Transformer Sizing Chart: Complete kVA Selection Guide

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Comprehensive transformer sizing reference with kVA ratings, full load amps, dimensions, and selection guidelines
for electrical engineers and contractors

Quick Sizing Formula

Single-Phase:
kVA = (Volts × Amps) ÷ 1000		Three-Phase:
kVA = (Volts × Amps × 1.732) ÷ 1000

Selecting the right transformer size is critical for electrical system reliability and efficiency. This comprehensive
transformer sizing chart provides all the data you need to choose the correct kVA rating for your
application.

💡 Rule of Thumb: Size your transformer at 125% of your calculated load to allow for future
expansion and prevent overloading. Never load a transformer above 80% of its rated capacity for continuous
operation.

Table of Contents

Single-Phase Transformer Sizing Chart

Standard single-phase transformer ratings with typical dimensions, weights, and efficiency:

kVADimensions (H×W×D)WeightFLA @ 480VFLA @ 240VEfficiencySpecifications
5 kVA22″×15″×10″80 lbs10.4 A20.8 A97.8%View
Details →
7.5 kVA24″×16″×11″100 lbs15.6 A31.2 A97.9%View
Details →
10 kVA26″×16″×12″120 lbs20.8 A41.7 A98.0%View
Details →
15 kVA30″×17″×13″170 lbs31.2 A62.5 A98.2%View
Details →
25 kVA37″×20″×16″285 lbs52.1 A104.2 A98.5%View
Details →
37.5 kVA40″×22″×17″350 lbs78.1 A156.2 A98.6%View
Details →
50 kVA43″×24″×18″400 lbs104.2 A208.3 A98.7%View
Details →
75 kVA46″×26″×20″500 lbs156.2 A312.5 A98.8%View
Details →
100 kVA50″×28″×22″650 lbs208.3 A416.7 A98.9%View
Details →
167 kVA55″×30″×24″860 lbs347.9 A695.8 A99.0%View
Details →
250 kVA60″×34″×28″1200 lbs520.8 A1041.7 A99.1%View
Details →

Three-Phase Transformer Sizing Chart

Standard three-phase transformer ratings with typical dimensions, weights, and efficiency:

kVADimensions (H×W×D)WeightFLA @ 480VFLA @ 208VEfficiencySpecifications
15 kVA30″×18″×14″190 lbs18.0 A41.6 A98.0%View
Details →
30 kVA35″×20″×16″280 lbs36.1 A83.3 A98.3%View
Details →
45 kVA38″×22″×17″360 lbs54.1 A124.9 A98.5%View
Details →
75 kVA43″×24″×20″500 lbs90.2 A208.2 A98.7%View
Details →
112.5 kVA47″×26″×22″650 lbs135.3 A312.3 A98.8%View
Details →
150 kVA50″×28″×24″800 lbs180.4 A416.4 A98.9%View
Details →
225 kVA55″×30″×26″1050 lbs270.6 A624.6 A99.0%View
Details →
300 kVA58″×32″×28″1300 lbs360.8 A832.8 A99.0%View
Details →
500 kVA64″×36″×32″1900 lbs601.4 A1388.0 A99.1%View
Details →
750 kVA70″×40″×35″2500 lbs902.1 A2082.0 A99.2%View
Details →
1000 kVA76″×44″×38″3200 lbs1202.8 A2776.0 A99.2%View
Details →
1500 kVA84″×48″×42″4500 lbs1804.2 A4164.0 A99.3%View
Details →
2000 kVA90″×52″×46″6000 lbs2405.6 A5552.0 A99.3%View
Details →
2500 kVA96″×56″×50″7500 lbs3007.0 A6940.0 A99.4%View
Details →

Full Load Amps Reference Table

Quick reference for transformer full load amps at common voltages:

Single-Phase Full Load Amps

kVA120V208V240V277V480V600V
541.724.020.818.110.48.3
1083.348.141.736.120.816.7
15125.072.162.554.231.225.0
25208.3120.2104.290.352.141.7
37.5312.5180.3156.2135.478.162.5
50416.7240.4208.3180.5104.283.3
75625.0360.6312.5270.8156.2125.0
100833.3480.8416.7361.0208.3166.7

Three-Phase Full Load Amps

kVA208V240V480V600V
1541.636.118.014.4
3083.372.236.128.9
45124.9108.354.143.3
75208.2180.490.272.2
112.5312.3270.6135.3108.2
150416.4360.8180.4144.3
225624.6541.3270.6216.5
300832.8721.7360.8288.7
5001388.01202.8601.4481.1
7502082.01804.2902.1721.7
10002776.02405.61202.8962.3

How to Size a Transformer

Step 1: Calculate Total Load

Add up the power requirements of all equipment:

  • List all loads (motors, lighting, HVAC, etc.)
  • Convert horsepower to kVA: HP × 0.746 ÷ PF ÷ Efficiency ≈ kVA
  • Account for power factor (typical 0.8-0.9)

Step 2: Apply Safety Factor

Add 25% safety margin for:

  • Future expansion
  • Motor starting currents
  • Harmonics
Example Calculation:
Total connected load: 18 kVA
Safety factor: × 1.25
Required capacity: 22.5 kVA
Answer: Select 25 kVA transformer

Step 3: Select Standard Size

Choose the next standard size above your calculated requirement:

Single-Phase Standard SizesThree-Phase Standard Sizes
5, 7.5, 10, 15, 25, 37.5, 50, 75, 100, 167, 250, 333 kVA15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000, 2500 kVA

Standard kVA Ratings (IEEE/ANSI)

CategoryStandard Sizes (kVA)Typical Application
Small (Residential)5, 7.5, 10, 15Homes, small offices
Medium (Commercial)25, 37.5, 50, 75, 100Retail, restaurants, small factories
Large (Industrial)150, 225, 300, 500Manufacturing, warehouses
Extra Large (Utility)750, 1000, 1500, 2000, 2500Substations, heavy industry

Frequently Asked Questions

What size transformer do I need for 100 amps at 480V?

For a single-phase load: kVA = (480 × 100) ÷ 1000 = 48 kVA. Select a 50 kVA transformer.
For a three-phase load: kVA = (480 × 100 × 1.732) ÷ 1000 = 83.1 kVA. Select a 112.5 kVA
transformer.

What is the full load current of a 75 kVA transformer at 480V?

Single-phase: 75,000 ÷ 480 = 156.2 amps
Three-phase: 75,000 ÷ (480 × 1.732) = 90.2 amps

Can I overload a transformer temporarily?

Most transformers can handle 150% overload for short periods (minutes to hours), but this reduces transformer life.
Avoid sustained overloading above 100%.

Why are transformers rated in kVA instead of kW?

kVA (apparent power) accounts for both real power and reactive power. Since transformers must handle both, they’re
rated in kVA regardless of load power factor.

Related Resources

Disclaimer: Data shown are typical reference values. Actual specifications vary by manufacturer. Always verify
with manufacturer datasheets before ordering or installation. Full load amps calculated using standard
electrical formulas. Last updated: January 2026

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