Enter your transformer rating, primary and secondary voltages, power unit, and phase configuration to calculate your CT ratio — plus get the primary current, secondary current, and turns ratio all in one shot.
CT Ratio
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Primary Current
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Secondary Current
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Turns Ratio
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The primary winding full load current is calculated using the formula: Ip = S/(√3 × Vp) for three-phase or Ip = S/Vp for single-phase, where S is the transformer rating and Vp is the primary voltage.
The secondary winding full load current is calculated using: Is = S/(√3 × Vs) for three-phase or Is = S/Vs for single-phase, where S is the transformer rating and Vs is the secondary voltage.
The turns ratio is calculated as the ratio of primary voltage to secondary voltage: Turns Ratio = Vp/Vs. This represents the ratio of turns in the primary winding to turns in the secondary winding.
CT ratio is the ratio of primary current to secondary current, typically expressed as X:1. It indicates how much the primary current is stepped down to the secondary current for measurement purposes.
Single-phase calculations use the simple formula I = S/V, while three-phase calculations include the √3 factor: I = S/(√3 × V). Three-phase systems are more common in industrial applications.
Choose based on your transformer size: VA for small transformers (under 1000 VA), kVA for medium transformers (1-1000 kVA), and MVA for large power transformers (above 1000 kVA).
CT ratio is crucial for protection relays, metering, and monitoring systems. It ensures that high primary currents are safely reduced to measurable secondary currents (typically 1A or 5A) for instruments and protective devices.