How it is calculated
The transformer kVA calculator determines the apparent power (kVA) required for a load and the minimum transformer rating needed to supply it safely. For a single-phase system, the load kVA is calculated as:
Load kVA (single-phase) = (V × I) / 1000
For a three-phase system, the formula includes the square root of three to account for the phase relationship:
Load kVA (three-phase) = (V × I × √3) / 1000
Where V is the voltage in volts (line-to-line for three-phase, line-to-neutral for single-phase), and I is the current in amperes. The result is the apparent power in kilovolt-amperes (kVA).
To find the required transformer kVA, the load kVA is divided by the transformer loading factor (expressed as a decimal):
Required transformer kVA = Load kVA / (L / 100)
Where L is the transformer loading percentage (e.g., 80% means 0.80). This ensures the transformer is not operated at full capacity, allowing for future load growth and preventing overheating.
Assumptions
- The load is continuous and linear (e.g., motors, heaters, lighting without significant harmonics).
- Voltage is measured line-to-line for three-phase systems and line-to-neutral for single-phase systems.
- A loading factor of 80% is typical for continuous loads to provide a safety margin and accommodate future expansion.
- The power factor is assumed to be 1.0 (unity), meaning the load is purely resistive. For loads with a lower power factor, use the power factor correction calculator to adjust.
Worked example
Consider a three-phase load with the following inputs:
- Phase: 3 (three-phase)
- Voltage (V): 480 V
- Current (I): 100 A
- Transformer loading (L): 80%
First, calculate the load kVA:
Load kVA = (480 × 100 × √3) / 1000 = (480 × 100 × 1.732) / 1000 = 83136 / 1000 = 83.14 kVA
Next, calculate the required transformer kVA:
Required transformer kVA = 83.14 / (80 / 100) = 83.14 / 0.80 = 103.92 kVA
Thus, the load requires 83.14 kVA, and a transformer rated at least 103.92 kVA should be selected to operate at 80% loading.
Practical tips
- Always use the line-to-line voltage for three-phase systems and line-to-neutral for single-phase to avoid calculation errors.
- The 80% loading factor is a common guideline, but consult local codes or the transformer manufacturer for specific recommendations.
- For non-linear loads (e.g., variable frequency drives, computers), consider derating the transformer further or using a transformer sizing calculator for more precise sizing.
- If you need to size cables for the transformer, use the cable size calculator to ensure proper conductor ampacity.
- Round up the required transformer kVA to the next standard size (e.g., 112.5 kVA, 150 kVA) for practical selection.
Related calculators
Calculators that go with this one:
- Energy usage calculator — Calculate the energy consumption and electricity cost of any appliance
- Solar panel angle calculator — Find the optimal tilt angle for your solar panels based on latitude and season
- Energy consumption calculator — Estimate the annual energy consumption and electricity cost of any appliance or device
- Voltage drop calculator — Calculate voltage drop in volts and percent for copper or aluminum wire, in AWG/kcmil or mm², for 120 V, 240 V
- Electricity cost calculator — Calculate the electricity cost of any appliance
- Solar panel kilowatt hour calculator — Estimate solar panel energy production and cost savings