Transformer Current Calculator

Q: Should I enter line-to-line or line-to-neutral voltage?

Enter line-to-line voltage for three-phase. That is what the √3 formula expects. For single-phase, enter the voltage measured across the winding, such as 240 V or 120 V.

Updated Aug 23, 2026 By Jehan Wadia

Introduction

This Transformer Current Calculator finds the current on both sides of a transformer. Type in the transformer rating (kVA), the primary voltage, and the secondary voltage. The tool gives you the primary full-load current and the secondary full-load current right away.

Pick single-phase or three-phase with one click. For three-phase, the math uses the √3 factor for you. You can also add a power factor to see true power (watts) and reactive power.

The calculator also shows the turns ratio, a step-by-step solution, a chart of current at different loads, and a table for 25%, 50%, 75%, and 100% loading. That makes it easy to size wires, fuses, and breakers.

Need to work backward? Use the "Solve for Missing Value" tab. Enter any two of these three — kVA, volts, or amps — and it solves for the one you leave blank. For related jobs, see the kVA to Amps Calculator, the Transformer Sizing Calculator, or the general Transformer Calculator.

How to use our Transformer Current Calculator

Enter your transformer's phase type, power rating, and both voltages. The calculator gives you the primary current, the secondary current, the turns ratio, and the power, plus step-by-step math, a chart, and a partial-load table.

Phase Configuration: Pick Single-Phase (1Ø) or Three-Phase (3Ø). Three-phase adds the √3 factor to the current math. This choice is used on both tabs. For a deeper look at three-phase math, try the 3 Phase Power Calculator.

Transformer Rating (Apparent Power): Type the rating from the nameplate, then choose VA, kVA, or MVA. The kVA Calculator and kVA to kW Calculator help if your nameplate lists different units.

Primary Voltage (V_p): Type the supply-side voltage of the input winding, then pick mV, V, or kV.

Secondary Voltage (V_s): Type the load-side voltage of the output winding, then pick mV, V, or kV.

Power Factor (PF): Optional. Leave it at 1.00 for a plain resistive load. Enter a smaller number (0.01 to 1.00) to also get true power and reactive power. See the PF Calculator for more on power factor.

Current Display Unit: Choose mA, A, or kA to set how the current results are shown. The Amp Calculator and Watts to Amps Calculator cover other current conversions.

Power Display Unit: Choose Auto-scale, VA/W, kVA/kW, or MVA/MW to set how the power results are shown.

Solve for Missing Value tab: Fill in only two of the three boxes — Apparent Power, Voltage, and Current — and leave the third one blank. The calculator finds the blank value and shows the work. This works much like the Amps to kVA Calculator in reverse.

Calculate and Reset: Results update as you type, but you can click Calculate to run it now. Click Reset to put the sample values back.

Transformer Current: What It Means

A transformer changes voltage from one level to another. It has two windings. The primary winding takes power in. The secondary winding sends power out. Transformer current is the amount of amps that flows in each winding when the transformer carries its full rated load.

How Transformer Current Is Found

Transformer size is given in VA, kVA, or MVA. This is apparent power (S). Once you know the power rating and the winding voltage, you can find the current with simple formulas:

Single-phase: I = S ÷ V
Three-phase: I = S ÷ (√3 × V)

The √3 (about 1.7321) is used for three-phase power because the three line voltages are spread apart in time. Leaving it out will give the wrong answer. For DC and simple resistive circuits, the Ohms Law Calculator covers the basic V, I, R relationship.

Primary and Secondary Currents Are Not Equal

Power in almost equals power out. So the side with the higher voltage has the lower current. A 15 kVA, 11,000 V to 400 V three-phase transformer pulls only about 0.79 A on the high side but gives about 21.65 A on the low side. This is why high voltage lines can use thin wires and low voltage feeders need thick ones. Long low-voltage runs also lose voltage along the way — check that with the Voltage Drop Calculator and the Wire Resistance Calculator.

Turns Ratio

The turns ratio is n = V_p ÷ V_s. If n is bigger than 1, the transformer steps voltage down. If n is smaller than 1, it steps voltage up. Current changes the other way, so I_p ÷ I_s = V_s ÷ V_p. You can double-check any ratio work with the Ratio Calculator.

Power Factor

Power factor (PF) tells how much of the apparent power does real work. True power is P = S × PF. Reactive power is Q = S × sin(cos^-1 PF). Note that PF does not change the full-load winding current. The current comes from the kVA rating, not the kW. That is why transformers are rated in kVA. To go from kilowatts back to amps, use the kW to Amps Calculator, and the Impedance Calculator helps with reactive loads.

Why These Numbers Matter

Wire sizing: conductors must carry the full-load current without getting too hot — see the Wire Size Calculator and Conduit Fill Calculator.
Breakers and fuses: protection is picked from the primary and secondary current.
Load checks: partial-load current rises straight with load, so 50% load means about 50% current. The Generator Sizing Calculator uses the same idea for backup power.
Meter readings: comparing measured amps to full-load amps shows how hard a transformer is working. See the Power Consumption Calculator and Electricity Cost Calculator to turn that into energy and dollars.

Things To Watch

Use line-to-line voltage for three-phase math. Inrush current when a transformer is first switched on can be many times the full-load current, so it is not the same as the values here. Also, real codes may ask you to add a safety margin above the calculated full-load amps before picking wire or breaker sizes. If you are working on smaller electronics instead of power gear, the Resistor Calculator, Voltage Divider Calculator, and Trace Width Calculator are better fits.

Formulas used

Primary full-load current

I_p = \frac{S}{k \times V_p},\quad k = \sqrt{3}\ (3\phi),\ k = 1\ (1\phi)

Secondary full-load current

I_s = \frac{S}{k \times V_s},\quad k = \sqrt{3}\ (3\phi),\ k = 1\ (1\phi)

Turns ratio

n = \frac{V_p}{V_s}

True (real) power

P = S \times PF

Reactive power

Q = S \times \sin\left(\cos^{-1} PF\right) = S \times \sqrt{1 - PF^2}

Apparent power from voltage and current (solve mode)

S = k \times V \times I,\quad k = \sqrt{3}\ (3\phi),\ k = 1\ (1\phi)

Voltage from apparent power and current (solve mode)

V = \frac{S}{k \times I},\quad k = \sqrt{3}\ (3\phi),\ k = 1\ (1\phi)

Values at partial loading (fraction of rated load)

S_L = S \times \frac{L}{100},\quad I_{p,L} = I_p \times \frac{L}{100},\quad I_{s,L} = I_s \times \frac{L}{100},\quad P_L = P \times \frac{L}{100}

Frequently asked questions

What does full-load current mean on this calculator?

Full-load current is the amps a winding carries when the transformer is loaded to 100% of its nameplate rating. It is the top number the windings are built for. Real amps will be lower any time the load is lighter.

Does the calculator include transformer losses or efficiency?

No. It treats power in as equal to power out, which is the standard way to find full-load amps. Real transformers lose a small amount of power as heat, so the true primary current is slightly higher than shown, usually by about 1-3%.

Why is my breaker bigger than the current shown here?

Electrical codes let you size transformer protection above the full-load amps. This covers inrush and normal load swings. Use the calculator number as your starting point, then apply the code multiplier for your job.

Should I enter line-to-line or line-to-neutral voltage?

Enter line-to-line voltage for three-phase. That is what the √3 formula expects. For single-phase, enter the voltage measured across the winding, such as 240 V or 120 V.

Can I use this for a 120/240 V single-phase transformer?

Yes. Click Single-Phase, then enter your rating and both voltages. For a 240 V secondary, use 240 as the secondary voltage. If you want the 120 V half-winding current, run it again with 120 V.

Does it work for delta and wye transformers?

Yes. Line current is the same formula for both delta and wye when you use line-to-line voltage. Note that inside a delta winding, the phase current is the line current divided by √3.

I only know kW, not kVA. What should I enter?

Divide kW by the power factor to get kVA. For example, 12 kW at 0.8 PF is 15 kVA. Enter that kVA in the rating box, then set the power factor field to 0.8.

Why does the current stay the same when I change the power factor?

Winding current comes from the kVA rating, not from watts. Power factor only changes how much of that power does real work. So the amps stay put, while the true power (kW) and reactive power (kvar) results change.

Why must I leave one box blank on the Solve tab?

The tool needs one unknown to solve for. Fill in two of the three boxes (power, voltage, current) and clear the third. If all three are filled, it cannot tell which one you want.

How do I find the current at half load?

Current rises straight with load, so half load is half the full-load amps. The partial-load table already lists 25%, 50%, 75%, and 100% for you, and the chart plots every 10% step.

Does this show inrush or short-circuit current?

No. It shows steady full-load current only. Inrush at switch-on can be 8 to 12 times higher for a few cycles. Fault current depends on transformer impedance (%Z), which this tool does not use.

Why does the chart have two separate y-axes?

Primary and secondary currents are often very different in size. Two axes let both lines be easy to read on one chart. The blue line is the primary side and the green line is the secondary side.

Is the turns ratio the real number of wire turns?

It is the ratio, not the count. A ratio of 27.5 means 27.5 primary turns for every 1 secondary turn. The actual turn counts depend on the core design, but they follow that same ratio.

Can I use this for an autotransformer?

You can find the line currents at each terminal the same way. But an autotransformer's winding carries only the difference between those currents, so do not use these numbers to size the internal winding.

Why would I pick mA or kA for the current unit?

Small control and signal transformers give tiny currents that read better in mA. Large power transformers can push thousands of amps, which read better in kA. Pick the unit that keeps the number easy to read.

Can I use this to pick wire size?

Use it to get the amps, then size wire from an ampacity table for your code and install type. Add margin for heat, bundling, and long runs. The calculator gives the current, not the finished wire size.

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