Transformer Rating Calculator
Transformers Trade Voltage for Current, Not Power
An ideal transformer doesn't create or destroy energy — it steps voltage up or down while current adjusts in the opposite direction, so the apparent power on both sides stays equal. That relationship is what lets a transformer's kVA rating be calculated directly from the secondary side's voltage and current, without needing to measure the primary at all.
The Formulas
VA Rating = Vs × Is
Ip = VA Rating / Vp
Vp and Vs are the primary and secondary voltages; Is is the secondary current. Because an ideal transformer conserves apparent power (Vp × Ip = Vs × Is), the primary current can be found once the VA rating is known.
A Worked Example
Stepping 480 V down to 120 V to supply 50 A on the secondary:
| Quantity | Value |
|---|---|
| Turns Ratio | 4.0 : 1 |
| VA Rating | 6000 VA |
| Primary Current (Ip) | 12.5 A |
A 4:1 step-down in voltage corresponds to a 1:4 step-up in current on the primary side — power in equals power out.
Standard Transformer Sizes
Distribution transformers aren't custom-built to an exact calculated figure; they're selected from standard manufactured sizes at or above the calculated VA requirement:
| Standard sizes (kVA) | |||||
|---|---|---|---|---|---|
| 10 | 15 | 25 | 37.5 | 50 | 75 |
| 100 | 167 | 250 | 333 | 500 | — |
Where This Calculation Matters
- Selecting a replacement transformer — sizing a unit to the actual secondary load rather than guessing at a rating.
- Panel and subpanel feeds — confirming a step-down transformer can supply the full connected load of the panel it feeds.
- Control circuit transformers — small transformers stepping 120V or 240V down to 24V for HVAC and control circuits still follow the same VA math.
- Estimating primary-side current — useful for sizing primary-side overcurrent protection and feeder conductors.
How to Use This Calculator
- Enter the Primary Voltage.
- Enter the Secondary Voltage.
- Enter the Secondary Current the load will draw.
- Select Calculate to see the required VA rating, turns ratio, and primary current.
Related Calculations
Check the resulting primary-side current against a safe conductor with the Wire Size Calculator, or see the same voltage/current relationship applied to plain resistive circuits with the Ohm's Law Calculator.