The equivalent circuit of a current transformer is shown:

VB = CT terminal voltage across external burden

IP = primary current, A

ZE = exciting impedance, ohm

IST = total secondary current, A (IE + IS)

RS = CT secondary winding resistance, ohm

IS = secondary load current, A

XL = leakage reactance (negligible in Class C CT), ohm

IE = exciting current, A

N = CT turns ratio

ZB = burden impedance, ohm (includes secondary devices and leads)

The secondary exciting voltage is given by:

The primary current is given by:

The ratio correction factor is given by:

The percent ratio error is given by: