Step up the voltage and the current drops by the same ratio — a transformer changes the balance between volts and amps, never the total power available (minus small real-world losses).
V₁/V₂ = N₁/N₂ = I₂/I₁ is a direct consequence of energy conservation, not a design choice — it’s exactly why long-distance power transmission uses very high voltage: the same power at higher voltage means proportionally lower current, and resistive losses scale with current squared.
The 'minus small losses' hides real numbers: core and copper losses put a distribution transformer at 97–99%, and a small one much lower. A transformer left energised also consumes core loss continuously with no load at all, which is why standby transformers are switched out.
At 0.6× the rule says 1 and the exact answer is 0.9814 — 1.9% high. It holds to within 2% from 0.46× to 0.88×, and drifts outside that.
Power out never quite equals power in. Iron loss is there the moment it is energised and does not care about load; copper loss goes as the square of it. That puts peak efficiency near two-thirds load, not full load, and makes a lightly loaded transformer the worst of both worlds — at a tenth of rating it is throwing away 6% of what you feed it.
The rule against the exact answer, computed across the range. Inside the shaded band the shortcut is close enough to use; outside it, reach for the calculator.
Estimate with the rule, then check it against the calculator that models it properly.
Open Transformer Calculator →Step up the voltage and the current drops by the same ratio — a transformer changes the balance between volts and amps, never the total power available (minus small real-world losses). V₁/V₂ = N₁/N₂ = I₂/I₁ is a direct consequence of energy conservation, not a design choice — it’s exactly why long-distance power transmission uses very high voltage: the same power at higher voltage means proportionally lower current, and resistive losses scale with current squared.