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.

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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

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

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

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-phase1.732 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 transformerThe 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.

Assumptions. Balanced load and line-to-line voltage assumptions behind this chart.

The table shows apparent power math and does not select a listed transformer size.
Spare capacity, load diversity, nonlinear loads, inrush, impedance, temperature rise, ventilation, and enclosure choice require project review.
Primary and secondary protection, grounding, and conductor sizing must be reviewed with the equipment selected for the job.

Code and standard notes. Planning limits that should be checked before final equipment selection.

Verify transformer selection, protection, grounding, ventilation, and equipment installation with the adopted NEC edition, manufacturer data, equipment listing, local amendments, and the Authority Having Jurisdiction (AHJ).
Coordinate transformer kVA with available fault current, impedance, primary and secondary overcurrent protection, conductor ampacity, and ventilation before procurement.

How to use this chart

1Convert load to kVAStart with load current, voltage, phase, and duty context before selecting a transformer kVA rating or standard equipment size.

2Label both sidesKeep primary voltage, secondary voltage, primary current, and secondary current separate so breaker and conductor decisions use the correct side.

3Add equipment constraintsAfter the calculator result, document spare capacity, inrush, impedance, temperature rise, enclosure, ventilation, grounding, and manufacturer instructions.

Worksheet checklist. Record source basis, review gaps, and assumptions before using the chart result.

Record primary and secondary voltagesWrite the serving voltage and load-side voltage so each current calculation uses the correct transformer side.
Document load mixIdentify motor load, lighting load, continuous load, nonlinear load, expected future capacity, and any load diversity assumption.
Verify equipment dataUse manufacturer ratings, temperature rise, impedance, ventilation, enclosure, grounding, and protection requirements before selecting equipment.

Common mistakes to avoid. Review these before turning chart current into an equipment decision.

Selecting transformer kVA from connected load alone without checking load diversity, future capacity, inrush, ventilation, impedance, and equipment rating.
Mixing primary and secondary current calculations without labeling which voltage side is being used for conductors and protection.
Treating a standard transformer size as complete without reviewing overcurrent protection, available fault current, temperature rise, and manufacturer installation instructions.

Frequently asked questions

These answers explain how to use the chart without turning a quick reference into a final design decision.

Is calculated kVA the transformer size I should buy?

No. Calculated kVA is the load basis. Actual transformer selection also considers standard ratings, spare capacity, inrush, impedance, temperature rise, ventilation, and protection.

Should I use primary or secondary voltage?

Use the voltage on the side where the current is known, and label it clearly. Primary and secondary currents are different when the voltages differ.

Why does impedance matter after sizing kVA?

Impedance affects available fault current and voltage behavior. It belongs in the protection and coordination notes even though it is not part of the basic kVA formula.