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 clears. It will go to default calculation as a 100:1 ratio.

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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,

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.

Note: The sensitive metering CT always comes with the 1 Amps secondary current.

Let us take a simple example of a current transformer which is having CT ratio of 2000:1, assume that the 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,

2000/1=1500/x

x=1500/2000=0.75A

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

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

Application of CT Ratio:

The CT ratio calculation is used in the protection relay current settings and energy meter electricity calculation.