Free Transformer Calculator – Turns Ratio, Currents & kVA Rating

How Transformer Turns Ratio, Currents and kVA Fit Together

A transformer calculator works out the relationship between the two windings of a transformer: how their voltages, currents and turns relate, and what apparent-power rating in kVA the transformer carries. Transformers change voltage from one level to another so power can travel efficiently and then be used safely, which is why they sit everywhere from the grid substation down to the small adapter charging a phone. This tool takes the primary and secondary voltage and returns the turns ratio, both winding currents, and the kVA rating for single-phase or three-phase units. For an ideal, lossless transformer a single identity ties everything together: Np ÷ Ns = Vp ÷ Vs = Is ÷ Ip. The turns ratio, written a, equals the primary voltage divided by the secondary voltage. Voltage and turns move together, but current moves the opposite way, so a transformer that halves the voltage doubles the available current. The apparent-power rating stays the same on both sides because an ideal transformer conserves power: kVA = V × I ÷ 1000 for single-phase, or kVA = √3 × V × I ÷ 1000 for three-phase, and you can compute it from either winding. When the secondary voltage is higher than the primary the device is a step-up transformer, when it is lower it is step-down, and when they match it is an isolation transformer at a 1:1 ratio. Work through an example. A transformer with 240 volts on the primary and 12 volts on the secondary has a turns ratio of 240 ÷ 12 = 20, written 20:1, which makes it a step-down transformer. If the primary draws 1 amp, the secondary delivers 1 × 20 = 20 amps, because current scales inversely with voltage. Rating that same transformer as single-phase at 240 volts and 20 amps gives 240 × 20 ÷ 1000 = 4.8 kVA. Switch to a three-phase unit at 400 volts and 100 amps and the rating becomes √3 × 400 × 100 ÷ 1000 = 69.28 kVA. Flip the voltages so the primary is 120 and the secondary is 240 and the ratio becomes 0.5, a step-up transformer that trades current for voltage. These calculations show up in many roles. An electronics hobbyist winding or choosing a small mains transformer checks the turns ratio needed to get 12 volts from a 240 volt supply. A solar or battery installer sizing an isolation transformer confirms the kVA rating covers the connected load. A maintenance technician who reads the primary current on a clamp meter uses the ratio to infer the secondary current without breaking into the low-voltage side. Seeing the ratio expressed both as a decimal and as a clean whole-number form such as 20:1 makes the result easy to sanity-check against a nameplate. The model assumes an ideal transformer with no core or copper loss, so it reports apparent power in kVA rather than real power in kW; adding real loading needs the load power factor, which a simple ratio does not capture. The kVA figure is the same on both windings by design, which is exactly why either winding can be used to rate the unit. A frequent mistake is mixing up which winding is primary — the primary is the input side connected to the source, and swapping the two inverts every ratio. All voltages and currents must be positive, and a zero would divide by nothing, so the tool guards against it. Treat the output as an engineering estimate; any transformer or mains work must follow local electrical code and be done by a qualified electrician. Everything runs in your browser and nothing is uploaded.

Transformer Calculator Formula & Method

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