Transformer Calculator
Determine the primary current draw of a transformer given its output voltage, output current, and efficiency. Use this when sizing transformers for power supplies, HVAC systems, or industrial equipment.
About this calculator
A transformer transfers electrical energy between circuits via electromagnetic induction. The key relationship is power conservation: primary input power equals secondary output power divided by efficiency. The formula used here is: Primary Current = (secondary_current × secondary_voltage) / (primary_voltage × (efficiency / 100)). For an ideal transformer (100% efficient), input power equals output power exactly. Real transformers lose energy as heat in windings and the core, so efficiency is always below 100%. Turns ratio relates voltages as N₁/N₂ = V₁/V₂, and currents inversely as I₁/I₂ = N₂/N₁. Knowing primary current is essential for selecting fuses, breakers, and wiring on the supply side.
How to use
Suppose a transformer has a primary voltage of 240 V, a secondary voltage of 24 V, delivers a secondary current of 5 A, and operates at 90% efficiency. Step 1 — multiply secondary values: 5 A × 24 V = 120 VA. Step 2 — multiply primary voltage by efficiency factor: 240 V × (90/100) = 216. Step 3 — divide: 120 / 216 ≈ 0.556 A. The transformer draws approximately 0.56 A on the primary side. This is slightly more than the ideal 0.5 A because 10% of input power is lost to heat.
Frequently asked questions
What is transformer turns ratio and how does it relate to voltage?
Turns ratio is the ratio of primary winding turns to secondary winding turns (N₁/N₂). It directly determines the voltage transformation: V₁/V₂ = N₁/N₂. A turns ratio of 10:1 steps voltage down by a factor of 10. Current transforms inversely, so a step-down transformer draws less current on the primary side than it delivers on the secondary side (ignoring losses).
How does transformer efficiency affect primary current draw?
Efficiency accounts for real-world losses including resistive heating in windings (copper losses) and magnetic losses in the core (iron losses). A lower efficiency means the primary must supply more power than the secondary delivers. For example, an 85% efficient transformer needs to draw 1/0.85 ≈ 1.18 times the ideal primary current. Always derate your supply-side wiring and protection devices to handle this extra current.
When should I use a transformer turns ratio calculator for an electrical project?
Use this calculator whenever you are selecting or verifying a transformer for a project — such as powering low-voltage lighting, control circuits, or audio equipment from a mains supply. It helps you confirm the primary current so you can choose correctly rated fuses and cables. It is also useful when checking whether an existing transformer can handle an increased load without overheating.