Transformer Sizing Calculator
Size a transformer based on total load VA, primary and secondary voltages, phase, and future growth allowance.
Results
Visualization
How It Works
Transformers convert voltage from one level to another (step-up or step-down). Sizing is based on the total connected load in VA (volt-amperes) plus a margin for future growth. Transformers come in standard kVA ratings, so you always select the next size up from your calculated requirement.
The Formula
Required kVA = (Total VA x Growth Factor) / 1000
Single Phase Amps = kVA x 1000 / Voltage
Three Phase Amps = kVA x 1000 / (Voltage x 1.732)
Single Phase Amps = kVA x 1000 / Voltage
Three Phase Amps = kVA x 1000 / (Voltage x 1.732)
Variables
- VA — Volt-Amperes -- the apparent power consumed by the load
- kVA — Kilovolt-Amperes -- transformer rating (1 kVA = 1,000 VA)
- 1.732 — Square root of 3 -- used in all three-phase power calculations
- Growth Factor — Multiplier for future load additions (1.25 = 25% growth allowance)
Example
A 5,000 VA load with 25% growth on a 480V to 120V single-phase transformer: Required = 5,000 x 1.25 / 1,000 = 6.25 kVA. Next standard size is 7.5 kVA. Secondary amps = 7,500 / 120 = 62.5A.
Tips
- Always include a growth factor of at least 20-25% -- adding capacity later is much more expensive than sizing up initially.
- Load the transformer between 50-80% for optimal efficiency and longest life.
- Dry-type transformers are used indoors and are more common in commercial buildings. Oil-filled transformers are for outdoor utility use.
- The primary overcurrent protection is sized per NEC 450.3 -- typically 125% of primary FLA for transformers over 9 amps.
- Transformer hum is normal but excessive noise may indicate overloading or loose laminations.