CTR and its primary turns(NP) and secondary turns(Ns) is as follows.

CT ratio formula - Number of turns as inputs

Now if we take the example of CT that has CTR 300/5, the secondary current will be 5 amperes for a primary current of 300 amperes. If we talk in terms of the number of turns, the secondary and primary will have 300 turns and one turn respectively. In other words, we can say the CT ratio of the secondary is also equal to the ratio of secondary turns to primary turns.

How to Calculate CT ratio from Voltage Ratio?

Now, we can establish one more relation of CT ratio with the primary and secondary voltage.

How to Calculate CT ratio from Voltage Ratio?

From the above, it is clear that the CT ratio is the inverse of the voltage ratio. The CT ratio is also inverse of turn ratio. If the voltage ratio is 1:40, then the CT ratio will be 40:1. CT ratio of 40: 1 means, that if CT’s primary current is 40 amperes, its secondary current is 1 ampere. The CT ratio is written as 40/1.

CT Ratio Calculator

Enter Primary and secondary current and enter calculate button, it will show the CT ratio.

Enter Primary and secondary turns and enter calculate button, it will show the CT ratio.

For clearing the value, press the reset button for clearing the value.

CT Ratio Calculation

A current transformer has a CT ratio of 600:5, if the primary current flowing through the CT is 200 amperes, what is the CT secondary current?

CTR = 600/5
= 120

We know,
CTR = Ip/Is
120 = 200/Is
Is = 200/120
Is = 1.66 Amps.

A CT is connected to an ammeter. The current flowing through the secondary CT is 1.5 amperes. The primary current is 1000 amperes. Calculate the CT ratio?

CT Ratio = Primary Current/ Secondary Current
CTR = Ip/Is
= 1000/2
CTR = 500:1

A CT has 25 turns and 1000 turns in its primary and secondary respectively. What is the CT ratio?

solved problem on CT Ratio calculation

CT Ratio = 400:1

The secondary of CT is short circuited and 100 volts is applied to its primary. The measured voltage in its secondary is 1 volt. What is the CT ratio.

CT Ratio = Vp/Vs
= 100/1
CT Ratio = 100:1

Application of CT Ratio:

The overcurrent and other current-related settings depend on the CT ratio. Therefore, one must consider the CT ratio for the protection relay current settings and, the current settings in the energy meter for energy calculation.

4 thoughts on “CT Ratio Calculator & CT Ratio Calculation Formula”

  1. Please could someone help me to determine the ctr in electrical supply to office facility. What are the main factors to be considered. And any other advise


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