Transformer Calculator
A transformer is the one device where "less voltage means more current" is the whole point. Size it wrong and you either overheat the windings or pay for capacity you'll never use. Enter the primary and secondary voltages, the load current on the low side, and whether it's single or three-phase to get the exact turns ratio, the primary current you must feed it, and the kVA rating to specify — plus a loss estimate from efficiency.
Transformer & Load
The Transformer Equations
In an ideal transformer, voltage scales with turns and power is conserved, so current scales inversely:
Np / Ns = Vp / Vs and Vp × Ip = Vs × Is
The apparent-power rating (kVA) is what you actually buy. Single-phase: kVA = V × I / 1,000. Three-phase: kVA = √3 × V_LL × I / 1,000 — the √3 appears because the three windings sit 120° apart, so their vectors don't simply add. Get the phase wrong and you mis-size by 73%.
Worked Example: A 240 V → 24 V Control Transformer
A 24 V thermostat relay draws 10 A. The turns ratio is 240 ÷ 24 = 10:1, so the secondary delivers 240 VA (24 × 10). On the primary that's only 240 ÷ 240 = 1 A. The unit is a 0.24 kVA transformer — you'd specify the next standard size, a 1 kVA dry-type. At 97% efficiency the input is about 247 W, so roughly 7 W is lost to heat (core + copper). Note the wire sizes: the 24 V side carries 10 A and needs thick conductors; the 240 V side carries 1 A and can be thin.
Sizing With Headroom
Never size a transformer to exactly the running load. Motor and inductive loads pull 5–10× their running current for a fraction of a second at startup, and that inrush must fit without saturating the core. A common rule is to rate at 1.25–1.5× the continuous VA, and always round up to a standard kVA. Undersizing causes overheating and premature failure; oversizing only costs a little upfront and runs cooler (longer life).
Frequently Asked Questions
What is a transformer turns ratio?
It's Np/Ns and equals Vp/Vs. A 240→24 V unit is 10:1, and it inverts the current relationship.
Why does current go up when voltage steps down?
Power is conserved (Vp·Ip = Vs·Is), so stepping voltage down 10× steps current up 10×. The VA product is identical on both sides.
How do I calculate transformer kVA?
Single-phase: V×I/1000. Three-phase: √3×V_LL×I/1000. Round up to the next standard rating with inrush headroom.
What causes transformer losses?
Core (iron) losses from magnetization plus copper (I²R) losses from windings. Small dry-types run ~95–98% efficient at full load.
Single-phase or three-phase formula?
Use three-phase only for 3φ supply and load (√3 factor). Residential/control transformers are usually single-phase.