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.
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.
Transformer Rating (Apparent Power): Type the rating from the nameplate, then choose VA, kVA, or MVA.
Primary Voltage (Vp): Type the supply-side voltage of the input winding, then pick mV, V, or kV.
Secondary Voltage (Vs): 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.
Current Display Unit: Choose mA, A, or kA to set how the current results are shown.
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.
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.
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.
Turns Ratio
The turns ratio is n = Vp ÷ Vs. 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 Ip ÷ Is = Vs ÷ Vp.
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.
Why These Numbers Matter
- Wire sizing: conductors must carry the full-load current without getting too hot.
- 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.
- Meter readings: comparing measured amps to full-load amps shows how hard a transformer is working.
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.