Transformer Sizing Calculator
Calculate transformer kVA from load amperage, voltage, and phase configuration — apply the NEC 125% continuous load rule and compare against standard transformer sizes with a PASS/FAIL check.
Load Details
A
Live Results
Recommended Transformer Size
15.0kVA
PASSSelected transformer: 45.0 kVA
Computed kVA
9.60kVA
Adjusted Load Amps
40.00A
Load Amps
40A
Voltage
240V
Phase
Single-Phase
Continuous Load
No
Transformer sizing follows NEC continuous load guidelines. Always verify final design with local electrical code and manufacturer specifications.
How to Use This Transformer Sizing Calculator
- Enter the load amperage. Input the total connected load current in amperes — use calculated branch-circuit totals, motor FLA from nameplates, or feeder demand load values. For motor-driven equipment, use the Motor FLA Calculator to determine full-load amps before entering the value here.
- Select system voltage. Choose the line-to-line or line-to-neutral voltage the transformer will serve: 120 V, 208 V, 240 V, 277 V, or 480 V. Match the voltage to your distribution system — common configurations include 120/240 V single-phase, 208Y/120 V three-phase, and 480Y/277 V three-phase commercial systems.
- Choose single-phase or three-phase. Select the transformer configuration that matches your electrical system. Single-phase transformers serve residential and light commercial loads; three-phase transformers serve commercial and industrial equipment. Three-phase kVA includes the √3 multiplier in the formula.
- Identify continuous vs non-continuous load. Check Continuous Load when the equipment is expected to run for 3 hours or more at full rated current — examples include HVAC equipment, lighting panels, EV supply equipment, and process loads. The NEC 125% rule increases adjusted load amps before kVA is calculated.
- Review computed kVA, recommended size, and PASS/FAIL. Compare the computed kVA against the recommended standard transformer size rounded up from the available size list. Select your planned transformer size to run a PASS/FAIL check — PASS means the selected kVA meets or exceeds the recommended minimum. Verify clearances, ventilation, and local code before finalizing.
Formulas & Example
Transformer kVA is calculated from adjusted load amperage (with the NEC 125% continuous load multiplier when applicable), then rounded up to the next standard kVA rating.
Adjusted Load (A) = Load Amps × (Continuous ? 1.25 : 1.00)
Single-Phase kVA = (Adjusted Load × Voltage) ÷ 1000
Three-Phase kVA = (Adjusted Load × Voltage × √3) ÷ 1000
Recommended kVA = smallest standard size ≥ Computed kVA
Standard transformer sizes (kVA):
15, 25, 37.5, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000Worked Example
A 40 A non-continuous load on a 240 V single-phase system:
Load Amps = 40 A
Voltage = 240 V
Phase = Single-Phase
Continuous Load = No
Adjusted Load = 40 × 1.00 = 40 A
Computed kVA = (40 × 240) ÷ 1000 = 9.6 kVA
Standard sizes ≥ 9.6 kVA: 15, 25, 37.5 …
Recommended Transformer = 15 kVAIf the same 40 A load were continuous, adjusted load would be 50 A (40 × 1.25), yielding 12.0 kVA — still requiring a 15 kVA transformer. Pair this tool with the Motor FLA, Load Calculation, and Breaker Size calculators to build a complete electrical design workflow.
Frequently Asked Questions
How do I size a transformer?â–¾
Transformer sizing starts with the connected load current in amperes, system voltage, and whether the system is single-phase or three-phase. Apply the NEC 125% multiplier for continuous loads (equipment running 3 hours or more), then calculate kVA using the appropriate formula: single-phase kVA = (amps × volts) ÷ 1000, or three-phase kVA = (amps × volts × √3) ÷ 1000. Round the result up to the next standard transformer kVA rating from the manufacturer's available sizes. Always verify conductor ampacity, overcurrent protection, and voltage drop on the secondary side.
What is kVA?â–¾
kVA (kilovolt-amperes) is the apparent power rating of a transformer — it represents the total capacity the unit can deliver without exceeding its thermal limits. Unlike kW (kilowatts), kVA does not account for power factor; transformers are rated in kVA because they must handle both real power and reactive power. A 45 kVA transformer can deliver up to 45,000 volt-amperes at its rated voltage, which translates to different ampere values depending on whether the system is 120 V, 240 V, 208 V, or 480 V.
What is the difference between single-phase and three-phase transformers?â–¾
Single-phase transformers have two windings (primary and secondary) and serve 120 V or 240 V loads common in residential and light commercial applications. Three-phase transformers use three windings and serve 208 V, 240 V, 277 V, or 480 V systems found in commercial and industrial facilities. The kVA formula differs: single-phase uses amps × volts ÷ 1000, while three-phase multiplies by √3 (approximately 1.732) because power is distributed across three conductors. Three-phase transformers are more efficient for large motor and HVAC loads.
What affects transformer sizing?â–¾
Key factors include total connected load amperage, system voltage, phase configuration, whether loads are continuous (NEC 125% rule), future expansion capacity, power factor of connected equipment, and ambient temperature derating. Harmonic loads from variable-frequency drives and LED lighting may require K-rated transformers. Installation constraints — clearances, ventilation, and indoor vs outdoor placement — also influence the practical size selection even when the calculated kVA is lower.
Should I oversize a transformer?â–¾
Moderate oversizing (10–25% above calculated kVA) is common practice to accommodate future load growth and prevent nuisance tripping during motor inrush. However, significantly oversizing a transformer increases no-load losses (core losses), reduces efficiency at light loads, and may require larger primary overcurrent protection. NEC and utility requirements may also limit maximum oversizing. Size to the calculated demand plus a reasonable margin, then confirm with the transformer manufacturer and local AHJ.
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