01
Single‑Phase
Standard kVA sizes:
- 1
- 1.5
- 2
- 3
- 5
- 7.5
- 10
- 15
- 25
- 37.5
- 50
- 75
- 100
- 167
- 200
- 250
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Transformer Reference Guide
How to Read a Transformer Sizing Chart
A transformer sizing chart is a lookup table, not a calculation. Find the kVA in the left column, then read across to the column that matches your voltage. The number you land on is the full‑load current in amps on that side of the transformer.
kVA → Amps
Look Up, Don't Calculate
Find the kVA in the left column, then read across to the column that matches your voltage. The number you land on is the full‑load current in amps on that side of the transformer.
Three‑Phase
Rules of Thumb
At 480 V: roughly
1.2 amps per kVA.
At 208 V: roughly
2.8 amps per kVA.
Single‑Phase
Rules of Thumb
At 480 V: roughly
2.1 amps per kVA.
At 240 V: roughly
4.2 amps per kVA.
Use the Table, Not the Rule of Thumb
The chart works because current follows a fixed relationship with kVA and voltage. A 75 kVA, 480 V three‑phase transformer draws 90.2 amps at full load; the same transformer at 208 V draws 208 amps. Lower voltage means higher current for the same kVA, which is why the secondary side of a step‑down transformer always carries the heavier conductor.
These are approximations. Use the tables below for any sizing decision rather than the rule of thumb.
Key Takeaways
How to Read the Chart
01
Find kVA, Read Voltage Column
Locate the transformer kVA in the left column, then read across to the voltage column that matches your system.
02
Lower Voltage = Higher Current
A 75 kVA transformer draws 90.2 A at 480 V but 208 A at 208 V. Lower voltage means more amps — and heavier conductors.
03
Three‑Phase Approximations
~1.2 A/kVA at 480 V • ~2.8 A/kVA at 208 V. Useful for quick checks — but always verify with the table.
04
Single‑Phase Approximations
~2.1 A/kVA at 480 V • ~4.2 A/kVA at 240 V. Use the table for final sizing decisions.
Transformer Reference Data
Transformer Full Load Current Chart (kVA to Amps)
The tables below list full‑load current in amps by kVA and voltage, for single‑phase and three‑phase transformers.
Single‑Phase Full Load Current
| kVA | 120 V | 208 V | 240 V | 277 V | 480 V | 600 V |
|---|---|---|---|---|---|---|
| 1 | 8.3 A | 4.8 A | 4.2 A | 3.6 A | 2.1 A | 1.7 A |
| 3 | 25 A | 14.4 A | 12.5 A | 10.8 A | 6.3 A | 5.0 A |
| 5 | 41.7 A | 24.0 A | 20.8 A | 18.1 A | 10.4 A | 8.3 A |
| 7.5 | 62.5 A | 36.1 A | 31.3 A | 27.1 A | 15.6 A | 12.5 A |
| 10 | 83.3 A | 48.1 A | 41.7 A | 36.1 A | 20.8 A | 16.7 A |
| 15 | 125 A | 72.1 A | 62.5 A | 54.2 A | 31.3 A | 25.0 A |
| 25 | 208 A | 120 A | 104 A | 90.3 A | 52.1 A | 41.7 A |
| 37.5 | 313 A | 180 A | 156 A | 135 A | 78.1 A | 62.5 A |
| 50 | 417 A | 240 A | 208 A | 181 A | 104 A | 83.3 A |
| 75 | 625 A | 361 A | 313 A | 271 A | 156 A | 125 A |
| 100 | 833 A | 481 A | 417 A | 361 A | 208 A | 167 A |
Three‑Phase Full Load Current
| kVA | 208 V | 240 V | 480 V | 600 V |
|---|---|---|---|---|
| 15 | 41.6 A | 36.1 A | 18.0 A | 14.4 A |
| 30 | 83.3 A | 72.2 A | 36.1 A | 28.9 A |
| 45 | 125 A | 108 A | 54.1 A | 43.3 A |
| 75 | 208 A | 180 A | 90.2 A | 72.2 A |
| 112.5 | 312 A | 271 A | 135 A | 108 A |
| 150 | 416 A | 361 A | 180 A | 144 A |
| 225 | 625 A | 541 A | 271 A | 217 A |
| 300 | 833 A | 722 A | 361 A | 289 A |
| 500 | 1,388 A | 1,203 A | 601 A | 481 A |
| 750 | 2,082 A | 1,804 A | 902 A | 722 A |
The pattern is consistent. Current halves each time
the voltage doubles for the same kVA, so a 300 kVA
transformer draws 361 amps at 480 V but
722 amps at 240 V. That difference drives conductor
and breaker sizing on each side of the transformer.
Transformer Sizing Reference
Motor to Transformer Sizing Chart (HP to kVA)
Motors are the trickiest load to size a transformer for, because a motor's starting current is several times its running current. The chart below lists the minimum transformer kVA for a three‑phase motor, along with the motor's full‑load current at 230 V and 460 V for reference.
| Motor HP | FLA @ 230 V | FLA @ 460 V | Min. Transformer kVA |
|---|---|---|---|
| 1/2 | 2.0 A | 1.0 A | 0.9 |
| 3/4 | 2.8 A | 1.4 A | 1.2 |
| 1 | 3.6 A | 1.8 A | 1.5 |
| 1-1/2 | 5.2 A | 2.6 A | 2.1 |
| 2 | 6.8 A | 3.4 A | 2.7 |
| 3 | 9.6 A | 4.8 A | 3.8 |
| 5 | 15.2 A | 7.6 A | 6.3 |
| 7-1/2 | 22 A | 11 A | 9.2 |
| 10 | 28 A | 14 A | 11.2 |
| 15 | 42 A | 21 A | 16.6 |
| 20 | 54 A | 27 A | 21.6 |
| 25 | 68 A | 34 A | 26.6 |
| 30 | 80 A | 40 A | 32.4 |
| 40 | 104 A | 52 A | 43.2 |
| 50 | 130 A | 65 A | 52 |
| 60 | 154 A | 77 A | 64 |
| 75 | 192 A | 96 A | 80 |
| 100 | 248 A | 124 A | 103 |
| 125 | 312 A | 156 A | 130 |
| 150 | 360 A | 180 A | 150 |
| 200 | 480 A | 240 A | 200 |
Two adjustments apply when the load is a motor.
If the motor has a service factor above 1.0, increase the
full‑load amps proportionally; a 1.15 service factor
adds 15%. If the motor starts more than once per hour,
add about 20% more kVA to cover the extra
inrush. These factors are why the minimum kVA in the chart
runs higher than the motor's running kVA alone would suggest.
Transformer Selection
Standard Transformer Sizes (kVA)
Transformers are only manufactured in standard ratings, so a calculated load almost never matches a catalog size exactly. You always round up to the next standard kVA.
02
Three‑Phase
Standard kVA sizes:
- 3
- 6
- 9
- 15
- 30
- 45
- 75
- 112.5
- 150
- 225
- 300
- 500
- 750
- 1000
- 1500
- 2000
- and larger
A calculated 63 kVA load rounds up to a 75 kVA transformer. A calculated 42 kVA single‑phase load rounds up to 50 kVA. Selecting the next standard size, not the exact calculated value, is the rule.
Sizing Guide
Transformer Sizing Rules and Worked Examples
Sizing a transformer follows a short sequence. List every connected load, sum their amperes or kVA, convert to kVA with the formula, then round up to the next standard size.
A
Formulas
Single‑phase: kVA = (V × I) ÷ 1000
Three‑phase: kVA = (V × I × √3) ÷ 1000
From kW: kVA = kW ÷ power factor
B
Three More Rules
Match the voltage: The primary voltage must equal the source, and the secondary must equal the equipment's required voltage.
Match the frequency: A transformer does not change 50 Hz to 60 Hz — supply and load must agree.
Derate for the environment: Dry‑type transformers lose roughly 1% of rating for each degree Celsius above 40°C ambient and about 3% per 1000 meters above 1000 m elevation.
Worked Example
Three‑Phase 208 V, 175 A → 75 kVA
A three‑phase 208 V load drawing 175 amps works out to:
kVA = 1.732 × 208 × 175 ÷ 1000 ≈ 63 kVA
That rounds up to the 75 kVA standard size.
A single‑phase 240 V load drawing 175 amps works out to 240 × 175 ÷ 1000 ≈ 42 kVA, which rounds up to 50 kVA.
Once you have the kVA, the chart above gives you the full‑load current for conductor and breaker sizing. For the complete workflow, including demand factor and NEC 450.3 overcurrent protection, use our transformer sizing calculator.
Power System Coordination
Transformer Sized, Now Size the Generator
A transformer and a generator are two halves of the same power system, and they are usually sized together. The transformer steps voltage up or down; the generator supplies the source. If the facility loses utility power, the generator must carry the same load the transformer serves, and it must start any motors in that load.
01
Same Load, Two Components
The transformer steps voltage up or down; the generator supplies the source. If the facility loses utility power, the generator must carry the same load the transformer serves.
02
Motors Force Both Larger
A motor that forces a larger transformer also forces a larger generator, because the generator must deliver the motor's starting surge without an excessive voltage dip. Size the transformer first with this chart, then size the generator to match the same load.
Let's talk
About Shandong Huali
Shandong Huali Electromechanical Co., Ltd. manufactures diesel generator sets, dust-suppression equipment, and industrial automation systems, with over 25 years of experience and exports to more than 20 countries. We publish free electrical reference tools to help engineers and buyers specify power systems correctly.
Frequently Ask Questions
Q: What size transformer do I need?
A: List every connected load, sum the current or kVA, convert to kVA with the formula, then round up to the next standard size. A 63 kVA three-phase load becomes a 75 kVA transformer. If the load includes a motor, check the motor HP to kVA chart to confirm the transformer can start it.
Q: How many amps does a transformer draw?
A: It depends on the kVA and voltage. A 75 kVA, 480 V three-phase transformer draws 90.2 amps at full load, while the same transformer at 208 V draws 208 amps. Use this transformer sizing chart to look up the full-load current for any kVA and voltage.
Q: What are the standard transformer kVA sizes?
A: Three-phase transformers come in 3, 6, 9, 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, and 1000 kVA and larger. Single-phase transformers come in 1, 2, 3, 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100, 167, 200, and 250 kVA. Always round up to the next standard size.
Q: How do I convert kVA to amps?
A: For single-phase, divide kVA times 1000 by the voltage. For three-phase, divide kVA times 1000 by the voltage times the square root of three (1.732). The full-load current tables in this chart run the formula for you.
Q: What size transformer do I need for a 10 hp motor?
A: A 10 hp three-phase motor needs a minimum transformer of about 11.2 kVA, which rounds up to the next standard size of 15 kVA. Add 20% if the motor starts more than once per hour, and increase the figure proportionally if the service factor is above 1.0.
Q: What is the difference between single-phase and three-phase transformer sizing?
A: Three-phase transformers use kVA = (V × I × √3) ÷ 1000 and are the standard for industrial loads. Single-phase transformers use kVA = (V × I) ÷ 1000 and draw more current per kVA, which is why they carry heavier conductors for the same rating.