Pick a calculation mode, set the transformer parameters, and click Calculate.
Percent of nameplate capacity used by your (margin-adjusted) demand across neighboring standard kVA sizes. Your recommended size is highlighted in gold; the dashed line marks 100% capacity.
Results are engineering estimates based on published NEC Table 450.3(B) overcurrent protection percentages (transformers 1000V and below), NEC 430.24 multiple-motor feeder sizing, NEC Table 220.42-style general lighting/receptacle demand tiers, NEC 210.20(A)/240.6(A) continuous-load and standard OCPD sizing, IEEE C57.91-2011 ambient-temperature derating, and IEEE C57.96 altitude derating. Occupancy-specific NEC 220.42 demand tables for non-dwelling uses and NEC Table 220.56 commercial-kitchen-equipment demand factors are not modeled. K-factor is offered as a specification selector, not a calculated derate. Voltage regulation and fault-current estimates use a simplified %Z-based approximation, not a full X/R calculation. Actual sizing varies with installation conditions, occupancy classification, and the adopted NEC edition. Verify with a licensed electrician or electrical engineer before any electrical work or equipment purchase.
Standard catalog kVA sizes with illustrative full-load amps at a common 480V 3Ø primary / 208V 3Ø secondary pairing. Your actual FLA depends on the voltages you enter above.
| Standard kVA | Primary FLA (480V 3Ø) | Secondary FLA (208V 3Ø) |
|---|
Illustrative reference only — recalculate with your actual primary/secondary voltages using the tool above.
Three real-world scenarios, one per mode. Click "Load This Example" to run it in the calculator above.
Why kVA, not kW
Apparent power (kVA) combines real power (kW, which does useful work) and reactive power (kVAR, drawn by inductive/capacitive loads but not converted to work). A transformer's windings and core carry the full apparent current regardless of power factor, so its heating and capacity limits are set by kVA. A lower power factor load needs a larger kVA transformer to deliver the same real kW — this is why the kW+PF mode divides by power factor rather than multiplying.
Standard sizes and rounding up
Manufacturers stock discrete kVA steps — 15, 25, 37.5, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000 kVA and larger. After computing the demand load and adding a growth margin, this calculator rounds up to the next standard size rather than a smaller one, so the transformer isn't operated above its nameplate rating.
NEC Table 450.3(B) — primary-only vs. primary-and-secondary protection
For transformers 1000V and below, primary-only protection limits the primary overcurrent device to 125% of primary full-load current — the secondary conductors must then be protected by other means (e.g., a panelboard main breaker). When a dedicated secondary device is also provided, sized at 125% of secondary FLA, the primary device can be sized up to 250% of primary FLA, since the secondary device now directly protects the downstream conductors. Both arrangements permit rounding up to the next standard OCPD size per NEC 240.6(A) where the calculated value falls between two standard sizes.
NEC 430.24 — sizing for multiple motors
A feeder or transformer supplying more than one motor is sized at the sum of every motor's full-load current, plus 25% of the single largest motor's full-load current — the extra 25% covers that motor's starting (inrush) current on top of the others already running. This calculator applies the same logic to the Motor category in Load Inventory mode.
Transformer types
Dry-Type — air-cooled, indoor-rated, no fluid to contain, typical impedance 2–6%, classed by winding temperature rise (80/115/150°C). Oil-Filled — liquid-immersed, higher continuous-duty capacity per frame size, typical impedance 4–7%, needs outdoor siting or a fire-rated vault per NEC 450. Pad-Mount — tamper-resistant outdoor enclosure, common for utility-side distribution feeding a building service. Vault-Type — indoor, higher-capacity installation in a fire-rated transformer vault per NEC 450.41–450.48.
Ambient temperature derating
Per IEEE C57.91-2011 Table 3, a transformer's usable kVA capacity drops by roughly 1.5% for every °C the average ambient runs above the standard 30°C (24-hour average) baseline — for example, a 100 kVA unit in a 40°C average ambient carries roughly 85% of its nameplate rating. This calculator applies the same factor in reverse: increasing the required nameplate kVA so the load is still comfortably carried.
Altitude derating
Above 1,000 m (about 3,300 ft), thinner air reduces convective cooling. IEEE C57.96 gives an approximate 0.5% capacity reduction for every 100m above that threshold, applied the same way as the ambient derating above.
K-factor and non-linear loads
Non-linear loads (VFDs, LED drivers, electronic ballasts, switch-mode power supplies, computer/data equipment) draw harmonic-rich current that causes extra eddy-current heating in a transformer's windings beyond what standard nameplate current accounts for. A K-rated transformer (K-1 through K-40, per UL 1561) is designed for that added heating. Because an honest derated-kVA number requires the load's actual harmonic spectrum — data rarely available at the sizing stage — this calculator treats K-factor as a specification choice: flag the non-linear load, pick the K-rating that matches its severity, and specify a K-rated transformer of at least the calculated kVA, rather than fabricating a derated number.
Voltage regulation and fault current (advanced/optional)
Transformer impedance (%Z) resists both normal load current and fault current. A simplified first-pass voltage-regulation estimate is %Z × power factor; a simplified fault-current estimate is secondary full-load amps ÷ (%Z ÷ 100). Both ignore the transformer's actual resistance/reactance (R/X) split, so treat them as ballpark figures — for coordination studies, arc-flash calculations, or precise voltage regulation, use the manufacturer's factory test report.