Transformer Calculator
A transformer's turns ratio, secondary voltage and current, and its kVA rating, for single-phase or three-phase, with an efficiency for real losses.
Result…
How it's calculated
a = Np ÷ Ns = Vp ÷ Vs · Vs = Vp × Ns ÷ Np Ideal: Is = Ip × Np ÷ Ns Single-phase S = V × I · Three-phase S = √3 × V(line) × I(line) Output = efficiency × input · kVA = VA ÷ 1000
A transformer's voltage changes in the same proportion as its turns: a winding with 10 times fewer turns gives 10 times lower voltage. Power in and out is (almost) the same, so the current changes the other way: stepping the voltage down by 10 steps the current up by 10. The turns ratio is Np divided by Ns, so a ratio above 1 is a step-down transformer and below 1 a step-up one. The kVA rating is the apparent power, volts times amps divided by 1000, multiplied by √3 for a three-phase transformer using line voltage and line current. Real transformers lose a few percent as heat in the core and windings: enter an efficiency below 100% to see the extra primary current or the lower output.
Your numbers, step by step
About the Transformer calculator
This transformer calculator works out the turns ratio, the secondary voltage, the current on each side and the kVA rating. Enter the primary and secondary turns, or both voltages if you know them, and say whether the supply is single-phase or three-phase. Then give the load current on the secondary side, or the current drawn from the supply, and an efficiency: 100% for an ideal transformer or the figure on a datasheet. The answer shows the voltage after the transformer, whether it steps up or down, the other current, the output and input in kVA and the power lost as heat. It helps when choosing a transformer for a lamp, a control circuit, a workshop machine or a building supply, and when checking textbook problems, with every step shown using your own numbers.
How to use it
- Under “I know”, pick “The turns” and enter “Primary turns Np” and “Secondary turns Ns”, or pick “Both voltages”.
- Enter the “Primary voltage” (line-to-line for three-phase) and choose the “Supply”.
- Under “Current I know”, choose the secondary (load) or primary (supply) current and enter it.
- Set the “Efficiency”, or leave it at 100% for an ideal transformer.
- Choose a transformer whose kVA rating is at least the “Output (rating)” figure.
Worked example
A single-phase transformer has 2,000 primary turns and 100 secondary turns on a 230 V supply. The turns ratio is 2,000 ÷ 100 = 20, so the secondary voltage is 230 × 100 ÷ 2,000 = 11.5 V: a step-down transformer. A 5 A load takes 11.5 × 5 = 57.5 VA, or 0.0575 kVA. At 95% efficiency the input is 57.5 ÷ 0.95 = 60.53 VA, so the primary current is 60.53 ÷ 230 = 0.263 A and about 3 VA is lost as heat.
Updated
Questions people ask
How do I calculate the secondary voltage of a transformer?
Multiply the primary voltage by the secondary turns and divide by the primary turns: Vs = Vp × Ns ÷ Np. A transformer with 10 times fewer secondary turns gives one tenth of the voltage.
What is the formula for transformer kVA?
For single-phase, kVA = volts × amps ÷ 1000. For three-phase, kVA = √3 × line voltage × line current ÷ 1000. The three-phase power calculator also turns kVA into kW at a power factor.
Why does the current go up when the voltage goes down?
A transformer passes on nearly all the power it takes in. If the voltage falls by a factor of 20, the current can rise by about 20 for the same power, less the small losses set by the efficiency.
What does the turns ratio tell me?
The turns ratio is Np ÷ Ns. Above 1 it is a step-down transformer, below 1 a step-up one, and exactly 1 is an isolation transformer that keeps the voltage the same.
Is it a mistake to size a transformer at exactly the load?
Usually, yes. Motors draw extra current as they start and loads grow over time, so many people allow some headroom. The datasheet gives the continuous kVA, how much overload it handles and for how long.
Does this work for any three-phase transformer?
It assumes both windings are connected the same way. A delta-star or star-delta transformer shifts the line voltage ratio by √3 and adds a phase shift. Have a qualified electrician choose and connect any mains transformer.