Current Transformer Ratio Calculator

A Current Transformer (CT) ratio tells you how a transformer scales down a high current to a safely measurable level — critical for selecting the right CT in electrical protection and metering systems. Enter your Transformer Rating, Power Unit, Primary Voltage, Secondary Voltage, and Phase Configuration into the Current Transformer Ratio Calculator to get the CT Ratio. Secondary outputs include Primary Current, Secondary Current, and Turns Ratio.

Power rating of the transformer

V

Primary winding voltage (higher voltage for step-down)

V

Secondary winding voltage (lower voltage for step-down)

Results

CT Ratio

--

Primary Current

--

Secondary Current

--

Turns Ratio

--

More Electrical & Electronics Tools

Frequently Asked Questions

How to calculate a transformer's primary full load current?

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.

How to calculate a transformer's secondary full load current?

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.

How to calculate a transformer's turns ratio?

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.

What is a current transformer (CT) ratio?

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.

What's the difference between single-phase and three-phase calculations?

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.

How do I choose the right power unit (VA, kVA, MVA)?

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).

Why is the CT ratio important in electrical systems?

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.