Why a transformer is rated in kVA
The losses inside a transformer come from two sources: resistive losses in the windings, which depend on current, and core losses, which depend on voltage. Neither depends on the phase relationship between them. That means the heating is set by the product of volts and amps — apparent power — rather than by the real power delivered.
The practical consequence is that a 75 kVA transformer can deliver 75 kW to a resistive load, but only 60 kW to a load at 0.8 power factor, because 60 ÷ 0.8 = 75 kVA. Sizing from a load schedule that lists kW without a power factor is a common way to end up with an undersized transformer.
Standard kVA ratings
| Rating (kVA) | Typical use | 208 V three-phase FLA | 480 V three-phase FLA |
|---|---|---|---|
| 15 | Small commercial, one suite | 42 A | 18 A |
| 30 | Small office or retail | 83 A | 36 A |
| 45 | 125 A service at 208 V | 125 A | 54 A |
| 75 | Small industrial, light process | 208 A | 90 A |
| 112.5 | 200 A service at 208 V | 312 A | 135 A |
| 150 | Light industrial | 416 A | 180 A |
| 225 | Medium industrial | 625 A | 271 A |
| 300 | Larger industrial | 833 A | 361 A |
| 500 | Heavy industrial | 1,388 A | 602 A |
Secondary current is what sets your panel bus rating, so 45 kVA at 208 V producing exactly 125 A is why that pair appears so often in commercial fit-outs.
The standard rating series is two lists, not one
Ask for "the next standard size" and you will get a different answer depending on whether the transformer is single-phase or three-phase. The two series meet at 15 kVA and again at 75 kVA and differ everywhere else. These are the ratings the US distribution-transformer efficiency rules are written around, which is why they are also the sizes that are actually stocked.
| Single-phase (kVA) | Three-phase (kVA) |
|---|---|
| 15 | 15 |
| 25 | 30 |
| 37.5 | 45 |
| 50 | 75 |
| 75 | 112.5 |
| 100 | 150 |
| 167 | 225 |
| 250 | 300 |
| 333 | 500 |
| — | 750 |
| — | 1000 |
Sizing a three-phase load against the single-phase column is a quiet way to specify a unit that nobody builds. A 26 kVA three-phase need falls below 30 kVA, so 30 kVA is the answer; the single-phase column has no 30 kVA at all, and its 25 kVA sits below the load. The same mistake reads differently at the top of the range: an 80 kVA need is satisfied by the 112.5 kVA three-phase unit, while the single-phase column offers 100 kVA, a size that does not appear in the three-phase series.
Two worked examples
1. A light industrial load of 65 kW at 0.85 power factor
Apparent power is 65 ÷ 0.85 = 76.5 kVA. With a 25% growth margin the design figure is 95.6 kVA, so the standard size chosen is 112.5 kVA. Its 208 V secondary full-load current is 312 A, which sets the minimum main bus rating of the downstream panel. Primary protection at 125% of the 480 V primary current of 135 A gives about 169 A.
2. A 40 kW resistive heating load at 0.98
Apparent power is 40 ÷ 0.98 = 40.8 kVA. With a 20% margin that is 49 kVA, so a 50 kVA unit would be marginal and 75 kVA is the sensible standard choice. This example shows how much the power factor changes the outcome: at 0.7 the same 40 kW would need 57 kVA at unity growth, and 68 kVA with the same 20% margin.
Both specifications above come from the three-phase column. Entering the same two loads in the calculator returns 100 kVA and 50 kVA, which is the merged-list behaviour described in the section above — take the kVA figure it prints and round it up on the three-phase column before you write it into a specification.
Third and fourth examples: working backwards from amps
3. An existing 200 A three-phase service at 208 V
Here the load is already expressed as a current, so apparent power comes first: √3 × 208 × 200 ÷ 1000 = 72.1 kVA. Adding the 25% growth margin gives 90.1 kVA. On the three-phase column that falls between 75 kVA and 112.5 kVA, so the specification is 112.5 kVA. Its 208 V secondary full-load current is 312 A, which leaves the existing 200 A service well inside the transformer's rating, and its 480 V primary full-load current is 135.3 A, so protection at 125% works out to 169.1 A.
4. A 100 A single-phase 240 V subpanel
No √3 appears here: 240 × 100 ÷ 1000 = 24.0 kVA, and with the same 25% margin, 30.0 kVA. The single-phase column has no 30 kVA, so the next size up is 37.5 kVA. Full-load current at 240 V is 156.3 A on both the primary and the secondary, because the voltage is the same on each side, and 125% of that is 195.3 A. This example happens to agree with the calculator exactly — 37.5 kVA is one of the sizes the merged list does contain — which is a reminder that the check above is about which series you are on, not about distrusting every answer.
Growth margin is not the continuous-load factor
The percentage field on this page is yours to choose, and it represents load you expect to add later. It is not the 125% continuous-load factor that NEC 210.20(A) applies when a branch circuit or feeder is sized for a continuous load. The two numbers answer different questions and they do not cancel each other out. A feeder carrying a continuous 60 kVA load is already calculated at 75 kVA before any growth allowance is added, so a transformer bought at exactly 75 kVA starts with nothing in reserve for the equipment installed the following year.
Keeping the two figures separate is what makes the decision defensible afterwards. Record the connected load, the power factor it was measured or assumed at, and the growth percentage applied. If the transformer turns out to be too small in three years, that record is what distinguishes a load that grew from an estimate that was wrong at the start — and those two failures call for different fixes.
What the calculation does not cover
- Motor starting inrush. A squirrel cage motor draws five to eight times its running current at start. A transformer sized only on running load may see a voltage dip during starting that trips other equipment. Where large motors are present, check the starting kVA.
- Harmonics. A standard transformer feeding variable frequency drives or switch-mode loads can overheat from harmonic losses even though its kVA rating is not exceeded. K-rated transformers and derating factors exist for this case.
- Ambient temperature and altitude. Standard ratings assume 30°C ambient and altitudes below 3,300 feet. A transformer in an unconditioned mechanical room needs derating.
- Impedance. A low-impedance transformer gives better voltage regulation but higher fault current, which raises the interrupting rating needed on the downstream equipment.
How this calculator is verified
Standard kVA ratings on this page and in the tables above follow the sizes the US distribution-transformer efficiency rules are written around, quoted separately for single-phase and three-phase. Full-load currents are derived directly from the kVA rating and voltage using the standard three-phase formula, and every cell in the rating table was recomputed rather than copied. Protection sizing follows NEC 450.3(B) at 125% of primary full-load current.
- 10 CFR Part 431, Subpart K — Distribution Transformers — the federal rule whose tables define the single-phase and three-phase kVA rating series used above.
- US Department of Energy — Distribution Transformers — background on those standards and why these are the stocked sizes.
- NFPA 70, National Electrical Code — Article 450 (transformers), Article 430 (motors), Article 220 (load calculation). Article 450.3(B) sets the protection percentages.
- UL Solutions — UL 1561, the product standard for dry-type general purpose transformers.
Standard ratings, rating tables and worked examples last verified: 2 November 2026.
Instruments and hardware for transformer work
Sizing is arithmetic, but the design load has to be measured and the installation has to be terminated correctly. These are the items that job needs.
- Clamp meter — The connected load is the starting point. Measuring actual running current beats trusting a nameplate that may be conservative or wrong.
- Power quality analyzer — Power factor is what converts the load to kVA. An analyzer gives the true figure for the design load, including the harmonic content a standard transformer may not tolerate.
- Infrared thermometer — A transformer running hot points to a sizing or loading problem. A thermal check confirms the loading without opening the enclosure.
- Torque screwdriver — Transformer and lug terminations have specified torque values. A torque screwdriver is how you meet them.
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