CT Ratio Calculator & CT Ratio Calculation Formula

CT ratio calculator:

Just enter the primary current and turn ratio, then press the calculate button to get the exact secondary current. Also, you can get a CT ratio along with the burden resistance value from this calculator.

For clearing the value, press the clear button, the value in the field automatically clear. It will go to default calculation as a 100:1 ratio.

CT Ratio Calculation:

Current transformers are the voltage step-up transformer which is used to reduce the current level from high to low. Since the direct measurement techniques are not suitable for higher current rating equipment. Therefore, the current transformer act as a current step down transformer which reduces the current with respect to the primary. Let us discuss how to calculate the CT ratio.

What is CT ratio:

CT ratio is nothing but a ratio between the primary current to the secondary current. It will be indicated in the CT nameplate details. Let us assume,

Learn More:   Watt to HP conversion Calculator

I(P) = Primary current in Amps

I(s) = Secondary current in Amps

V(p) = Primary Voltage in Volts

V(s) = Secondary Voltage in Volts

CT ratio formula:


CT ratio = I(P) / I(s) ————-1

Example: 1000:1. Here 1000 means it is a primary current and 1 means secondary current. Also, we can have rewritten as 1000/1.

CT secondary current will be always 1 or 5 to get easy calculation as well as we can create small rating secondary circuits to withstand up to 5 Amps.

Let we take a simple example of a current transformer which is having CT ratio of 2000:1, assume that CT primary has 1500Amps, calculate the secondary current with respect to the primary.

Consider the unknown secondary current as x in Amps,

Apply our CT ratio formula,



Therefore, the CT secondary reads 0.75 Amps

How to Calculate the CT ratio from voltage ratio:

As you know the transformer equation,

V(p) * I(P) = V(s) * I(s)

From that Voltage, the ratio will be

Learn More:   kW to kVA Conversion Calculator Real Power to Apparant Power

V(p) / V(s) = I(s) / I(P) ———— 2

Look at the formula 1 and 2

The CT ratio is the inverse of the voltage ratio. If your transformer has the voltage ratio is 1:25 means, the CT ratio would be 25:1. It means if the CT has primary current 25 Amps, you get 1 Amp as output. The current will be reduced considerably.


Please enter your comment!
Please enter your name here