Transformer Sizing Calculator

Size a single-phase or three-phase transformer from a simple load or a full Article 220 multi-item load inventory — lighting, receptacles, motors, HVAC, and other categories rolled up with their own NEC demand factors, then rounded to the next standard catalog kVA. Every result pairs primary and secondary full-load amps with NEC Table 450.3(B) overcurrent protection sizing for primary-only or primary-and-secondary arrangements, so you leave with both the transformer size and its breaker or fuse rating. A transformer-type reference panel compares dry-type, oil-filled, pad-mount, and vault-rated units. Optional site and load conditions let you apply an altitude derating for high-elevation installations and select a K-factor rating for non-linear loads like VFDs, LED drivers, and electronic ballasts, when those conditions apply. Results include a loading/headroom chart, worked examples, and a print view that captures the chart and results exactly as they appear on screen. Built for electricians, electrical engineers, and facility managers sizing distribution transformers for commercial and industrial installations.

Pick a calculation mode, set the transformer parameters, and click Calculate.

Phase

Protection

Type

Growth Margin

Quick Presets

Primary Supply Voltage (select or enter custom)

Secondary (Load) Voltage (select or enter custom)

Load Current (amps, at secondary voltage)

Connected Load (kW)

Power Factor

Optional — Site, Load & Design Conditions

Installation above 3,300 ft (1,000 m)?

Installation Altitude (feet above sea level)

Elevated ambient temperature (site average above 30°C / 86°F)?

Average Ambient Temperature (°C, 24-hour average)

Significant non-linear load (VFDs, LED drivers, electronic ballasts, computer/data loads)?

K-Factor

Advanced: estimate voltage regulation & fault current from transformer impedance?

Transformer Impedance (%Z, from nameplate)

Results

Recommended Size

—

kVA

Primary FLA

—

Secondary FLA

—

Primary OCPD

—

Secondary Protection

—

Nameplate Loading—

Formula: —

Standard kVA Sizes — Loading

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.

Neighboring Sizes Recommended Size 100% Capacity

🌀 Sizing for motor loads? Starting (inrush) current can be 6–8× running current for a few cycles and is a common reason transformers get oversized — check it with the Motor Starting Current Estimator before finalizing your kVA.

🔌 Need the downstream panel breaker sized too? Use the Circuit Breaker Sizing Calculator for the branch circuits fed by this transformer's secondary.

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 edition of the National Electrical Code (NFPA 70). Verify with a licensed electrician or electrical engineer before any electrical work or equipment purchase.

How to Use the Transformer Sizing Calculator

Choose the Calculation Mode

Pick Simple Load for a single voltage/current value, kW + Power Factor for a connected kW load, or Load Inventory to add multiple categorized loads with NEC-style demand factors.

Set the Transformer Parameters

Select phase, primary supply voltage, secondary (load) voltage, protection arrangement (primary-only or primary-and-secondary), transformer type, and a growth/safety margin.

Enter the Load and Optional Conditions

Enter the load for your chosen mode. If applicable, check any optional conditions — high altitude, elevated ambient temperature, non-linear (K-factor) load, or transformer impedance for an advanced estimate.

Calculate, Review, and Print

Click Calculate to see the recommended standard kVA size, primary/secondary FLA, NEC 450.3(B) overcurrent protection sizing, and a loading chart. Use Print to capture results and the chart exactly as they appear on screen.

Comprehensive Guide to Transformer Sizing

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.

Worked Examples

Three real-world scenarios, one per mode. Click "Load This Example" to run it in the calculator above.

1. Simple Load — Small Commercial, 90A at 208V, 480V Primary

A panel measures 90A at 208V three-phase secondary, fed from a 480V primary, with primary-only protection and a 20% growth margin.

kVA = 208 × 90 × 1.732 ÷ 1000 = 32.4 kVA → with 20% margin = 38.9 kVA → next standard size = 45 kVA. Primary FLA = 45,000 ÷ (480 × 1.732) = 54.1A → 125% = 67.7A → 70A primary OCPD.

2. kW + PF — Industrial, 180 kW at 0.85 PF, Primary + Secondary Protection

A plant load measures 180 kW at 0.85 power factor, 480V three-phase throughout (unit substation), with dedicated secondary protection and a 25% growth margin.

kVA = 180 ÷ 0.85 = 211.8 kVA → with 25% margin = 264.7 kVA → next standard size = 300 kVA. Secondary FLA = 300,000 ÷ (480 × 1.732) = 360.8A → 125% = 451A → 450A secondary OCPD. Primary FLA equals secondary FLA here (same voltage) → 250% = 902A → 1000A primary OCPD.

3. Load Inventory — Office Building, Mixed Categories

An office building has 18 kVA of general lighting/receptacles, a 22 kVA rooftop HVAC unit, and two motors (15 HP ≈12 kVA and 7.5 HP ≈6 kVA), 480V→208V three-phase, primary-only protection, 20% margin.

Lighting: 18 kVA connected → 100% of first 3 + 35% of next 15 = 3 + 5.25 = 8.25 kVA demand. Motors: 12 + 6 = 18 kVA connected → +25% of largest (12) = 21 kVA demand. HVAC: 22 kVA (100%). Total demand = 8.25 + 21 + 22 = 51.25 kVA → ×1.20 margin = 61.5 kVA → next standard size = 75 kVA.

Quick Reference Table

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.

Frequently Asked Questions

How do I size a transformer for my load?

Determine the load's apparent power in kVA (from voltage and current, from kW and power factor, or from a full multi-item load inventory with NEC-style demand factors), add a growth/safety margin of roughly 20-25% for future load additions, and round up to the next standard catalog kVA size — commonly 15, 25, 37.5, 45, 75, 112.5, 150, 225, 300, 500, 750, or 1000 kVA and larger.

What is the difference between primary-only and primary-and-secondary protection?

Under NEC Table 450.3(B), primary-only protection limits the primary overcurrent device to 125% of the transformer's primary full-load current, and the secondary conductors must be protected by other means. When a dedicated secondary overcurrent device is also provided, sized at 125% of secondary full-load current, the primary device can be sized more generously, up to 250% of primary full-load current, since the secondary device now directly protects the downstream conductors.

Why does my transformer need to be derated at altitude?

Transformers rely on the surrounding air to carry heat away from the core and windings. At higher elevations the air is less dense, so it removes less heat for the same airflow. IEEE C57.96 addresses this with an approximate 0.5% reduction in usable kVA capacity for every 100 meters of elevation above 1,000 meters (about 3,300 feet).

What is a K-factor transformer and when do I need one?

Non-linear loads such as variable-frequency drives, LED drivers, electronic ballasts, and computer/data equipment draw current in harmonic-rich pulses rather than a clean sine wave, which causes extra eddy-current heating in a transformer's windings beyond what its standard nameplate current accounts for. A K-rated transformer (K-1 through K-40) is designed and UL-listed to handle that additional heating. K-factor is a specification choice made from the type and concentration of non-linear load present, not a number calculated from a standard transformer's rating.

Why is a transformer rated in kVA and not kW?

kVA is apparent power — the combination of real power (kW, which does useful work) and reactive power (kVAR, drawn by inductive and capacitive loads but not converted to work). A transformer's windings and core must carry the full apparent current regardless of power factor, so its heating and capacity limits are set by kVA, not by the kW portion alone. A lower power factor load needs a larger kVA transformer to deliver the same real kW.

How are multiple motors combined when sizing a transformer or feeder?

NEC 430.24 sizes a feeder or service supplying multiple motors at the sum of all the motors' full-load currents plus 25% of the single largest motor's full-load current in the group, to account for that motor's starting current on top of the others already running.