Challenge: Size the Transformer for This Hospital — It's Not Just kVA

The Problem

A new 200-bed regional hospital. The maximum demand study shows:

The project electrical engineer specifies a 2,000 kVA transformer with 5% impedance. Is this correct?

The Challenge

Identify at least four reasons why a 2,000 kVA transformer is wrong for this application, and determine the correct size.

The Solution

Problem 1: Harmonic Derating

LED lighting (280 kVA) and IT/GPO loads (420 kVA) are high-harmonic sources. Combined, they represent 700 kVA of non-linear load — 36% of total.

Per IEEE C57.110, a transformer supplying significant harmonic loads must be derated. With a K-factor of approximately 7 (typical for mixed LED/IT load profile), the transformer must be derated to approximately 85% of nameplate capacity.

2,000 kVA × 0.85 = 1,700 kVA effective capacity — insufficient for 1,960 kVA demand.

Problem 2: Motor Starting Voltage Dip

The largest HVAC chiller has a 200 kW DOL-start compressor motor. Starting current: approximately 6× FLC = ~1,800A at 415V = ~1,300 kVA.

Voltage dip during motor starting:

ΔV% = (S_start / S_tx) × Z_tx% = (1,300 / 2,000) × 5% = 3.25%

AS/NZS 61000.3.3 and most hospital standards require voltage dips to be less than 3% for sensitive medical equipment. At 3.25%, imaging equipment (MRI, CT) may produce artefacts or require recalibration.

Increasing transformer size to 2,500 kVA: ΔV% = (1,300 / 2,500) × 5% = 2.6% — passes.

Problem 3: Essential Services Redundancy

Hospital electrical systems require redundancy per AS/NZS 3009 (Electrical installations — Emergency power supplies in hospitals). Essential services (theatres, ICU, emergency department) must have:

• N+1 transformer configuration or automatic changeover from utility to generator
• If using a single transformer, it must be rated for the essential load with adequate margin for generator synchronisation transients

Essential services typically represent 60–70% of hospital demand. If the single transformer fails, the generator must support ~1,300 kVA. The transformer itself needs headroom above calculated demand for transient load acceptance.

Problem 4: Future Load Growth

Hospital load typically grows 2–3% per year as services expand and technology evolves. Over a 20-year transformer life:

1,960 kVA × (1.025)^20 = 3,211 kVA

Even with conservative 1.5% growth: 1,960 × (1.015)^20 = 2,638 kVA

The Correct Specification

Recommended: 2,500 kVA transformer (or 2×1,600 kVA in parallel for redundancy), with K-factor rating or de-rated accordingly.

The 2,000 kVA specification was undersized by 25% once real-world factors were considered.

Run your own analysis: Use the Transformer Calculator and Maximum Demand Calculator together.

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Frequently Asked Questions

How do I size a transformer?

Size based on maximum demand (not connected load), diversity factors per AS/NZS 3000 Table C2, and future growth (typically 125-150% of current load). Don't forget power factor correction and harmonics per IEEE C57.110.

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