Transformer Sizing Calculator

Use our transformer sizing calculator to obtain the minimum kVA needed to power your load.

Apparent power rating needed36 VAtransformers are rated in VA rather than watts, because the heating depends on current regardless of the load's power factor

Secondary voltage12 V240 V × 30 ÷ 600

Turns ratio20:1step-down

Volts per turn400 mVthe same on both windings — this is what a transformer actually holds constant

Impedance ratio400:1the SQUARE of the turns ratio — which is what makes transformers useful for impedance matching

Secondary current3 A

Primary current154.6392 mAstepping voltage down steps current up — a transformer transfers power, it does not create it

Secondary power36 W

Primary power37.1134 Wincluding 3% loss in the core and windings

Power lost as heat1.1134 W

Load reflected to the primary1.6 kΩwhat the source actually sees

No-load secondary voltage12.48 V4% regulation means the output sags by this much between no load and full load

Voltage sag under load480 mV

The formula

V_s/V_p = N_s/N_p; I_s/I_p = N_p/N_s; power is conserved

Impedance goes as the square

A transformer holds volts per turn constant across both windings, which is where the turns ratio comes from. Voltage scales with the ratio and current inversely, so power is conserved apart from losses — a transformer moves power between voltage-current combinations, it never creates any.

Impedance, though, transforms as the square of the turns ratio. A 20:1 step-down makes a 4 Ω speaker look like 1600 Ω to the primary, which is the whole basis of impedance matching in audio and radio work and a result that catches people out when they reason only about voltage.

Two practical notes. Transformers are rated in VA, not watts, because the winding heating depends on current whatever the load's power factor — a reactive load draws current without consuming power, and the transformer still has to carry it. And regulation describes how much the secondary voltage sags from no load to full load: a transformer specified at its loaded voltage will read appreciably higher with nothing connected, which is normal rather than a fault.