### 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

### SC Series Ratio Modifications

**Primary Turn Ratio Modifications**

Formula:Ka = Kn x Nn /Na

Where:Ka = Actual Transformer RatioThe 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.)

Kn = Nameplate Transformer Ratio

Na = Actual Number of Primary Turns

Nn = Nameplate Number of Primary Turns

Square Case Transformer Polarity

**Secondary Turn Ratio Modification**

The current ratio between the primary and secondary winding is defined by the following formula:Where:Ns x Is = N<p x IpIp = Primary CurrentThe 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.

Is = Secondary Current

Np = No. of turns on the Primary Winding

Ns = No. of turns on the Secondary Winding

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.