Transformer Sizing Calculator
Size a transformer from kW, kVA or amps with spare capacity, pick the next standard kVA rating and get primary and secondary full-load currents and the turns ratio.
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About this toolWhat is the Transformer Sizing Calculator?
Give this calculator the load you need to serve, in kW with a power factor, in kVA or in amps at the secondary voltage, and it works out the kVA a transformer must supply, adds a growth margin and rounds up to the next standard rating. It then lists the full-load current on the primary and secondary sides and the voltage (turns) ratio.
You can choose North American ratings (the 15, 30, 45, 75, 112.5, 150, 225 kVA sequence for three-phase and 25, 37.5, 50, 75, 100, 167 kVA for single-phase) or the IEC series (100, 160, 250, 315, 400, 630 kVA and so on). A small table shows the sizes either side with their currents and loading so you can see what one size up costs in headroom.
The full-load currents are what the primary and secondary protection, conductors and switchgear are built around, so they are worth having on the same page as the kVA.
- Load from kW and PF, kVA or amps
- Spare capacity and next standard rating (ANSI or IEC lists)
- Primary and secondary full-load and actual load currents
- Voltage (turns) ratio
- Comparison table of neighbouring sizes
How to use the Transformer Sizing Calculator
- Choose single-phase or three-phase.
- Enter the load as kW with power factor, as kVA, or as amps at the secondary voltage.
- Set the spare capacity you want, for example 20% for growth.
- Enter the primary and secondary voltages (line to line for three-phase) and choose North American or IEC ratings.
- Read the standard size, how heavily it is loaded today, the full-load currents and the voltage ratio.
Worked examples
Small commercial load, 480 to 208 V
Input
Three-phase, 48 kW at PF 0.8, 20% spare, 480 V to 208 V
Output
60 kVA load, 72 kVA required, 75 kVA standard (80% loaded); 90.2 A primary, 208.2 A secondary
48 ÷ 0.8 = 60 kVA. With 20% spare the next standard rating is 75 kVA.
Single-phase shop heater bank
Input
Single-phase, 18 kVA, 25% spare, 480 V to 240 V
Output
22.5 kVA required, 25 kVA standard (72% loaded); 52.1 A primary, 104.2 A secondary
A 2:1 ratio halves the voltage and doubles the current.
IEC distribution transformer
Input
Three-phase, 420 kVA, 20% spare, 11 kV to 400 V, IEC ratings
Output
504 kVA required, 630 kVA standard (67% loaded); 33.1 A primary, 909.3 A secondary
500 kVA would be just short of 504 kVA, so the next R10 rating up is chosen.
Background and insight
Transformers are rated in kVA rather than kW because their heating depends on current and voltage, not on the power factor of the load. Full-load current is I = kVA × 1000 ÷ V for single-phase and I = kVA × 1000 ÷ (√3 × V(LL)) for three-phase. A 75 kVA three-phase unit stepping 480 V down to 208Y/120 V carries 90.2 A on the primary and 208.2 A on the secondary.
The load figure should be the calculated demand, after any diversity your code allows, not the sum of nameplates. Designers then add spare capacity, commonly 20 to 25%, so that the unit runs at about 80% of its rating today. That margin covers growth, keeps losses and temperature rise down, and leaves room for motor starting. Harmonic-rich loads (computers, drives, LED drivers) may need a K-factor rated transformer or a further derating.
For an ideal transformer the voltage ratio equals the turns ratio, a = Vp ÷ Vs = Np ÷ Ns, and current goes the other way: Is = Ip × a. In three-phase units the per-winding ratio depends on the connection; a delta-wye 480 to 208Y/120 V transformer has 480 V on each delta winding and 120 V on each wye winding, a winding ratio of 4:1, even though the line-to-line ratio is 2.31:1. Ratings follow ANSI/IEEE C57.12 series standards in North America and IEC 60076 elsewhere; protection follows NEC 450.3 or your local code.
Common use cases
Specifying a step-down transformer
Choose the kVA for a 480 to 208/120 V panel feeding receptacles and lighting.
Machine isolation transformers
Size a control or isolation transformer for imported equipment that needs a different voltage.
Checking an existing unit
See how loaded a transformer is by entering the measured secondary amps.
Planning protection
Get primary and secondary full-load amps as the starting point for breakers and conductors.
Tips and limitations
- Use demand load after diversity, not connected load, or the transformer will be oversized and run inefficiently.
- Large motors may need a bigger transformer to limit voltage dip on starting even if the running load fits.
- High ambient temperature or altitude reduces capacity; check the manufacturer derating.
- For non-linear loads, ask for a K-rated or harmonic-mitigating transformer.
- Primary protection on transformers up to 1,000 V follows NEC Table 450.3(B), typically 125% of primary current, and secondary conductors have their own tap rules in 240.21(C).
Frequently asked questions
How do I calculate what size transformer I need?
Add up the demand load in kVA (kW ÷ power factor), add a growth margin of about 20 to 25%, and choose the next standard rating at or above that figure.
How many amps is a 75 kVA transformer?
Three-phase at 480 V primary it is 90.2 A; at 208 V secondary it is 208.2 A. Single-phase 75 kVA at 240 V would be 312.5 A.
Why are transformers rated in kVA instead of kW?
Because losses and heating depend on current and voltage regardless of power factor. The transformer cannot know what PF its load will have, so the rating is apparent power.
What are the standard transformer kVA sizes?
Common three-phase ratings in North America are 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750 and 1000 kVA; single-phase are 10, 15, 25, 37.5, 50, 75, 100 and 167 kVA. IEC sizes follow 100, 160, 250, 315, 400, 500, 630, 800, 1000 kVA.
Should a transformer run at 100% load?
It can, by design, but most engineers aim for about 80% at today's load to leave room for growth and keep temperature rise and losses moderate.
How do you find the turns ratio of a transformer?
Divide the primary voltage by the secondary voltage. 480 V to 240 V is 2:1. For three-phase units, the winding ratio also depends on whether each side is delta or wye.
Is there an in-depth guide to the Transformer Sizing Calculator?
Yes. “Transformer Sizing: kVA, Full-Load Amps and Standard Ratings” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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