Transformer Sizing Calculator — Required kVA and the Next Standard Rating

Background.

Sizing a transformer is two separate problems, and most calculators only solve the first. The first is arithmetic: turn the load into apparent power, because a transformer is rated in kVA and not in kilowatts, and add whatever spare capacity the design calls for. The second is that you cannot order the answer. Transformers are built in a fixed ladder of sizes, and a requirement of 45.03 kVA does not buy you a 45.03 kVA transformer — it buys you a 75 kVA one, because 45 is a standard size and the next one up is 75.

This page does both. Describe the load in amps at the secondary voltage, in kilowatts with a power factor, or in kVA if you have already worked it out; pick single-phase or three-phase; add a growth allowance; and it returns the smallest standard rating that covers the requirement, along with the full-load current on both sides of that rating.

The sizes come from ANSI C57.12.50-1981, Table 1, 'Self-Cooled Kilovolt-Ampere Sizes'. Single-phase: 1, 3, 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100, 167, 250, 333, 500. Three-phase: 15, 30, 45, 75, 112.5, 150, 225, 300, 500. Note how far apart the steps are at the top — from 45 to 75 is a 67 % jump — which is why the utilisation figure this page reports is often much lower than people expect, and why it is not evidence of over-sizing.

Two scope limits sit beside every result rather than in a footnote. The standard cited covers ventilated dry-type distribution transformers with high voltage 601–34 500 V and low voltage 120–600 V, and it stops at 500 kVA; anything larger is a different standard and this page refuses to answer rather than extrapolate a size that may not exist. And the 1981 edition is old — later editions and individual manufacturers list ratings it does not — so confirm the size against a current catalogue before ordering.

Finally, the part that matters most. Choosing a kVA number is not designing an installation. In the United States, transformer installation and overcurrent protection are governed by NFPA 70, the National Electrical Code, Article 450; the adopted edition varies by jurisdiction and local amendments govern. **No figure on this page is taken from the NEC** — the growth allowance is your own design choice and is labelled as such — and this page sizes neither overcurrent protection nor conductors nor clearances. A licensed electrician or engineer must sign off before anything is installed.

What is transformer sizing calculator?

Transformer capacity is quoted in kilovolt-amps rather than kilowatts because the two limits inside a transformer are current, which heats the windings, and voltage, which stresses the insulation. Neither cares about the phase angle between voltage and current, so a transformer that can deliver 100 kVA delivers it whether the load is a resistive heater at unity power factor drawing 100 kW or a motor bank at 0.8 power factor drawing 80 kW. Converting a load to kVA therefore comes first: from amps it is V × I ÷ 1000 for single-phase and √3 × V × I ÷ 1000 for three-phase, using the line-to-line voltage; from kilowatts it is kW ÷ power factor. Adding spare capacity comes second and is a judgement rather than a formula — how much a building's load is expected to grow, how much of the connected load runs at once, how much margin the owner wants to pay for. Landing on a rating comes third, and is not a judgement at all: the sizes exist or they do not. That is why this page separates the required figure from the recommended rating and reports both, along with how much headroom the gap between them actually gives you.

How to use this calculator.

1. 01Choose how you know the load — amps, kilowatts with a power factor, or kVA — and whether the system is single- or three-phase.
2. 02Enter the load. In amps mode it is measured at the secondary voltage; in kilowatt mode it is real power, so the power factor matters.
3. 03Enter the secondary and primary voltages, line-to-line for three-phase. The primary voltage does not change the kVA, only the primary current.
4. 04Enter the spare capacity you want. This is a design decision, not a code figure — enter 0 if you want the bare requirement.
5. 05Read the recommended standard rating. Then read the required kVA beside it: if the two are far apart, that is the spacing of the standard ladder, not over-sizing.
6. 06Check the two full-load currents. They are what the conductors and protection on each side have to be rated for — sizing those is a separate job this page does not do.
7. 07Confirm the rating against a current manufacturer catalogue, and have a licensed professional review the installation.

The formula.

Start by converting the load into apparent power, because that is the only thing a transformer is rated in. Take the worked example: 100 A on a three-phase 208 V secondary. Three-phase apparent power is √3 × V × I, so 1.7320508 × 208 × 100 = 36026.66 VA, or 36.0266567974 kVA. Had this been single-phase, the √3 would simply not be there.

If the load is given in kilowatts instead, divide by the power factor: 36 kW at 0.8 power factor is 45 kVA, and it is the 45 that the transformer has to be able to deliver. Getting this backwards — sizing a transformer from kilowatts and ignoring the power factor — under-sizes it by exactly the power factor, which for a motor load is a 20 to 25 % error.

Now add the spare capacity. With 25 % the requirement is 36.0266567974 × 1.25 = 45.0333209968 kVA. And here is where the example earns its place: the three-phase ladder contains 45 kVA, and the requirement misses it by 0.033 kVA — under a tenth of a percent. There is no 45.05 kVA transformer, so the answer is 75 kVA, the next size up. A load 0.07 % over a standard size costs an entire size.

That is also why the utilisation figure comes out at 48.04 %: the load is not half of what the transformer can do because anyone over-specified it, but because the standard ladder has nothing between 45 and 75. The headroom figure, 66.54 %, says the same thing about the requirement rather than the load.

The full-load currents come from the chosen rating, not from the load: 75 kVA at 208 V three-phase is 75000 ÷ (1.7320508 × 208) = 208.18 A on the secondary, and 75000 ÷ (1.7320508 × 480) = 90.21 A on the 480 V primary. Their ratio is 480/208, the voltage ratio, exactly as an ideal transformer requires. Those are the numbers the conductors and overcurrent devices on each side have to be chosen against — and choosing them is a different calculation that this page does not perform.

Rounding happens once, at the return boundary, and the headroom band reads the unrounded value, so a requirement that is a hair over a standard size is treated as over it even when it displays as equal.

What this page refuses to do. Above 500 kVA the cited table has no entries, and rather than invent a size the calculator stops and says so: units of 501 kVA and larger are covered by a separate standard that this page does not implement. That refusal is deliberate. A size that does not exist is worse than no answer, because it looks like one.

A worked example.

Frequently asked questions.

References& sources.

1. [1]ANSI C57.12.50-1981, 'American National Standard Requirements for Ventilated Dry-Type Distribution Transformers, 1 to 500 kVA, Single-Phase, and 15 to 500 kVA, Three-Phase, with High-Voltage 601 to 34 500 Volts, Low-Voltage 120 to 600 Volts' (Secretariat: IEEE; approved by ANSI 15 August 1980; published 1981). Sole source of the rating ladder implemented. §4.1.2: 'Self-cooled (AA) kilovolt-ampere sizes shall be as shown in Table 1.' Table 1 'Self-Cooled Kilovolt-Ampere Sizes' lists, single-phase: 1, 3, 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100, 167, 250, 333, 500; three-phase: 15, 30, 45, 75, 112.5, 150, 225, 300, 500. §4.1.1 adds that the ratings 'are continuous and based on not exceeding a 150°C average winding temperature rise, as measured by resistance (220°C limiting temperature)'. The foreword notes that units of 501 kVA and larger are covered by ANSI C57.12.51-1981, which is why this page refuses above 500 kVA. PDF; text extracted locally. Retrieved 2026-07-29.
2. [2]OpenStax (Rice University), University Physics Volume 2, §15.6 'Transformers'. Source of the power relation underlying the full-load current outputs: for an ideal transformer 'i_P(t)v_P(t) = i_S(t)v_S(t)', so the primary and secondary full-load currents at a given rating stand in the inverse ratio of the voltages — which is what this page's two current outputs satisfy. Also the source of the real-world caveat that 'a good transformer can have losses as low as 1% of the transmitted power', meaning the primary side draws slightly more than the secondary delivers. Retrieved 2026-07-29.
3. [3]ANSI C57.12.51-1981, 'American National Standard Requirements for Ventilated Dry-Type Power Transformers, 501 kVA and Larger, Three-Phase, with High-Voltage 601 to 34 500 Volts, Low-Voltage 208Y/120 to 4160 Volts'. Named in the foreword of C57.12.50-1981 as the standard covering units above the range this page implements. Cited to identify precisely what is out of scope: the calculator refuses requirements above 500 kVA and points the reader here rather than extrapolating a rating. Bibliographic reference only — the document itself was not opened, and no figure on this page comes from it. Retrieved 2026-07-29 via the C57.12.50-1981 foreword.