Electrical reference chart
Transformer Sizing Chart
Use this transformer sizing chart after the calculator result to document load kVA, primary and secondary current, spare capacity, impedance, inrush, ventilation, and protection review before selecting equipment.
Quick reference table
Transformer sizing starts with kVA math, but the calculator worksheet should also keep load mix, primary and secondary voltage, phase, continuous duty, inrush, impedance, temperature rise, enclosure, ventilation, grounding, overcurrent protection, manufacturer data, adopted NEC rules, and AHJ review visible.
Transformer kVA planning examples
| Load current | 240 V single phase | 208 V three phase | 480 V three phase |
|---|---|---|---|
| 25 A | 6.0 kVA | 9.0 kVA | 20.8 kVA |
| 50 A | 12.0 kVA | 18.0 kVA | 41.6 kVA |
| 100 A | 24.0 kVA | 36.0 kVA | 83.1 kVA |
| 200 A | 48.0 kVA | 72.1 kVA | 166.3 kVA |
Transformer result follow-up worksheet
| After kVA result | Document next | Why it matters |
|---|---|---|
| Calculated kVA near a standard size | Spare capacity, continuous duty, and load diversity | A minimum math result may not be a practical equipment selection |
| Primary and secondary currents differ | Label each side voltage and conductor path | Protection and conductor sizing use side-specific current |
| Motor or nonlinear load present | Inrush, harmonics, K-rated needs, and voltage dip concerns | Load type can affect transformer choice beyond base kVA |
| Indoor or tight location | Temperature rise, enclosure, clearances, and ventilation | Heat and accessibility can control installation planning |
How to use this chart
Convert load to kVA
Start with load current, voltage, phase, and duty context before selecting a transformer kVA rating or standard equipment size.
Label both sides
Keep primary voltage, secondary voltage, primary current, and secondary current separate so breaker and conductor decisions use the correct side.
Add equipment constraints
After the calculator result, document spare capacity, inrush, impedance, temperature rise, enclosure, ventilation, grounding, and manufacturer instructions.
Formula basis
Single phase: kVA = V x A / 1000. Three phase: kVA = 1.732 x V x A / 1000.
- V is the transformer-side voltage used for the current calculation.
- A is load current on the same side of the transformer.
- kVA is apparent power before spare capacity, standard equipment size, or protection review is added.
Worked examples
100 A at 480 V three-phase
1.732 x 480 x 100 / 1000 = about 83.1 kVA before spare capacity, standard transformer rating, ventilation, impedance, or protection is selected.
208 V secondary with 45 kVA transformer
The secondary current is much higher than the primary current when voltage steps down, so conductor and breaker reviews must label which side of the transformer the calculator result represents.
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Related charts
kVA to Amps Chart
Use this kVA to amps chart: 45 kVA at 480V 3-phase = 54.1A; 15 kVA at 240V 1-phase = 62.5A.
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Screen conductor size from calculated load, copper or aluminum material, terminal rating, derating, voltage drop, and equipment notes.
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Frequently asked questions
These answers explain how to use the chart without turning a quick reference into a final design decision.