What Is a Transformer?
A transformer is a device that transforms voltage. It takes an input (primary) voltage and produces a different output (secondary) voltage. We can use it to create a step-up or a step-down in voltage — for example, lowering the high voltage from power lines so it can be safely used inside a house. Our transformer sizing calculator lets you obtain the minimum single-phase or three-phase transformer size (in kVA) required to power a given load.
How Does a Transformer Change Voltages?
Transformers rely on magnetic induction. A changing magnetic flux induces a voltage in a conductor, as described by Faraday's law of induction:
emf = −dΦ/dt
An alternating current in the primary coil creates a continuously changing magnetic field in the shared core. By Faraday's law, this changing flux induces an electromotive force in the secondary coil. Because the induced voltage is proportional to the number of turns the flux passes through, a secondary coil with more turns produces a higher voltage (step-up), and one with fewer turns produces a lower voltage (step-down). Lenz's law sets the direction of the induced current so that it opposes the change in flux. Note that transformers only work with AC — a steady DC current produces a static field that induces no secondary voltage.
What Does kVA Stand For on a Transformer?
kVA stands for kilovolt-amperes — a unit of apparent power. Transformers are rated in kVA rather than kilowatts (kW) because a transformer's capacity is limited by voltage and current, not by the power factor of the load it serves. Apparent power (kVA) combines the real power (kW) actually consumed and the reactive power (kVAR) that circulates in inductive or capacitive loads:
kVA = kW / power factor
A purely resistive load has a power factor of 1, so 1 kW = 1 kVA. Motors and other inductive loads have a power factor below 1 (typically 0.8–0.95), so they require more kVA than their kW rating suggests.
How Do You Calculate kVA for a Transformer?
The apparent power depends on whether the system is single-phase or three-phase:
- Single-phase: kVA = (V × I) / 1000
- Three-phase: kVA = (√3 × V × I) / 1000
where V is the voltage (line-to-line for three-phase) and I is the load current in amperes. If you know the load as power instead of current, first convert it to apparent power using kVA = kW / power factor.
How to Calculate Transformer Load Capacity
Working backward from a transformer's kVA rating, you can find the maximum load current it can supply:
- Single-phase: I = (kVA × 1000) / V
- Three-phase: I = (kVA × 1000) / (√3 × V)
In practice, transformers should not be loaded to 100% of their nameplate rating continuously. A safety margin (commonly 20–25%) is added so the transformer runs cooler, lasts longer, and has headroom for future load growth and inrush currents. This calculator applies your chosen margin and then rounds up to the next standard transformer size (e.g., 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100, 150 kVA…).
Metric and American Unit Systems
Voltage (volts) and current (amperes) are the same SI units worldwide. The main difference between the metric and American systems is how mechanical/real load is expressed:
- Metric: load power entered in kilowatts (kW); typical voltages 230 V (single-phase) and 400 V (three-phase).
- American: load power entered in horsepower (HP), automatically converted with 1 HP = 745.7 W; typical voltages 120/240 V (single-phase) and 208/480 V (three-phase).
FAQs
What size transformer do I need for my load?
Enter your supply voltage and either the load current (amps) or the load power (kW or HP) with its power factor. The calculator returns the required apparent power in kVA, applies your safety margin, and recommends the next standard transformer size that can safely supply the load.
Should I size a transformer in kVA or kW?
Always size in kVA. Transformers are limited by current and voltage (apparent power), so a transformer rated in kW would not account for the reactive power drawn by inductive loads such as motors.
Why add a safety margin?
Continuous operation at full nameplate rating shortens transformer life through heat. A 20–25% margin keeps the unit cooler, accommodates inrush and overload currents, and leaves room for future expansion.
Single-phase or three-phase?
Single-phase transformers are common in residential and light commercial settings. Three-phase transformers are used for industrial machinery and larger commercial loads, delivering more power for the same current thanks to the √3 factor in the apparent-power formula.
Does power factor affect transformer sizing?
Yes. A lower power factor means more apparent power (kVA) is needed for the same real power (kW). For example, a 10 kW load at a power factor of 0.8 needs 12.5 kVA, while the same 10 kW at 1.0 needs only 10 kVA.