Find what size transformer a load needs. Enter the load as voltage and current, as kW with a power factor, or as kVA, choose single- or three-phase, add a growth margin, and get the required kVA, the next standard transformer size and its primary and secondary full-load currents.
Choose single- or three-phase and how you will describe the load.
Enter the secondary (load) voltage, line-to-line for three-phase, and the current, kW and power factor, or kVA.
Enter the primary voltage to get the supply-side current.
Set a margin for future growth and continuous loads (25% is a common choice).
Press Calculate to see the required kVA, the recommended standard size and the full-load amps.
Transformer sizing formulas
Quantity
Single-phase
Three-phase
Apparent power
kVA = V × I / 1000
kVA = √3 × VLL × I / 1000
From real power
kVA = kW / PF
Full-load current
I = kVA × 1000 / V
I = kVA × 1000 / (√3 × VLL)
Required size
kVArequired = load kVA × (1 + margin), rounded up to a standard rating
Full-load amps of common three-phase sizes
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
For single-phase, divide kVA × 1000 by the voltage: a 25 kVA transformer is 104 A at 240 V and 52 A at 480 V; 50 kVA is 208 A at 240 V.
Sizing tips
Use the real demand: add up nameplate loads, then apply realistic demand factors; not every load runs at full power at once.
Motors: starting current can be 5–7 times running current. A transformer sized only for running kVA may cause large voltage dips when big motors start.
Non-linear loads: computers, LED drivers and variable-frequency drives create harmonics that heat transformers; consider a K-rated or oversized unit.
Environment: high ambient temperatures, altitude and enclosed rooms reduce capacity; manufacturers publish derating factors.
Single-phase loads on a three-phase transformer: balance them across the phases, because the most loaded phase sets the limit.
Frequently asked questions
How do I calculate the kVA a transformer needs?
For single-phase, kVA = volts × amps / 1000. For three-phase, kVA = √3 × line-to-line volts × amps / 1000 (√3 ≈ 1.732). If you know real power instead, kVA = kW / power factor. Then add a margin and pick the next standard size up.
What size transformer do I need for 100 amps at 240 V single-phase?
240 × 100 / 1000 = 24 kVA. With a 25% margin that becomes 30 kVA, so the next standard single-phase size is 37.5 kVA. Without any margin a 25 kVA unit would run at 96% load, which leaves no room for growth.
Why add a margin when sizing a transformer?
Transformers run hotter and age faster near full load, loads tend to grow, and continuous loads such as heating or lighting run for hours. A 20–25% margin keeps the transformer at roughly 80% loading or less. Motor starting, harmonic-rich loads (which may need a K-rated transformer) and high ambient temperatures can justify more.
How do I find transformer full-load current?
Full-load amps = kVA × 1000 / V for single-phase, or kVA × 1000 / (√3 × V) for three-phase, using the voltage of the winding you are interested in. A 75 kVA three-phase transformer is 75,000 / (1.732 × 208) ≈ 208 A on a 208 V secondary and about 90 A on a 480 V primary.
What are standard transformer kVA sizes?
Common single-phase ratings include 1, 1.5, 2, 3, 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100, 167, 250, 333 and 500 kVA. Common three-phase ratings include 3, 6, 9, 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000, 2500 kVA and up. Available sizes vary by manufacturer and type (dry-type, pad-mounted, pole-mounted).
What is the difference between kVA and kW for a transformer?
kVA is apparent power, the product of voltage and current, and it is what heats the windings, so transformers are rated in kVA. kW is the real power doing work; kW = kVA × power factor. A 50 kVA transformer can only deliver 40 kW to a load with a 0.8 power factor.
Does this replace an engineer or the electrical code?
No. It is a planning tool for the basic kVA calculation. Overcurrent protection, conductor sizing, grounding, impedance, inrush and installation rules are governed by the National Electrical Code (Article 450 covers transformers) and local codes, and should be checked by a qualified electrician or engineer.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.