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3 Phase Transformer Full-Load Current
Enter the transformer kVA and secondary voltage. The calculator returns the full-load current per line conductor, the primary current, and the recommended NEC 450 breaker size.
Enter Transformer Parameters
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Full-Load Current
Calculation Results
Secondary Full-Load Current
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Per line conductor
Primary Current
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Transformer kVA
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Recommended Breaker (125%)
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Min. Conductor (75°C)
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Formula used: Amps = (kVA × 1000) ÷ (1.732 × V). Results are per line conductor in a balanced three-phase system.
Based on NEC 450.3(B) for transformer protection. Breaker size = 125% of full-load current, rounded up to standard size. Always verify with local code and a qualified engineer.
Reference Data
3 Phase Transformer Amperage Chart
Full-load current for every standard kVA size from 3 to 2000 kVA at common voltages. Includes formula, primary vs secondary current, single-phase comparison, and medium-voltage values.
How to Calculate Transformer Full-Load Amps
The full-load current is derived from the formula: Amps (3-phase) = (kVA × 1000) ÷ (1.732 × V). For single-phase, use Amps = (kVA × 1000) ÷ V. The 1.732 factor (√3) is the reason three-phase current differs from single-phase at the same kVA and voltage.
| kVA | 208 V | 240 V | 400 V | 415 V | 480 V | 600 V |
|---|---|---|---|---|---|---|
| 3 | 8.3 | 7.2 | 4.3 | 4.2 | 3.6 | 2.9 |
| 6 | 16.7 | 14.4 | 8.7 | 8.3 | 7.2 | 5.8 |
| 9 | 25.0 | 21.7 | 13.0 | 12.5 | 10.8 | 8.7 |
| 15 | 41.6 | 36.1 | 21.7 | 20.9 | 18.0 | 14.4 |
| 25 | 69.4 | 60.1 | 36.1 | 34.8 | 30.1 | 24.1 |
| 30 | 83.3 | 72.2 | 43.3 | 41.7 | 36.1 | 28.9 |
| 45 | 125.0 | 108.3 | 64.9 | 62.6 | 54.1 | 43.3 |
| 50 | 138.8 | 120.3 | 72.2 | 69.6 | 60.1 | 48.1 |
| 75 | 208.2 | 180.4 | 108.3 | 104.3 | 90.2 | 72.2 |
| 100 | 277.6 | 240.6 | 144.3 | 139.1 | 120.3 | 96.2 |
| 112.5 | 312.3 | 270.6 | 162.4 | 156.5 | 135.3 | 108.3 |
| 150 | 416.0 | 360.8 | 216.5 | 208.7 | 180.4 | 144.3 |
| 225 | 624.3 | 541.3 | 324.8 | 313.0 | 270.6 | 216.5 |
| 300 | 832.5 | 721.7 | 433.0 | 417.4 | 360.8 | 288.7 |
| 500 | 1,388 | 1,203 | 722 | 695.6 | 601 | 481 |
| 750 | 2,082 | 1,804 | 1,083 | 1,043 | 902 | 722 |
| 1000 | 2,776 | 2,406 | 1,443 | 1,391 | 1,203 | 962 |
| 1500 | 4,164 | 3,608 | 2,165 | 2,087 | 1,804 | 1,443 |
| 2000 | 5,552 | 4,811 | 2,887 | 2,782 | 2,406 | 1,925 |
Standard kVA sizes: 3, 6, 9, 15, 25, 30, 45, 50, 75, 100, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000. Amps are per line conductor in a balanced three-phase system. Values are based on the formula I = kVA × 1000 ÷ (1.732 × V).
Primary vs Secondary Current
The same kVA flows through both primary and secondary, but current differs because voltage differs. Higher voltage → lower current; lower voltage → higher current. Always size each side separately at its own voltage.
| Transformer kVA | Primary Voltage | Primary Current | Secondary Voltage | Secondary Current |
|---|---|---|---|---|
| 45 kVA | 480 V | 54.1 A | 208 V | 125.0 A |
| 75 kVA | 480 V | 90.2 A | 208 V | 208.2 A |
| 100 kVA | 480 V | 120.3 A | 208 V | 277.6 A |
| 112.5 kVA | 480 V | 135.3 A | 208 V | 312.3 A |
| 150 kVA | 480 V | 180.4 A | 208 V | 416.0 A |
| 300 kVA | 4160 V | 41.6 A | 480 V | 360.8 A |
Key rule: Secondary current is always higher than primary current for a step-down transformer. The secondary feeder and breaker must be sized for the secondary current, not the primary current.
Single-Phase vs Three-Phase Transformer Amps
At the same kVA and voltage, three-phase current is lower by a factor of √3 (1.732). Mixing up the formulas leads to gross over- or under-sizing.
| Configuration | Formula | Full-Load Amps (75 kVA, 240 V) |
|---|---|---|
| Single-Phase | kVA × 1000 ÷ V | 312.5 A |
| Three-Phase | kVA × 1000 ÷ (1.732 × V) | 180.4 A |
Why the difference? Three-phase power is distributed across three windings, delivering more power at the same current. Always confirm the phase count before using the formula.
Transformer kVA to Amps at Other Voltages
Medium-voltage primaries and other voltages are common in large plants and utility distribution. The formula works for any voltage; the table below gives examples at 2400 V and 4160 V.
| kVA | 2400 V (A) | 4160 V (A) |
|---|---|---|
| 300 | 72.2 | 41.6 |
| 500 | 120.3 | 69.4 |
| 1000 | 240.6 | 138.8 |
| 1500 | 360.8 | 208.2 |
| 2000 | 481.1 | 277.6 |
Pattern: Current falls as voltage rises. A 1000 kVA transformer draws 1,203 A at 480 V but only 138.8 A at 4160 V. For any other voltage, apply the formula I = kVA × 1000 ÷ (1.732 × V).
Application Guide
Transformer Sizing, Protection & Generator Pairing
Practical guidance for selecting the right transformer, sizing breakers per NEC 450, and pairing transformers with diesel generators for complete power systems.
How to Size a 3-Phase Transformer Method
Choosing the transformer starts with the load. Convert it to kVA, apply a safety factor, then round up to the next standard size. The full-load current table then tells you what the installed transformer draws.
- Step 1 – Find the load in kVA: kVA = 1.732 × V × A ÷ 1000. If the load is in kW, divide by the power factor: kVA = kW ÷ PF. A 100 kW load at 0.8 PF needs 125 kVA of transformer capacity.
- Step 2 – Apply a safety factor: For well-known loads use 1.0. For uncertain loads or future growth use 1.25. Multiply the calculated kVA by this factor.
- Step 3 – Round up to the next standard size: A 71.9 kVA calculation becomes 75 kVA. A 120 kVA calculation becomes 150 kVA. Standard sizes: 3, 6, 9, 15, 25, 30, 45, 50, 75, 100, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000 kVA.
| Load (kVA) | Safety Factor | Required kVA | Standard Transformer |
|---|---|---|---|
| 71.9 | 1.0 | 71.9 | 75 kVA |
| 71.9 | 1.25 | 89.9 | 100 kVA |
| 120.0 | 1.0 | 120.0 | 150 kVA |
| 120.0 | 1.25 | 150.0 | 150 kVA |
| 250.0 | 1.0 | 250.0 | 300 kVA |
| 250.0 | 1.15 | 287.5 | 300 kVA |
Motor loads get special treatment: Motors draw 3–6 times their running current at startup. If a motor is more than 10% of the total load, double its kVA contribution in the sizing calculation, or oversize the transformer by 125–150%. Continuous loads should run at 80–90% of transformer rated capacity for long service life.
Transformer Fusing & Breaker Sizing NEC 450
Per NEC Article 450.3(B), transformers rated 1000 V or less are protected as follows: primary-only protection at 125% of primary FLA; secondary-only protection at 125% of secondary FLA; or primary at 250% and secondary at 125% when both are used.
| Transformer | Primary FLA (480 V) | 125% | Recommended Breaker |
|---|---|---|---|
| 45 kVA | 54.1 A | 67.7 A | 70 A |
| 75 kVA | 90.2 A | 112.8 A | 125 A |
| 112.5 kVA | 135.3 A | 169.1 A | 175 A |
| 150 kVA | 180.4 A | 225.5 A | 250 A |
| 300 kVA | 360.8 A | 451.0 A | 500 A |
Standard overcurrent device sizes: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500 A. Verify against the specific edition of your code and confirm with a qualified engineer.
Generator-to-Transformer Sizing Pairing
Pair a transformer with a kVA rating at least equal to the generator's kVA at the site power factor. The transformer carries the full generator output, so its full-load current is read directly off the generator's output current.
| Generator kVA | Output (415 V) | Transformer kVA | Secondary Cable (mm²) |
|---|---|---|---|
| 8 kVA | 11.1 A | 9 kVA | 4 |
| 30 kVA | 41.7 A | 30 kVA | 10 |
| 100 kVA | 139.1 A | 100 kVA | 35 |
| 250 kVA | 347.8 A | 250 kVA | 120 |
| 500 kVA | 695.6 A | 500 kVA | 2×240 |
| 1000 kVA | 1,391 A | 1,000 kVA | 4×240 |
| 2000 kVA | 2,782 A | 2,000 kVA | 8×240 |
| 4000 kVA | 5,565 A | 4,000 kVA | 16×240 |
Cable sizes are planning values for XLPE copper conductors at roughly 125% of full-load current. The wire size chart carries the full ampacity and voltage-drop method. Generator and transformer are both rated in kVA, so the pairing is direct.
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The Right Transformer Size Starts Here
The full-load current of a 3-phase transformer decides everything downstream: the breaker, the cable, the switchgear, and the generator. This chart gives you that number for every standard kVA at every common voltage.
Frequently Ask Questions
Q: How many amps does a 45 kVA 3-phase transformer draw?
A: At 480 V a 45 kVA three-phase transformer draws 54.1 A. At 208 V it draws 125.0 A. Current depends on voltage, so the same transformer has a different amperage on each side. Read the current at the voltage you actually use. At 480 V the math is 45,000 ÷ (1.732 × 480) = 54.1 A. At 208 V it is 45,000 ÷ (1.732 × 208) = 125.0 A.
Q: How many amps is a 75 kVA 3-phase transformer?
A: At 208 V, 75 kVA draws 208.2 A. At 240 V it draws 180.4 A. At 480 V it draws 90.2 A. The 208 V secondary carries the most current, which is why the secondary feeder on a 75 kVA step-down transformer is heavier than the primary feeder.
Q: How many amps does a 112.5 kVA transformer draw at 480V?
A: A 112.5 kVA three-phase transformer at 480 V draws 135.3 A: 112,500 ÷ (1.732 × 480). At a 208 V secondary it draws 312.3 A. Use the primary current for the primary breaker and feeder, and the secondary current for the secondary equipment.
Q: What is the difference between primary and secondary current?
A: The primary side is the high-voltage side and carries less current. The secondary side is the low-voltage side and carries more current, for the same kVA. A 45 kVA transformer at 480 V primary / 208 V secondary carries 54.1 A on the primary and 125.0 A on the secondary. Always size each side at its own voltage.
Q: Why is a transformer rated in kVA instead of kW?
A: Because a transformer carries current regardless of the load’s power factor. kW = kVA × PF, so a 100 kVA transformer at 0.8 power factor delivers only 80 kW of real power, but still carries the full current. The winding heating depends on current, not on real power, so the rating is in kVA. Generators are rated the same way for the same reason.
Q: How do I size a transformer for my load?
A: Convert the load to kVA: for three-phase, kVA = 1.732 × V × A ÷ 1000, or divide the kW by the power factor. Apply a safety factor of 1.0 for a known load or 1.25 for an unknown one, then round up to the next standard size. A 71.9 kVA load becomes a 75 kVA transformer; with a 1.25 factor it becomes 100 kVA.
Q: What size breaker protects a 45 kVA transformer?
A: At a 480 V primary, the 45 kVA full-load current is 54.1 A, and 125 percent of that is 67.7 A, so the recommended primary breaker is 70 A per NEC 450.3(B). The secondary breaker is sized from the secondary current at the secondary voltage.
Q: Can a 3-phase transformer run on single phase?
A: Only if it is specifically designed for it, and not at full rating. A three-phase transformer fed from a single-phase supply delivers only a fraction of its kVA, typically around a third to a half. The unused windings must also be handled correctly. For single-phase loads, buy a single-phase transformer sized with the single-phase formula.