Transformer Calculator - Turns Ratio, Voltage, kVA to Amps

A transformer calculator built around one ratio

Wind 500 turns on one side of a core and 100 on the other, feed it 120 V, and 24 V comes out: the voltages follow the turns, and the currents go the opposite way. This tool solves that ratio for whichever number is missing, the secondary or primary voltage, the secondary or primary turns, or just the turns ratio from two voltages. Add the VA or kVA rating and it gives the full-load current on both sides, single-phase or three-phase, with the losses if you know the efficiency.

5 : 1

120 V down to a 24 V thermostat circuit

1.667 A

what a 40 VA, 24 V secondary may deliver

208.18 A

full load of 75 kVA on a 208 V three-phase secondary

30 : 1

a 7,200 V line down to 240 V at the house

What each field expects

What to find - secondary voltage, primary voltage, secondary turns, primary turns, or the turns ratio from two voltages when the turn counts are unknown, which is the usual case with a bought transformer.
Primary and secondary voltage - RMS values from the nameplate, such as 120 V in and 24 V out. For three-phase use line-to-line voltages, for example 480 V and 208 V.
Primary and secondary turns - only for the turn modes. When the answer is not a whole number, the result also shows what the nearest whole turn counts would really give.
Rating and its unit (optional) - VA for small transformers, kVA for building and utility units.
Phases - needed with a rating, because the current formula differs: S ÷ V for single-phase, S ÷ (√3 × V) for three-phase.
Efficiency (optional) - in percent. Without it the primary current is the ideal one, with no losses.
Read the result - the answer in the colored box, both voltages, the turns and the ratio, then the currents and losses when a rating was entered.

Everything the ratio decides

Both windings share the same changing magnetic flux, so each turn has the same voltage induced in it. That is the whole reason V1 ÷ V2 = N1 ÷ N2: a winding with five times as many turns carries five times the voltage. Power in has to equal power out in an ideal transformer, so the current does the reverse. A 5 : 1 step-down that delivers 1.667 A at 24 V draws only a fifth of that, 333.3 mA, from the 120 V side.

There is a third quantity the ratio controls, less often mentioned. A resistance hung on the secondary looks, from the primary, like that resistance multiplied by the square of the turns ratio. An 8 Ω speaker behind a 10 : 1 output transformer appears as 800 Ω to the amplifier tube driving it. Matching those impedances is what audio output transformers are for.

A transformer's rating is apparent power in volt-amperes, not watts. The windings heat up with current whatever the power factor of the load, so the nameplate limits VA. On a resistive load such as a heater or an incandescent bulb, VA and watts are the same number.

Full-load current for common kVA ratings

These are the currents each rating allows at full load, rounded to two decimals, from I = S ÷ V for single-phase and I = S ÷ (√3 × V) for three-phase. The calculator gives the same figures for any rating you type.

Rating	Single-phase 240 V	Single-phase 120 V	Three-phase 480 V	Three-phase 208 V
15 kVA	62.50 A	125.00 A	18.04 A	41.64 A
25 kVA	104.17 A	208.33 A	30.07 A	69.39 A
45 kVA	187.50 A	375.00 A	54.13 A	124.91 A
75 kVA	312.50 A	625.00 A	90.21 A	208.18 A
150 kVA	625.00 A	1,250.00 A	180.42 A	416.36 A
300 kVA	1,250.00 A	2,500.00 A	360.84 A	832.72 A

The three-phase columns are why a 75 kVA unit is a common size for a small commercial building: at 208 V it supplies a little over 200 A per line. The breaker and conductors for that current are a separate question, sized by the electrical code rather than by this arithmetic.

Transformers around a North American house

The ratios below are the nameplate voltages divided, which is what the calculator's ratio mode returns. Real turn counts are a little different, because manufacturers add turns to the secondary to make up for the voltage drop under load.

Where	Voltages	Ratio
Utility transformer on the pole	7,200 V to 240 V	30 : 1
Furnace or air conditioner controls	120 V to 24 V	5 : 1
Doorbell	120 V to 16 V	7.5 : 1
Low-voltage landscape lighting	120 V to 12 V	10 : 1
Tube amplifier heater winding	120 V to 6.3 V	19.0476 : 1
Step-up for 240 V equipment on a 120 V outlet	120 V to 240 V	1 : 2

Five transformer facts that catch people out

It does nothing with steady DC. Only a changing current changes the flux, so a battery on the primary gives 0 V on the secondary once the first instant has passed, while the primary winding, with only its wire resistance to limit it, can overheat.

A new small transformer reads high. With nothing connected, a secondary labeled 12 V often measures noticeably more, because the label is the voltage at full load. Small units lose a larger share to winding resistance, so the gap is widest on them.

Frequency matters in one direction. The flux in the core grows as the frequency drops, so a 60 Hz design run on 50 Hz at the same voltage carries 20% more flux and may saturate and overheat. A 50 Hz design on 60 Hz is generally fine.

Impedance scales with the square. A 10 : 1 ratio turns 8 Ω into 800 Ω, not 80 Ω. Voltage and current each change by the ratio, and impedance is their quotient, so the ratio counts twice.

Big ones are remarkably efficient. Large power transformers lose well under 1% of what passes through them, while a small plug-in unit may waste 10 to 20%. With 85% efficiency, a 40 VA unit draws 47.06 VA and turns 7.06 W into heat at full load.

People who end up here, and what they type

HVAC technicians checking a replacement control transformer: ratio mode, 120 and 24 V, 40 VA single-phase, which gives 1.667 A for the thermostat circuit.

Electricians sizing a building transformer: ratio mode, 480 and 208 V, 75 kVA three-phase at 98%, for 208.18 A on the secondary and 92.052 A on the primary.

Hobbyists rewinding a transformer: secondary turns mode with 120 V, 16 V and 1,000 primary turns. The answer, 133.33, is not buildable, so the result adds that 133 turns give 15.96 V and 134 give 16.08 V.

Physics students with a textbook problem: 10 primary and 20 secondary turns with 240 V out means 120 V in, a 1 : 2 step-up.

Quick questions about ratios, VA and turns

How do I find the turns ratio of a transformer I cannot open?

Apply a known AC voltage to the primary with nothing on the secondary, measure the secondary, and divide. The ratio you get is slightly lower than the nameplate voltages suggest, because the no-load secondary reads high.

How many amps can a 50 VA transformer supply?

Divide the rating by the secondary voltage: 50 VA at 12 V is 4.167 A, and at 24 V it is 2.083 A. The rating already refers to the output, so there is no need to take the efficiency off again.

Can a transformer be used backwards?

In principle yes: a 120 V to 24 V unit fed with 24 V gives about 120 V. The output comes out a little lower than the ratio suggests, because the extra turns meant to compensate for load drop are now on the wrong side.

Why does the three-phase formula divide by √3?

A balanced three-phase load carries S = √3 × V_line × I_line. Solving for the current gives S ÷ (√3 × V). The √3, about 1.732, comes from the 120° spacing between phases.

Is kVA the same as kW?

Only at a power factor of 1. kW = kVA × power factor, so a 75 kVA transformer feeding loads at a power factor of 0.8 delivers 60 kW of real power while running at its full current.

What does a 1:1 transformer do if it does not change the voltage?

It isolates. The secondary has no conductive connection to the supply, so touching one output wire and ground completes no circuit through you. Isolation transformers are used on test benches for that reason.

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