Before selecting a transformer, you need to know how much load it will serve.
The basic calculation uses voltage and amps to find the required kVA. Then you choose a transformer size that meets or exceeds that calculated load.
Quick Transformer Sizing Example
Example: a 240V single-phase load drawing 80 amps.
For a single-phase load, use:
Volts × amps ÷ 1,000 = kVA
Now calculate the required transformer capacity:
240 volts × 80 amps = 19,200 VA
19,200 VA ÷ 1,000 = 19.2 kVA
The load requires at least 19.2 kVA.
Transformers come in standard kVA sizes, so you move up to the next size that can handle the load.
For this example:
Recommended transformer size = 25 kVA
Final Results for This Example
- System: 240V single-phase
- Load current: 80 amps
- Calculated load: 19,200 VA
- Required transformer capacity: 19.2 kVA
- Recommended transformer size: 25 kVA
How Sparky Toolbox Helps
Instead of doing the VA and kVA math by hand, enter the voltage, amps, and phase into Sparky Toolbox.
The Transformer Sizing Calculator can calculate both single-phase and three-phase loads. It shows:
- Calculated load in amps
- Required kVA to meet the load demand
- Recommended transformer kVA size
That gives you a quick starting point when planning equipment, building a material list, or discussing transformer size with a customer.
Important Reminder
This is one simple example. Transformer selection can change based on phase, system voltage, continuous load, starting current, future load, manufacturer ratings, installation conditions, and other job details.
Always verify equipment information, project requirements, approved plans, manufacturer instructions, and local inspection requirements before installation.