What the result means
Start from the load in kVA (or amps and voltage), add a spare capacity allowance for future circuits, and the calculator picks the smallest standard transformer at or above that number.
Full-load amps on the primary and secondary are based on the standard size you'll install, which is what you need when laying out feeders and protection.
How it's calculated
- Load kVA = V × I ÷ 1,000 (× 1.732 for three-phase)
- Required kVA = load kVA × (1 + spare% ÷ 100)
- FLA = standard kVA × 1,000 ÷ (V or 1.732 × V)
Single-phase sizes: 1, 1.5, 2, 3, 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100, 167 kVA.
Three-phase sizes: 3, 6, 9, 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000 kVA.
Worked example
A 208 V three-phase load of 100 A, 480 V primary, 25% spare.
- Load = 1.732 × 208 × 100 ÷ 1,000 = 36.03 kVA
- Required = 36.03 × 1.25 = 45.03 kVA
- 45 kVA is just short, so the next standard size is 75 kVA
- Secondary FLA = 75,000 ÷ (1.732 × 208) = 208.2 A
Specify a 75 kVA transformer.
Reference table
The values below are computed from the same calculator functions used above. They let you compare common sizes and operating points without replacing the interactive calculation or its assumptions.
| kVA | 120 V | 240 V | 480 V |
|---|---|---|---|
| 1 | 8.33 | 4.17 | 2.08 |
| 1.5 | 12.50 | 6.25 | 3.13 |
| 2 | 16.67 | 8.33 | 4.17 |
| 3 | 25.00 | 12.50 | 6.25 |
| 5 | 41.67 | 20.83 | 10.42 |
| 7.5 | 62.50 | 31.25 | 15.63 |
| 10 | 83.33 | 41.67 | 20.83 |
| 15 | 125.00 | 62.50 | 31.25 |
| 25 | 208.33 | 104.17 | 52.08 |
| 37.5 | 312.50 | 156.25 | 78.13 |
| 50 | 416.67 | 208.33 | 104.17 |
| 75 | 625.00 | 312.50 | 156.25 |
| 100 | 833.33 | 416.67 | 208.33 |
| 167 | 1391.67 | 695.83 | 347.92 |
Reference values are examples based on the stated assumptions. Verify manufacturer data, conductor ampacity and local requirements before using a result.
Sources and assumptions
Single-phase sizes: 1, 1.5, 2, 3, 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100, 167 kVA. Three-phase sizes: 3, 6, 9, 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000 kVA. Calculator formulas are published with their units so a technician can check a result instead of treating it as a black box. Where a code table, manufacturer specification or field measurement is required, use the applicable local source.
Last reviewed . Spot an error? Email support@tiermenu.com.
FAQ
Why add spare capacity?
Loads grow, and transformers run cooler and last longer below full load. 25% is a common default; adjust to the project.
Why did 45.03 kVA jump to 75 kVA?
45 kVA is the next size down and is just below the requirement, so the calculator moves to the next standard size.
Does this size the primary breaker?
No. Overcurrent protection depends on your code's transformer protection rules and is not calculated.
Are these the only sizes available?
They are common standard ratings. Manufacturers offer others; check availability with your supplier.
Using a reference result in the field
A calculator is most useful when the inputs match the job being checked. Where a field asks for a length, confirm whether it is one-way or total; where it asks for current, confirm whether the load is continuous or intermittent. The unit label beside the result is part of the result; do not carry a number into another calculation without checking its units.
Reference rows are provided for comparison, not as universal design limits. They use the values and conditions stated with each table. A different conductor material, temperature, power factor or load type can change a field result. Where the page calls out a code check, equipment rating or manufacturer table, that check remains part of the job.
Use the interactive inputs for the exact conditions at hand, then review the displayed formula and assumptions. If a value looks unexpected, change one input at a time and compare it with a known point in the reference table. Keep the measured readings and job-specific notes with the work record so another technician can understand how the result was obtained.
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