transformer Sizing Calculator

Transformer KVA Sizing Calculator
Calculate required KVA, next standard transformer size, and full load amps

Transformer Sizing Results

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Recommended KVA Size
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Calculated KVA Required
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Full Load Amps (Secondary)
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Full Load Amps (Primary)
Note: This result is a planning estimate based on your inputs. It does not account for harmonic loads, transformer impedance, inrush current, fault current requirements, ambient temperature derating, altitude derating, or specific NEC overcurrent protection sizing per Article 450. Standard KVA sizes follow NEMA and ANSI standards. Always have final transformer specifications reviewed by a licensed electrical engineer before procurement. For commercial electrical design and installation, contact FSG's electrical team.
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Already know your transformer KVA rating and need to find the full load amps? Use the reverse calculator below, useful for sizing breakers, conductors, and overcurrent protection devices for an existing or specified transformer.

Transformer Full Load Amps Calculator
Given a KVA rating, calculate full load amps and breaker sizing

Overrides the KVA dropdown above

Full Load Amps Results

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Full Load Amps
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80% Continuous Rating
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Min. Breaker Size (NEC)
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KVA Used
Note: Full load amps are calculated from the KVA rating and voltage per standard transformer formulas. Breaker sizing shown follows NEC Article 450 (125% of full load amps for most transformer primary protection). Actual overcurrent protection sizing depends on transformer impedance, secondary conductor length, and specific NEC table requirements. Always verify with a licensed electrical engineer or refer to NEC Article 450 for final protection sizing. For conductor sizing, see our Watts to Amps Calculator.
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How to Use the Transformer KVA Calculator

The transformer KVA sizing calculator above determines the minimum transformer size required to serve a given electrical load. Enter your load in amps or kilowatts, select the secondary voltage and phase, choose a power factor, and apply a safety factor. The calculator returns the calculated KVA required, the next standard NEMA transformer size up, full load amps on both the primary and secondary, and a compliance check against the 80% NEC continuous load rule.

The full load amps calculator works in reverse, given a transformer KVA rating from a nameplate or specification sheet, it returns the full load amps at your voltage and phase. This is the calculation used to size secondary overcurrent protection, conductors, and distribution panels downstream of a transformer. For complete electrical load calculations including panel sizing and circuit loads, see our Electrical Load Calculator and Watts to Amps Calculator.

The Transformer KVA Formula

KVA (kilovolt-amperes) is the unit of apparent power -- the total power a transformer must handle including both real power (kW) and reactive power (kVAR). The relationship between KVA, voltage, current, and phase follows these standard formulas:

Single-Phase Transformer Formulas

KVA = (Volts x Amps) / 1,000 Amps = (KVA x 1,000) / Volts KVA = kW / Power Factor

Three-Phase Transformer Formulas

KVA = (Volts x Amps x 1.732) / 1,000 Amps = (KVA x 1,000) / (Volts x 1.732) KVA = kW / Power Factor

The 1.732 factor is the square root of 3, applied to three-phase calculations to account for the phase relationship between the three conductors. Volts in three-phase calculations is the line-to-line voltage.

Standard NEMA Transformer KVA Sizes

Commercial and industrial transformers are manufactured in standard KVA sizes per NEMA and ANSI standards. Always size up to the next standard size above the calculated requirement -- never install a transformer below its calculated KVA requirement. The calculator above automatically selects the next standard size for you.

Standard KVA SizeTypical ApplicationFull Load Amps (480V 3PH)Full Load Amps (208V 3PH)
15 KVASmall office tenant, branch panel feed18 A42 A
25 KVASmall commercial space, lighting panel30 A69 A
37.5 KVAMedium office suite, retail space45 A104 A
50 KVAMedium commercial tenant, HVAC feed60 A139 A
75 KVALarge office floor, light manufacturing90 A208 A
100 KVALarge commercial tenant, data room feed120 A278 A
112.5 KVAMost common commercial size135 A312 A
150 KVALarge floor distribution, industrial feed180 A416 A
225 KVALarge commercial building, campus feed271 A625 A
300 KVAIndustrial facility, large campus361 A833 A
500 KVALarge industrial, data center, campus601 A1,388 A
750 KVALarge manufacturing, hospital902 A2,082 A
1,000 KVALarge campus, utility-scale commercial1,203 A2,776 A

NEC Requirements for Transformer Sizing and Protection

The National Electrical Code (NEC / NFPA 70) Article 450 governs transformer installation, overcurrent protection, and sizing requirements for commercial and industrial applications. Key requirements that affect transformer selection include:

NEC RequirementRuleApplication
Continuous Load Rule125% of continuous loadLoads energized for 3 hours or more require transformer sized at 125% of calculated load
Primary Overcurrent ProtectionNEC 450.3Primary protection typically sized at 125% of primary full load amps for supervised installations
Secondary Overcurrent ProtectionNEC 450.3(B)Secondary conductors must be protected within the distances specified in NEC 240.21(C)
VentilationNEC 450.9Transformers must have adequate ventilation -- dry-type transformers have specific clearance requirements
Transformer LocationNEC 450.13Accessibility requirements for inspection and maintenance vary by transformer type and location
80% Loading RuleBest practiceIndustry best practice limits continuous transformer loading to 80% of nameplate KVA for longevity

Worked Example: Sizing a Transformer for a Commercial Tenant Space

A three-phase commercial tenant space at 208V with a measured load of 200 amps. Power factor is 0.85 (typical mixed commercial load). Safety factor of 115% per standard commercial practice.

Inputs

  • Phase: Three-Phase
  • Load: 200 amps at 208V secondary
  • Power Factor: 0.85
  • Safety Factor: 115%

Calculation

KVA Required = (Volts x Amps x 1.732) / 1,000 = (208 x 200 x 1.732) / 1,000 = 72.1 KVA (calculated) With 115% safety factor: = 72.1 x 1.15 = 82.9 KVA required Next standard NEMA size above 82.9 KVA = 112.5 KVA transformer Primary FLA at 480V (3PH): = (112.5 x 1,000) / (480 x 1.732) = 135 amps

Frequently Asked Questions

KVA (kilovolt-amperes) is apparent power -- the total power a transformer must handle. kW (kilowatts) is real power -- the power actually consumed by the load. The ratio between them is the power factor. For a load with a power factor of 0.85, a 100 kW load requires 100 / 0.85 = 117.6 KVA of transformer capacity. Transformers are rated in KVA because they must handle apparent power regardless of power factor. Always size transformers in KVA, not kW.
Industry best practice limits continuous transformer loading to 80% of the nameplate KVA rating. This preserves transformer life by limiting heat buildup, provides headroom for load growth, and aligns with the NEC 125% continuous load rule for overcurrent protection. A 112.5 KVA transformer continuously loaded above 90 KVA is operating above the 80% threshold and may experience accelerated insulation degradation. The safety factor dropdown in the KVA sizing calculator accounts for this -- selecting 125% ensures the transformer operates at or below 80% of nameplate under continuous load.
For single-phase transformers: FLA = (KVA x 1,000) / Volts. For three-phase transformers: FLA = (KVA x 1,000) / (Volts x 1.732). For example, a 112.5 KVA three-phase transformer at 480V has a primary FLA of (112,500) / (480 x 1.732) = 135 amps. At 208V secondary, the same transformer delivers (112,500) / (208 x 1.732) = 312 amps. The full load amps calculator above handles both single and three-phase calculations automatically.
At 208V three-phase, a 200-amp panel requires: KVA = (208 x 200 x 1.732) / 1,000 = 72.1 KVA calculated. With a 115% safety factor, that rounds to 82.9 KVA -- requiring a 112.5 KVA transformer (next standard NEMA size up). At 480V three-phase, a 200-amp panel requires: (480 x 200 x 1.732) / 1,000 = 166.3 KVA calculated, sizing up to a 225 KVA transformer with safety factor applied. Use the KVA sizing calculator above to run your specific voltage and load combination.
Power factor is the ratio of real power (kW) to apparent power (KVA). A load drawing 100 kW at 0.85 power factor requires 117.6 KVA of transformer capacity. A load drawing 100 kW at 1.0 power factor (purely resistive) requires exactly 100 KVA. Most commercial loads -- motors, variable frequency drives, fluorescent and HID lighting, and mixed building loads -- have power factors between 0.75 and 0.95. Using an inaccurate power factor will result in undersized transformer selection. If your facility has significant motor loads, low power factor can be corrected with capacitor banks, which reduces your KVA demand and can lower utility demand charges.
Dry-type transformers use air or solid insulation for cooling and are the standard choice for indoor commercial and industrial applications. They require no special containment, are lower maintenance, and are available in standard KVA sizes from 1 KVA to 2,500 KVA. Liquid-filled transformers use transformer oil or other dielectric fluid for cooling and insulation. They are more efficient at large KVA ratings, better suited for outdoor installations, and are the standard for utility-scale and substation applications above 2,500 KVA. For most commercial building and industrial facility applications, dry-type transformers are specified. FSG's electrical team can advise on transformer type selection for your specific application.

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Disclaimer: Results are planning estimates only and do not constitute engineered specifications. Transformer sizing must account for harmonic distortion, inrush current, ambient temperature, altitude, impedance, and NEC overcurrent protection requirements per Article 450. All transformer installations must be designed and reviewed by a licensed electrical engineer and installed by a licensed electrical contractor in accordance with the National Electrical Code (NFPA 70) and applicable local codes. Standard KVA sizes referenced per NEMA ST-20 and ANSI C57 standards.