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How Do I Modify the Ratio of My Current Transformer?

RC, SC & CT Current Transformer Ratio Modifications

The current ratio between the primary and secondary winding is defined by the following formula:

Ns x Is = Np x Ip

Where:

Ip = Primary Current
Is = Secondary Current
Np = No. of turns on the Primary Winding
Ns = No. of turns on the Secondary Winding

Example:
On a 300:5 current transformer,

Is = 5 Amps when Ip = 300 Amps,
      the number of primary turns is 1.
Ns x 5 = 300 x 1
Ns = 60

The ratio of the current transformer can be modified by altering the number of secondary turns. Forward or backwinding the secondary leads through the window of the current transformer will add or subtract secondary turns respectively.

By adding secondary turns the same primary current will result in a decrease in secondary current output. By subtracting secondary turns the same primary current will result in greater secondary output.

Example:

On a 300:5 current transformer, if a 325:5 current ratio is desired it is necessary to add five turns to the secondary winding.
Ns x 5 = 325 x 1
Ns = 65

Deducting 5 secondary turns will create a transformer with a current ratio of 275:5.
Ns x 5 = 275 x 1
Ns = 55
Diagram showing modification of CT series currnet transformer by adding secondary turns.
To add secondary turns, the white leads should be wound through the CT from the side opposite the polarity mark.
Diagram showing how to modify ratios by subtracting secondary turns.
To subtract secondary turns, the lead from the left terminal or the white lead should be wound through the CT from the same side as the polarity mark.
Diagram showing current transformer polarity.
When wired as shown, the voltage on the white lead will be in phase with the voltage on the power lead connecting the line to the load.
 

SC Series Ratio Modifications

Primary Turn Ratio Modifications

Formula:
Ka = Kn x Nn /Na
Where:
Ka = Actual Transformer Ratio
Kn = Nameplate Transformer Ratio
Na = Actual Number of Primary Turns
Nn = Nameplate Number of Primary Turns
The ratio of the current transformer can be modified by adding more primary turns to the transformer. By adding primary turns, the current required to maintain five amps on the secondary is reduced. (Example: A 100:5 current transformer designed for one primary turn.)
Square Case Transformer Polarity
Illustration of a Square Case Current Transformer with 2 primary turns.
Illustration of a Square Case Current Transformer with 4 promary turns.
 

Secondary Turn Ratio Modification

The current ratio between the primary and secondary winding is defined by the following formula:
Ns x Is = N<p x Ip
Where:
Ip = Primary Current
Is = Secondary Current
Np = No. of turns on the Primary Winding
Ns = No. of turns on the Secondary Winding
The ratio of the current transformer can be modified by altering the number of secondary turns. Forward or backwinding the secondary leads through the window of the current transformer will add or subtract secondary turns respectively.

By adding secondary turns the same primary current will result in a decrease in secondary current output. By subtracting secondary turns the same primary current will result in greater secondary output.
Diagram showing modification of SC series currnet transformer by adding secondary turns. 
To subtract secondary turns, the lead from the X1 terminal should be would through the CT from the same side as H1.
Diagram showing how to modify ratios by adding secondary turns. 
To add secondary turns, the lead from the X1 terminal should be wound through the CT from the side opposite H1.