Transformer · kVA · Standard size
Transformer Sizing Calculator
Enter the load in kW, kVA or amps. See the required kVA, the next standard transformer size and the full-load current, then carry the size on to protection and conductors.
By Allen Tan · Updated · How results are sourced
1Load
Total connected or demand load you want the transformer to carry.
2Secondary
Line-to-line voltage.
More detail (optional)Spare capacity 0%, primary not entered. Tap to change.
Extra capacity added on top of the load, for growth or your own design margin. Left at 0 unless you set one.
Only used to show the primary current. It does not change the size.
Standard size to use
112.5 kVA
89% loaded, three-phase, 480 V secondary
- Required kVA
- 100 kVA
- Secondary current at 112.5 kVA
- 135.3 A
- Secondary current at your load
- 120.3 A
No spare capacity is included. Add it under More detail if the load may grow.
Motor starting, harmonics and ambient temperature are not considered.
Next, with 112.5 kVA filled in
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https://kvcalc.com/tools/transformer-sizing-calculator/
The link carries the full calculation, so anyone opening it lands on the same inputs and the same result.
How to size a transformer
Four steps take a load to a transformer rating.
- Put the load in kVA. From kW, divide by the power factor. From amps, use the formula below.
- Single-phase kVA: volts × amps ÷ 1000. Three-phase kVA: 1.732 × line-to-line volts × amps ÷ 1000.
- Add the spare capacity you have decided on. The tool leaves it at 0 until you enter one.
- Pick the smallest standard size at or above the result. Loading is the load divided by that size.
Example: 100 kW at 0.9 power factor is 111.1 kVA. The next three-phase size is 112.5 kVA, so the transformer runs at 89% of its rating. At 480 V its full-load secondary current is 135.3 A.
Standard transformer sizes
These are the ratings Schneider Electric publishes for its low-voltage and medium-voltage transformers (FAQ FA91532, last modified 2024-10-07). The company notes that some smaller sizes exist only on stocked product. This is one manufacturer’s list, not a standards text, so another catalog may differ.
| Type | Standard kVA ratings |
|---|---|
| Single phase, low voltage | 1, 1.5, 2, 3, 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100, 167, 200, 250, 333 |
| Three phase, low voltage | 3, 6, 9, 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750 |
| Three phase, medium voltage | 1000, 1500, 2000, 2500, 3000, 3750, 5000 |
| Single phase above 333 kVA | 500, 667, 833 (DOE liquid-immersed table) |
Transformer sizing chart: full-load amps by kVA
Calculated from the formulas above with 1.732 for three-phase, and rounded to 0.1 A. The sizes are taken from the lists in the previous section. Voltages are line-to-line for three-phase.
Three phase
| kVA | 208 V (A) | 240 V (A) | 480 V (A) | 600 V (A) |
|---|---|---|---|---|
| 15 | 41.6 | 36.1 | 18.0 | 14.4 |
| 30 | 83.3 | 72.2 | 36.1 | 28.9 |
| 45 | 124.9 | 108.3 | 54.1 | 43.3 |
| 75 | 208.2 | 180.4 | 90.2 | 72.2 |
| 112.5 | 312.3 | 270.6 | 135.3 | 108.3 |
| 150 | 416.4 | 360.9 | 180.4 | 144.3 |
| 225 | 624.6 | 541.3 | 270.6 | 216.5 |
| 300 | 832.7 | 721.7 | 360.9 | 288.7 |
| 500 | 1,387.9 | 1,202.8 | 601.4 | 481.1 |
| 750 | 2,081.9 | 1,804.3 | 902.1 | 721.7 |
| 1000 | 2,775.8 | 2,405.7 | 1,202.8 | 962.3 |
Single phase
| kVA | 120 V (A) | 240 V (A) | 480 V (A) |
|---|---|---|---|
| 5 | 41.7 | 20.8 | 10.4 |
| 10 | 83.3 | 41.7 | 20.8 |
| 15 | 125.0 | 62.5 | 31.3 |
| 25 | 208.3 | 104.2 | 52.1 |
| 37.5 | 312.5 | 156.3 | 78.1 |
| 50 | 416.7 | 208.3 | 104.2 |
| 75 | 625.0 | 312.5 | 156.3 |
| 100 | 833.3 | 416.7 | 208.3 |
| 167 | 1,391.7 | 695.8 | 347.9 |
| 250 | 2,083.3 | 1,041.7 | 520.8 |
| 333 | 2,775.0 | 1,387.5 | 693.8 |
What the result is not. It sizes the kVA rating from the load you enter. It does not account for motor starting, harmonics, ambient temperature, altitude, impedance or voltage drop. Use it to pick a starting size, then check the manufacturer data.
Sources.Schneider Electric FAQ FA91532: standard single phase and three phase kVA ratings;10 CFR 431.196, Energy conservation standards (eCFR) for the single-phase ratings above 333 kVA. The kVA and current formulas are the standard power relations. Results are an aid for checking a design, not a substitute for an engineer’s review. Checked on 2026-10-08.
Questions people actually ask
How do I size a transformer?
Convert the load to kVA, add any spare capacity you want, then pick the next standard size at or above that number. For single-phase, kVA = volts x amps / 1000. For three-phase, kVA = 1.732 x line-to-line volts x amps / 1000. For example, a 300 A three-phase load at 480 V is 1.732 x 480 x 300 / 1000 = 249.4 kVA, so the next size in the list is 300 kVA.
How do I convert kW to kVA for transformer sizing?
Divide kW by the power factor. A 100 kW load at 0.9 power factor is 100 / 0.9 = 111.1 kVA, which rounds up to a 112.5 kVA three-phase transformer. At 0.8 power factor the same 100 kW is 125 kVA and the next size is 150 kVA. Take the power factor from the equipment nameplate or data sheet.
How much spare capacity should I add?
The tool does not assume one, because the right figure depends on your growth plans, the owner specification and the engineer of record. Enter the percentage you have chosen under More detail and the required kVA rises by that amount before the size is picked. Left at 0, the result is sized to the load alone.
What are the standard transformer sizes?
This tool uses the ratings Schneider Electric publishes for its Square D transformers. Single-phase low voltage: 1, 1.5, 2, 3, 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100, 167, 200, 250 and 333 kVA. Three-phase low voltage: 3, 6, 9, 15, 30, 45, 75, 112.5, 150, 225, 300, 500 and 750 kVA. Three-phase medium voltage: 1000, 1500, 2000, 2500, 3000, 3750 and 5000 kVA. Other manufacturers can list different sizes, so check the catalog for the unit you plan to buy.
Why is there no 100 kVA three-phase size?
It is not in the three-phase list this tool uses, which goes 75, 112.5, 150. A 100 kVA load therefore lands on 112.5 kVA, running at about 89% of its rating. Another manufacturer may offer a different set, so confirm with the supplier.
How do I calculate transformer full-load current?
Single-phase: amps = kVA x 1000 / volts. Three-phase: amps = kVA x 1000 / (1.732 x line-to-line volts). A 112.5 kVA transformer at 208 V three-phase carries 112,500 / (1.732 x 208) = 312.3 A at full rating. Use the primary voltage for the primary current and the secondary voltage for the secondary current.
Does the primary voltage change the transformer size?
No. kVA is the same on both sides, apart from losses. The primary voltage only changes the primary current, which is why it is an optional field. Enter it under More detail if you want to see the primary current for sizing the primary protection.
What does this calculator not take into account?
It does not model motor starting current, harmonic loads, ambient temperature, altitude, duty cycle or impedance. It also does not decide whether the load is continuous, which matters for the overcurrent device and the conductors but not for the kVA rating used here. For those, use the fuse and breaker and conductor calculators linked in the results, and the manufacturer data.
Where do the sizes above 333 kVA single-phase come from?
Schneider lists single-phase low-voltage sizes up to 333 kVA. For larger single-phase loads the tool uses the ratings in the DOE liquid-immersed efficiency table (10 CFR 431.196): 500, 667 and 833 kVA. The result says so when it applies. These are liquid-immersed unit ratings, so confirm availability for your application.
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