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Home › Insights › How to size a transformer: a practical guide to kVA selection

Power & Distribution

# How to size a transformer: a practical guide to kVA selection

Entogo May 30, 2026 Updated September 5, 2026

![Utility distribution substation with three-phase power transformers — selecting the correct kVA rating for a North American electrical load](https://entogo.ca/_astro/entogo-buyer-line-utility-electrical-transformer-substation.CknnJLeO_2kbHKd.webp)

In short

How to convert a real electrical load into the right transformer kVA: the calculation, the standard ANSI/IEEE rating ladder, and the margins for continuous duty, future growth, ambient and altitude that sit between calculated load and nameplate.

A transformer is sold by one number — its kVA rating — and getting that number right is the quiet decision that the rest of an electrical design leans on. Undersize it and the windings run hot, insulation ages early, and there is no headroom for the next load. Oversize it and capital is tied up in capacity that never gets used, while the unit spends its life on the inefficient end of its loss curve, dragging power factor down with it. The whole exercise of **kVA selection** is matching the rating you buy to the load you actually have, with the right margins and the right deratings — no more, no fewer.

This is the calculation, the standard rating ladder you round to, and the adjustments that sit between a calculated load and a nameplate.

## Why transformers are rated in kVA, not kW

The first thing to settle is what kVA means. **kVA is apparent power** — the product of voltage and current the transformer has to carry. **kW is real power** — the portion that does useful work — and the two are linked by the power factor:

> kW = kVA × power factor (PF)

A transformer is rated in kVA rather than kW because the heating that limits it is driven by **current**, and current tracks apparent power regardless of how much of it is converted to real work. A 400 kW load running at 0.8 power factor draws the current of a 500 kVA load (400 ÷ 0.8), and it is 500 kVA of transformer that load needs. Sizing from a kW figure without dividing by the power factor is the single most common way a transformer ends up under-rated.

## The core calculation: from load to kVA

Once the load is expressed in apparent power, the arithmetic is short. The form depends only on whether the system is single- or three-phase.

| You know | Single-phase | Three-phase |
| --- | --- | --- |
| Voltage and current | kVA = (V × I) ÷ 1000 | kVA = (√3 × VLL × I) ÷ 1000 |
| Real power and power factor | kVA = kW ÷ PF | kVA = kW ÷ PF |
| Apparent power directly | kVA = VA ÷ 1000 | kVA = VA ÷ 1000 |

Here VLL is the line-to-line voltage and I is the line current. The √3 (≈ 1.732) is the only thing that distinguishes the three-phase case — it comes from the geometry of three phases 120° apart, not from anything physical about the transformer.

## Standard kVA ratings: you round up, you don’t invent

You cannot order a 437 kVA transformer. Liquid-immersed and dry-type distribution transformers are built to a **preferred series of ratings** defined by the ANSI/IEEE C57.12 family of standards, and a custom kVA between two rungs of the ladder costs more and takes longer for no benefit. The rule is simple: **calculate the requirement, then round up to the next standard rating.**

| Phase | Standard kVA ratings (ANSI/IEEE C57.12) |
| --- | --- |
| Single-phase | 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, 750, 1000, 1500, 2000, 2500 |

(Larger power units continue at 3750, 5000, 7500 and 10,000 kVA. International projects following **IEC 60076** round to that standard’s own preferred series, which is similar but not identical.)

Notice that the ladder gets coarser as it climbs — the step from 500 to 750 kVA is 50%. That spacing is why the calculation is worth doing carefully near the top of a range: a small overrun on a 500 kVA requirement pushes a project to a 750 kVA unit, and a small saving keeps it at 500.

## The five adjustments between calculated load and nameplate

The calculated kVA is a starting point, not the answer. Five adjustments sit between it and the rating you actually specify. One pulls the number down; four push it up. Knowing the direction of each is what keeps a spec honest, because the temptation is to stack margins until the transformer is twice the size it needs to be.

| Adjustment | Direction | Typical magnitude | Governing reference |
| --- | --- | --- | --- |
| Demand & diversity | ↓ reduces | Load-dependent | NEC (NFPA 70) Article 220 |
| Continuous-duty (80%) | ↑ adds | ÷ 0.8 (+25%) | NEC 210.19 / 215.2 / 450.3 |
| Future growth | ↑ adds | +15% to +25% | Project / owner requirement |
| Ambient & altitude | ↑ adds | ~0.3% per 100 m above 1000 m | IEEE C57.12.00 |
| Harmonics (K-factor) | ↑ adds | K-4 to K-20 rating | ANSI/IEEE C57.110 |

**Demand and diversity (down).** Connected load is not coincident load — not every motor, oven and machine runs at once. NEC Article 220 lets you apply demand factors so the transformer is sized for the realistic peak, not the arithmetic sum of every nameplate on the one-line. This is the adjustment that keeps a building from being grossly over-supplied.

**Continuous-duty margin (up).** The NEC requires conductors and overcurrent protection to be rated at **125% of a continuous load** — one drawing current for three hours or more. The working inverse is the familiar **80% rule**: do not load a transformer above 80% of nameplate continuously. A 40 kVA continuous load therefore wants a 50 kVA transformer (40 ÷ 0.8). The margin exists to keep the winding insulation off its thermal limit during sustained operation.

**Future growth (up).** Where load is expected to climb, a 15–25% allowance is cheap insurance — but only where growth is genuinely expected. Gross oversizing is not free: fixed **no-load (core) losses** are paid every hour the unit is energised regardless of load, so a chronically under-loaded transformer is an efficiency liability, not a conservative choice.

**Ambient and altitude (up).** Standard ratings assume a **30°C average ambient (40°C maximum)** and an altitude up to **1000 m (3300 ft)**. Hotter air or thinner air both reduce a transformer’s ability to shed heat. Per IEEE C57.12.00, a self-cooled unit is derated about **0.3% for every 100 m above 1000 m**; forced-air-cooled designs derate faster.

| Altitude | Self-cooled kVA factor |
| --- | --- |
| ≤ 1000 m (3300 ft) | 1.00 (no derating) |
| 1500 m | 0.985 |
| 2000 m | 0.97 |
| 3000 m | 0.94 |
| 4000 m | 0.91 |

**Harmonics (up).** Non-linear loads — variable-frequency drives, UPS rectifiers, LED drivers, the switch-mode supplies in IT and data-center gear — inject harmonic currents that heat a transformer beyond what its sinusoidal rating anticipates. ANSI/IEEE C57.110 defines the derating, and the answer is usually a **K-factor-rated** transformer (K-4 for light electronic load, K-13 for general IT, up to K-20 for drive-heavy buses) rather than a larger standard unit. The data-center power chain leans on this hard — covered in detail in [Power equipment for AI data centers](https://entogo.ca/insights/ai-data-center-power-equipment-2026/).

## A worked example, end to end

A light-industrial facility takes a 480 V three-phase service. After applying NEC Article 220 demand factors, the engineer measures a coincident demand of **440 amps** at the transformer secondary. Walking the ladder:

1. **Apparent power.** kVA = (√3 × 480 V × 440 A) ÷ 1000 = **366 kVA**.
2. **Continuous-duty margin.** The load runs all shift, so apply the 80% rule: 366 ÷ 0.8 = **457 kVA** minimum nameplate.
3. **Altitude.** The site sits at 1500 m, so divide by the 0.985 derating factor: 457 ÷ 0.985 = **464 kVA** required.
4. **Round up.** The next standard three-phase rating above 464 kVA is **500 kVA** — which also leaves roughly 9% of inherent growth headroom before the 80% line is reached again.

The facility gets a standard **500 kVA** transformer. Note what did *not* happen: the demand factor already trimmed the connected load to a realistic peak, so there was no need to bolt an extra “safety factor” on top of the 80% margin. Each adjustment answered one specific question. Stacking them blindly is how a 366 kVA load ends up on a 1000 kVA transformer.

Run the same ladder with your own numbers — change the voltage or the measured demand and the apparent power updates; apply the 80% continuous-duty margin and any altitude factor to that figure before rounding up to a standard rating.

**Step 1 — apparent power from voltage and current** — `kVA = sqrt(3) * V * I / 1000` with V = 480 V, I = 440 A → **kVA = 365.8 kVA**

## Two ratings that change the picture: cooling and efficiency

**One transformer can carry several kVA ratings.** A unit with forced-air cooling is rated at its self-cooled base (ONAN / AA) and again at a higher forced-cooled value (ONAF / FA) when fans run — often 33% above base. If a load peaks occasionally but sits well below most of the time, specifying the fan-cooled stage can meet the peak on a smaller, cheaper base unit rather than buying a larger transformer outright.

**Efficiency is now a floor, not a choice.** In the United States, distribution transformers must meet the minimum efficiencies in **DOE 10 CFR Part 431** (current levels mandatory since 2016; a 2024 final rule raises them further, with compliance required from April 2029). Efficiency is set at a defined load point, which is one more reason to size near the real load: a transformer run far below its rating misses the loading where it was designed to be most efficient.

## A specification checklist

The kVA number is the headline, but a transformer order is under-specified without the rest of the nameplate. A complete request settles:

- **kVA rating** and **phase** (single / three-phase)
- **Primary and secondary voltage** (and the system grounding)
- **Frequency** (60 Hz in North America, 50 Hz for IEC markets)
- **Standard family** — ANSI/IEEE C57 (NEC/NEMA downstream) or IEC 60076
- **Cooling class** and **temperature rise** (65°C for modern liquid-immersed; 80/115/150°C for dry-type)
- **Impedance (%Z)** — sets fault current and parallel-operation behaviour
- **Taps** — typically ±2 × 2.5% for voltage adjustment
- **BIL** (basic insulation level) for the voltage class
- **K-factor** if the load is harmonic-rich
- **Ambient and altitude** if the site is outside the standard basis

## Where Entogo fits

Entogo manufactures liquid-immersed, dry-type and pad-mounted transformers, prefabricated substations and switchgear in its own source factory with a vertically integrated supply chain. Units are designed and built to ANSI/IEEE C57 or IEC 60076 — UL/CSA certifiable on request — and European-standard (IEC/CE) catalogue equipment ships in an average of [12 weeks](https://entogo.ca/about/manufacturing/), within 36 weeks even when a product requires new UL or other North-American certification.

The sizing decisions above turn directly into a specification. An online [transformer sizing calculator](https://entogo.ca/tools/transformer-sizing-calculator/) turns a load straight into a standard kVA, the primary and secondary full-load current and the overcurrent protection, and the [transformer configurator](https://entogo.ca/products/transformer-quote/) runs the kVA, voltage, cooling and K-factor selection in five steps. The full [transformer and substation range](https://entogo.ca/products/category/transformers-substations/) covers the standard ratings discussed here. Once the rating is chosen, the next decision is how to protect it — see [sizing transformer overcurrent protection](https://entogo.ca/insights/transformer-overcurrent-protection-fault-current/) for NEC 450.3(B) / CEC Section 26 and the available fault current — and for the supply-side context that makes early sizing worth getting right, see [how long transformer lead times run in 2026](https://entogo.ca/insights/transformer-lead-times-north-america-2026/).

Sizing a transformer is not a hard calculation — it is a disciplined one. Get the load honest, apply each margin once for a reason you can name, round up to a standard rating, and the number you buy will be the number the system needed.

- Transformers
- kVA sizing
- Specification
- NEC
- ANSI/IEEE C57
- Power & Distribution

Glossary: [kVA rating](https://entogo.ca/glossary/#kva-rating) [K-factor](https://entogo.ca/glossary/#k-factor) [Air-cooled vs. liquid-cooled BESS](https://entogo.ca/glossary/#air-cooled-vs-liquid-cooled-bess) [Demand factor](https://entogo.ca/glossary/#demand-factor) [60 Hz vs. 50 Hz](https://entogo.ca/glossary/#60-hz-vs-50-hz) [Temperature rise](https://entogo.ca/glossary/#temperature-rise)

FAQ

## Common questions

- **How do I calculate what size transformer I need?**: Start from the load, not the transformer. Convert the load to apparent power in kVA — for a three-phase system, kVA = (√3 × line-to-line volts × line amps) ÷ 1000; from a kW figure, kVA = kW ÷ power factor. Then add margin so the transformer is not loaded above about 80% of nameplate continuously, apply any ambient or altitude derating, and round up to the next standard ANSI/IEEE kVA rating. A measured three-phase demand of roughly 366 kVA, for example, lands on a standard 500 kVA unit once the continuous-duty margin is included.
- **What is the 80% rule for transformer sizing?**: The U.S. National Electrical Code (NFPA 70) requires conductors and overcurrent devices to be sized at 125% of a continuous load — one that runs for three hours or more. The practical inverse is that a transformer feeding a continuous load should not be loaded above 80% of its nameplate kVA. So a 40 kVA continuous load calls for a 50 kVA transformer (40 ÷ 0.8 = 50), not a 40 kVA one. The rule protects the winding insulation from sustained heating at full nameplate.
- **What is the difference between kVA and kW when sizing a transformer?**: kVA is apparent power; kW is real (working) power, and kW = kVA × power factor. Transformers are rated in kVA — not kW — because winding heating is driven by current, which tracks apparent power regardless of how much of it does useful work. If you only know the real load in kW, divide by the power factor to recover the kVA the transformer must actually carry. A 400 kW load at 0.8 power factor needs 500 kVA of transformer, not 400.
- **Should I oversize a transformer for future growth?**: A modest allowance — commonly 15% to 25% headroom — is good practice where load is expected to grow, and it costs little because standard ratings are coarsely spaced. But gross oversizing is a real penalty — an under-loaded transformer runs at poor power factor and spends its life on the inefficient part of its loss curve, since fixed no-load (core) losses are paid 24/7 regardless of load. Size for the realistic demand plus a defined growth margin, then round up to the next standard rating — don't stack arbitrary safety factors.
- **Do I need to derate a transformer for altitude or temperature?**: Standard ANSI/IEEE and IEC ratings assume a 30°C average ambient (40°C maximum) and an altitude up to 1000 m (3300 ft). Above 1000 m the thinner air cools less effectively, so a self-cooled transformer is derated roughly 0.3% per 100 m of additional elevation per IEEE C57.12.00; forced-air-cooled units derate faster. Sites that run hotter than the standard ambient, or that sit at altitude, need the nameplate kVA increased to compensate before rounding up to a standard size.

Keep reading

## Related insights

[![Public DC fast EV charging station with transmission lines and grid infrastructure behind it](https://entogo.ca/_astro/dc-fast-charging-site-transformer-switchgear-sizing.oT4SXZBz_15uHXg.webp) EV Charging ### What size transformer and switchgear does a DC fast charging site need? A DC fast charging site is sized from AC input kVA, not DC output kW, and EVSE loads carry no demand factor. Four 150 kW plus eight 350 kW dispensers draw 3,615 kVA, needing 5,000 kVA of transformer and two 3,000 A mains at 480 V. View](https://entogo.ca/insights/dc-fast-charging-site-transformer-switchgear-sizing/)[![Distribution transformer and secondary conductors feeding low-voltage distribution equipment, governed by NEC 240.21(C) tap rules and CEC Rule 26-256](https://entogo.ca/_astro/transformer-secondary-conductors-nec-240-21c-tap-rules.BX_tOclr_UXL9N.webp) Power & Distribution ### How long can transformer secondary conductors be? NEC 240.21(C) tap rules and CEC 26-256 Transformer secondary conductors get no next-size-up rule. NEC 240.21(C) allows 10 ft and 25 ft runs with ampacity floors of one-tenth and one-third of the primary device times the voltage ratio; CEC 26-256 instead demands 125% of rated secondary current at any length. View](https://entogo.ca/insights/transformer-secondary-conductors-nec-240-21c-tap-rules/)[![Three-phase distribution transformer and low-voltage switchgear at a North American facility substation](https://entogo.ca/_astro/choosing-a-facility-distribution-voltage.B10infhs_Z2obSGL.webp) Power & Distribution ### 208, 480, or 600 volts? Choosing a facility distribution voltage North American facilities are served at 120/208, 277/480, or 347/600 volts. A higher voltage moves the same power at lower current, cutting conductor size and losses while adding transformer and safety cost. Here is how to match the voltage to the load. View](https://entogo.ca/insights/choosing-a-facility-distribution-voltage/)

Project inquiry

## Turn the brief into a specified system.

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