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Free calculator

Use this calculator to

  • Size a single-phase or three-phase transformer in kVA from the connected load
  • Convert kW to kVA using the load power factor
  • Add spare capacity for growth and round up to a standard rating
  • Check a planned 100, 250 or 500 kVA unit against the actual load

Transformer Size Calculator

Size a transformer (kVA) for single-phase or three-phase loads with a safety margin.

kW
PF
%
Result
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kW
PF
%
Result
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Formulas

  • kVA required = kW / PF
  • With margin: kVA_sized = kVA required × (1 + spare%/100)
  • Standard sizes: 1, 1.5, 2, 3, 5, 7.5, 10, 15, 25, 50, 75, 100, 167, 250, 333, 500 kVA

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Common scenarios

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For business

Why this matters for businesses

If your site takes an HV (11 kV or higher) supply, you own the transformer, you maintain the transformer, and you pay the energy losses across it for the life of the contract. Sizing matters in two directions. Undersized, and you cap demand artificially, run hot, shorten insulation life, and risk a forced outage at the worst possible moment. Oversized, and you carry copper and iron losses every hour of every year for a load that never materialises. Either way the cost compounds across a fifteen to twenty year asset life, and the wrong call shows up on every monthly invoice.

Transformer sizing also drives the conversation with the DNO at connection time. The kVA you contract sets your Maximum Import Capacity, which then sets the available demand charges on your DUoS bill and the headroom you carry for future expansion. Sites planning a major electrification project (heat pumps replacing gas boilers, EV charging fleets, new production lines) need to walk the transformer sizing forward by five to ten years, not just the day one load. The cost difference between a one-shot uplift and two sequential uplifts can run into six figures across an I&C estate.

Purely Energy works on the procurement side of every HV-fed client we manage, but the transformer is where engineering and contract economics meet. Embedded transformer condition monitoring, regular oil sampling, and a documented load factor inform the next contract renewal, the next MIC review, and the case for embedded generation or storage. Get the kVA sizing right at the design stage and the rest of the asset's working life is significantly cheaper. Use this calculator as the first cut, then take the numbers to a competent HV designer for the formal selection.

Common questions

How do I size a transformer for my load?

Convert the load to apparent power: kVA = kW / power factor. Add spare capacity for growth, then round up to the next standard rating. A 72 kW three-phase load at 0.9 power factor needs 80 kVA; a 25 percent margin makes it 100 kVA, a standard unit. Sizing right matters in both directions: undersized units overheat, while heavily oversized units waste money through no-load losses.

Why are transformers rated in kVA rather than kW?

The manufacturer cannot know the power factor of the load you will connect, and the heating inside a transformer is driven by current and voltage, the apparent power, not by real power alone. Rating in kVA makes the limit unambiguous whatever the load. You convert to real power yourself: a 250 kVA transformer supplying loads at 0.9 power factor can deliver up to 225 kW.

How much spare capacity should I include?

20 to 30 percent is typical for commercial work. It absorbs load growth, motor starting, and harmonic-rich loads such as drives and IT equipment that heat a transformer beyond the simple kVA sum. Persistent operation near nameplate rating shortens insulation life: every 6 to 8°C of extra winding temperature roughly halves it. If significant expansion is expected, jumping a size class is usually cheaper than replacing the unit later.

What are the standard transformer sizes?

This calculator rounds your margined figure up through the steps 15, 25, 50, 75, 100, 167, 250, 333, 500, 750, 1000, 1500 and 2000 kVA, so a 400 kVA requirement becomes a 500 kVA unit rather than an overstressed 333 kVA one. Note that UK distribution transformers follow the IEC preferred series, commonly 315, 500, 630, 800 and 1000 kVA for commercial sites, so confirm the exact rating with your DNO or transformer supplier.

Does this work for single-phase and three-phase supplies?

Yes, and the arithmetic is identical: kVA = kW / power factor for both, because the phase arrangement is already absorbed in the kVA rating. Use the single-phase tab for small or domestic-scale loads and the three-phase tab for the 400 V supplies that feed most UK commercial premises. If a new or upgraded transformer changes your maximum demand, check your Agreed Supply Capacity (ASC) with your DNO at the same time.

Related calculators

The conversions that procurement and engineering use alongside this one.

Transformer Size Calculator (kVA) | Purely Energy