Transformer Sizing for Buildings: kVA Calculation Guide

The transformer is the single most expensive — and least forgiving — component in a building's electrical system. Size it too small and it overheats, ages prematurely and eventually fails under the very load it was bought to serve. Size it too large and the owner pays for capacity, losses and switchgear they will never use, every day, for thirty years.

This guide walks through how building transformers are actually sized: the difference between kVA and kW, how connected load becomes demand load, where diversity and growth allowances come in, and how the final number is rounded to a standard rating.

kVA vs kW: Why Transformers Are Rated in kVA

Transformers are rated in kilovolt-amperes (kVA), not kilowatts (kW), and the distinction matters. kW measures real power — the work actually done. kVA measures apparent power — the total current and voltage the transformer must physically carry, including the reactive component drawn by motors, drives and other inductive loads.

The two are linked by the power factor:

kVA = kW ÷ power factor — so a 900 kW load at 0.9 power factor is a 1,000 kVA load on the transformer

A transformer sees current, and current follows kVA. That is why sizing a transformer from the kW figure alone understates the duty — typically by 10–20% in a normal commercial building, and by more where the power factor is poor.

Step 1: Establish the Demand Load, Not the Connected Load

The connected load — every circuit breaker's worth of equipment added together — is not what the transformer will ever see. Buildings never run everything at once. The working figure is the maximum demand: the connected load reduced by demand and diversity factors that reflect how the building actually operates.

Typical reductions applied in a building load schedule include:

• Lighting — high demand factor, since most lighting runs together during occupied hours
• Small power and socket outlets — substantially diversified, since only a fraction draw current simultaneously
• HVAC plant — assessed against the peak season, since chillers and heating rarely peak together; the mechanical demand comes straight from the cooling load calculation
• Lifts, pumps and intermittent plant — heavily diversified by duty cycle

The exact factors come from the applicable code — NEC demand factors in North American practice, or the diversity allowances used in IEC/BS-based designs — and from engineering judgement about the building type. Getting this step right is the heart of the electrical design; everything after it is arithmetic.

Step 2: Convert to kVA and Add Growth

With the maximum demand established in kW, convert to kVA using the expected power factor (0.85–0.95 is typical for modern commercial buildings with power factor correction). Then add a growth allowance — commonly 10–20% depending on the client's brief and how likely the building is to densify. Data-heavy tenancies, future fit-outs and EV charging provision are the usual reasons the allowance earns its keep.

Step 3: Round to a Standard Rating

Transformers come in standard sizes, so the calculated figure is rounded up to the next rating. In IEC-based markets the preferred series runs 315, 400, 500, 630, 800, 1,000, 1,250, 1,600, 2,000 and 2,500 kVA; North American practice uses 300, 500, 750, 1,000, 1,500, 2,000 and 2,500 kVA. Above the largest single rating, the design moves to multiple transformers — which is often preferable anyway for redundancy and maintenance.

A Worked Example

1. Connected load from the schedule: 1,800 kW
2. After demand and diversity factors: maximum demand 1,150 kW
3. At 0.9 power factor: 1,150 ÷ 0.9 = 1,278 kVA
4. Plus 15% growth allowance: 1,470 kVA
5. Selected rating: 1,600 kVA (IEC series)

Note how far the final rating sits from the 1,800 kW connected figure. A naive sizing from connected load at unity power factor would have landed on 2,000 kVA — a full size class larger, purchased and fed for no benefit.

The example also shows where power factor correction earns its place. Improving the same building from 0.9 to 0.95 drops the demand to 1,211 kVA — and with the growth allowance, to 1,392 kVA, still inside the 1,600 kVA unit with more margin to spare. Capacitor banks are routinely the cheapest kVA a project ever buys, which is why the correction strategy belongs in the sizing calculation rather than after it.

The Cost of Oversizing — and the Risk of Undersizing

Oversizing feels safe, but it is not free. A transformer's core (no-load) losses run 24 hours a day regardless of load, so an oversized unit wastes energy every hour of its life. It also costs more to buy, demands more substation space, imposes higher inrush current on the upstream network, and — lightly loaded — can operate at a worse efficiency point than a correctly sized unit.

Undersizing is worse. Sustained overload drives winding temperatures up, and transformer insulation life falls steeply with temperature — a unit run hot ages at a multiple of its design rate. The failure arrives years later, unannounced, as the most disruptive outage a building can have.

One Transformer or Two?

Past roughly 1,500–2,000 kVA of demand, the question shifts from "which rating" to "how many units". Splitting the load across two transformers buys resilience — with the right switchboard arrangement, either unit can carry the essential load while the other is maintained or replaced — and it keeps fault levels and unit sizes within comfortable limits. Critical facilities push further to full N+1 redundancy, where each transformer alone can carry the entire demand. The premium is real, but so is the arithmetic on the other side: a hospital or data floor does not get to wait the months a failed large transformer can take to replace.

Dry-Type or Oil-Filled?

The sizing method is identical either way, but the technology choice affects the room around the number. Cast-resin dry-type units are the default indoors — no oil containment, simpler fire separation, happy in a basement substation. Oil-filled units are cheaper per kVA and quieter at large ratings but bring bunding, fire-rating and ventilation requirements that belong in the substation layout from day one, not discovered at authority approval stage.

Utility Transformer or Private Substation?

Smaller buildings are usually fed at low voltage from the utility's own transformer, and the sizing exercise above informs the connection application rather than a purchase. Larger buildings take a medium-voltage connection into a private substation, and the developer owns the transformer decision outright — along with the switchgear, protection and room sizing that follow from it. Either way, the demand calculation is the document the utility will ask for first.

Conclusion

Transformer sizing is a chain: connected load, demand factors, power factor, growth, standard rating. Each link is simple, but the judgement sits in the middle two — and errors there are bought in copper and iron and paid for over decades. If you need a load schedule and transformer sizing prepared properly, that is exactly the work our electrical team does remotely, to your local code.