Hospital Backup Power Sizing in Coastal Kerala

Scenario

A 250-bed tertiary care hospital in Kochi, Kerala—located at sea level with high ambient temperatures (avg. 34°C) and high humidity—required a diesel generator for critical life-support and emergency lighting systems. Constraints included strict compliance with ISO 8528-1 and Indian Standard IS 12615, limited rooftop space for installation, and zero tolerance for voltage/frequency deviation during motor starting (e.g., MRI chillers, HVAC compressors). The hospital’s electrical team provided verified load data from a 72-hour energy audit.

Given Data

• Voltage: 400 V
• Frequency: 50 Hz
• Continuous Power Demand: 185 kW
• Starting Power Demand (Surge): 520 kVA
• Power Factor: 0.82
• Safety Factor: 1.25 (increased from default due to coastal derating and future ICU expansion)

Calculation

The Generator Sizing Calculator uses the following logic:

1. Convert starting power to equivalent kW: starting_power_kW = starting_power × power_factor = 520 kVA × 0.82 = 426.4 kW
2. Determine base capacity requirement: max(continuous_power, starting_power_kW) = max(185, 426.4) = 426.4 kW
3. Apply safety factor: generator_capacity = 426.4 kW × 1.25 = 533.0 kW
4. Round up to next standard generator rating (per ISO 8528): 625 kW (as 533 kW is not commercially available; nearest compliant ratings are 500 kW and 625 kW — 500 kW fails surge margin by 33 kW, so 625 kW selected).

Result and Decision

A 625 kW, 400 V, 50 Hz, three-phase diesel generator (ISO Class G2, continuous duty) with integrated soft-start sequencing for HVAC motors was procured. It was derated by 8% during commissioning per manufacturer’s altitude/temperature correction curve (validated at 34°C, 0 m ASL), confirming actual output ≥ 575 kW — sufficient for the 533 kW design load.

Lesson

Always apply site-specific derating after sizing — never before. A generator sized for nominal conditions may fall short in tropical environments unless ambient-corrected capacity is verified post-selection, not assumed.