What is the formula for Transformer Copper Loss?

The Transformer Copper Loss formula is Pcu = Ip² × Rp + Is² × Rs, where Pcu—Total copper loss (W); Ip, Is—Primary/secondary RMS currents (A); Rp, Rs—Primary/secondary winding resistances (Ω).

Transformer Copper Loss

Q: How do you calculate Transformer Copper Loss?

Use the formula Pcu = Ip² × Rp + Is² × Rs. Copper (I²R) loss is the heat dissipated in the resistance of the transformer windings as load current flows. Enter the values you know in the calculator above and it solves for the remaining variable.

Pcu = Ip² Rp + Is² Rs

Calculator

Copper Loss

Primary Current

Primary Resistance

Secondary Current

Secondary Resistance

Result

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Formula

Pcu = Ip² × Rp + Is² × Rs

Description

Copper (I²R) loss is the heat dissipated in the resistance of the transformer windings as load current flows. It is the load-dependent part of transformer loss: at full load it dominates, and at no load it is essentially zero. Because it grows with the square of current, it sets the transformer’s thermal limit and efficiency at high load.

Variables

Pcu — Total copper loss (W)
Ip, Is — Primary/secondary RMS currents (A)
Rp, Rs — Primary/secondary winding resistances (Ω)

Practical Notes

Copper loss is measured by the short-circuit test. Combined with the no-load (iron) loss it gives total loss; maximum efficiency occurs where copper loss equals iron loss.

Related Concepts

All Magnetics & Transformers formulas →

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