What is the formula for Transformer Current Ratio?

The Transformer Current Ratio formula is Ip / Is = Ns / Np, where Ip—Primary current (A); Is—Secondary current (A); Np—Primary turns; Ns—Secondary turns.

Transformer Current Ratio

Q: How do you calculate Transformer Current Ratio?

Use the formula Ip / Is = Ns / Np. In a transformer the current ratio is the inverse of the turns ratio: the winding with more turns carries less current. Enter the values you know in the calculator above and it solves for the remaining variable.

Ip / Is = Ns / Np

Calculator

Primary Current

Secondary Current

Primary Turns

Secondary Turns

Result

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Formula

Ip / Is = Ns / Np

Description

In a transformer the current ratio is the inverse of the turns ratio: the winding with more turns carries less current. This follows from conservation of apparent power (Vp·Ip = Vs·Is) combined with the voltage being proportional to turns. A step-down transformer that lowers voltage therefore raises the available current proportionally.

Variables

Ip — Primary current (A)
Is — Secondary current (A)
Np — Primary turns
Ns — Secondary turns

Practical Notes

Use this to size winding wire gauge: the high-current (low-turns) winding needs thicker wire. The current ratio assumes an ideal transformer; magnetizing current adds a small extra primary current.

Related Concepts

All Magnetics & Transformers formulas →

Transformer EMF Equation Transformer VA Rating Transformer Voltage Regulation Transformer Current Ratio Transformer Copper Loss Transformer Iron (Core) Loss Transformer Turns-per-Volt Transformer Impedance Matching

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