Before choosing a transformer, you need to know how much capacity the load requires.
The main information needed is:
- Load voltage
- Load current in amps
- Whether the load is single-phase or three-phase
Those numbers are used to calculate the transformer capacity in kilovolt-amperes, also called kVA.
Single-Phase Transformer Capacity
For a single-phase load, multiply volts by amps and divide by 1,000.
Volts × amps ÷ 1,000 = kVA
Quick Example
Say you have a 240V single-phase load drawing 80 amps.
240 volts × 80 amps = 19,200 VA
19,200 VA ÷ 1,000 = 19.2 kVA
The load requires at least:
19.2 kVA
Since transformers come in standard sizes, you would move up to the next transformer size that can handle the load.
For this example:
Recommended transformer size = 25 kVA
Three-Phase Transformer Capacity
Three-phase transformer sizing uses a different calculation.
Volts × amps × 1.732 ÷ 1,000 = kVA
The 1.732 factor is used for three-phase loads.
For example, if you have a 208V three-phase load drawing 50 amps:
208 volts × 50 amps × 1.732 = 18,012.8 VA
18,012.8 VA ÷ 1,000 = 18.0 kVA
The calculated load requires at least:
18.0 kVA
Since 18 kVA is not a standard transformer size, move up to the next standard size.
Recommended transformer size = 25 kVA
Why Capacity Matters
A transformer that is too small may not handle the connected load properly.
A transformer that is much larger than needed can add unnecessary cost and take up more space.
Calculating the expected load first gives you a better starting point before choosing equipment.
How Sparky Toolbox Helps
The Sparky Toolbox Transformer Sizing Calculator can calculate both single-phase and three-phase loads.
Enter the voltage, load current, and phase. The calculator shows:
- Required kVA to meet the load demand
- Recommended transformer kVA size
That makes it easier to estimate transformer capacity before selecting equipment or building a material list.
Important Reminder
This is a basic sizing estimate. Transformer selection can change based on continuous loads, motor starting current, future load, system voltage, manufacturer ratings, installation conditions, available fault current, and project requirements.
Always verify equipment data, approved plans, manufacturer instructions, and local requirements before installation.