Transformer kVA Chart

Twenty-eight standard transformer ratings from 0.25 to 5,000 kVA, with the full-load current each produces at four common voltages.

Data verified 2026-09-29 · based on IEEE C57.12.00-2021

Standard Transformer Ratings and Full-Load Current

Standard ratings follow a preferred-number series, which is why 37.5, 75 and 167 kVA appear alongside 50, 100 and 150 — the intermediate sizes exist so a transformer can be matched to a load without excessive oversizing. Above 500 kVA the steps widen sharply and the rating is often specified rather than selected.

Current falls as voltage rises for the same kVA: a 75 kVA transformer draws 208 A at 208 V three-phase and 90 A at 480 V three-phase. The transformer is the same device; only the winding ratio differs, and that is why the same kVA can serve very different loads depending on the distribution voltage.

These are full-load currents for sizing the transformer's own circuit. The primary protection, secondary conductors and secondary overcurrent device are each sized separately, and the primary device is typically not the same rating as the secondary.

Sizing a Transformer

Transformers are sized on the connected load, not on the sum of the nameplate ratings of everything that might run. Three corrections matter.

Diversity — not all loads run at once. A building with 200 kVA of connected load might peak at 120 kVA, and a transformer sized at 200 kVA would be running at 60% of capacity, which is inefficient but also gives room for growth.

Motor starting — a large motor draws several times its full-load current during starting, causing a voltage dip. Transformer impedance, not just rating, determines how deep that dip is, so a marginally sized transformer can cause starting problems even though its continuous rating is adequate.

Harmonics — non-linear loads such as variable-frequency drives and switch-mode supplies cause additional heating that the kVA rating does not capture. Where a significant share of the load is non-linear, the transformer is derated or a K-rated unit is specified.

A transformer loaded to about 50 to 75% of rating usually runs most efficiently and has the least temperature rise for its life. Loading to 100% continuously is permitted but shortens insulation life and leaves no margin for growth.

Frequently Asked Questions

How do I calculate transformer full-load current?

For single-phase: I = kVA × 1000 ÷ V. For three-phase: I = kVA × 1000 ÷ (√3 × V). A 75 kVA transformer at 208 V three-phase gives 75,000 ÷ (1.732 × 208) = 208 A.

What are the standard transformer kVA sizes?

0.25, 0.5, 0.75, 1, 1.5, 2, 3, 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100, 150, 167, 225, 300, 500, 750, 1000, 1500, 2000, 2500, 3750 and 5000 kVA. The intermediate sizes at 37.5, 75 and 167 let a transformer be matched to a load without gross oversizing.

Why are there 37.5 and 167 kVA transformers?

Because the rating series follows preferred numbers rather than doubling steps. The intermediate sizes fill the gap between 25 and 50, and between 150 and 225, so a load that does not justify the next size up can still be served — inefficient to oversize a three-phase transformer by 50%.

How much can a transformer be overloaded?

Distribution transformers are designed for a small short-term overload, but continuous operation above nameplate rating shortens insulation life — the relationship is roughly a halving of life for every 8 to 10 °C of sustained additional temperature rise. Continuous loading above rating is not good practice.

What is the difference between kVA and kW for a transformer?

kVA is apparent power and kW is real power; the difference is the load's power factor. Transformers are rated in kVA because their heating depends on current, which is apparent power — not on how much of that power does useful work.

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Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

Data Sources

Cross-checked against:

• The rating series checked against standard transformer catalogues and the IEEE preferred ratings
• Every current computed from the kVA and voltage at build time, in both single and three-phase form
• Four values held as known-value checks spanning small and large ratings

Derived values — the following values on this page are calculated, not taken directly from the standard:

Ratings are standard sizes and currents are full-load figures for the transformer's own circuit. Sizing a transformer requires the connected load with diversity, and primary and secondary protection are sized separately under NEC Article 450.

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