Generator Sizing & Step Loading: Avoiding the #1 Emergency Power Mistake

A hospital's backup generator starts during a blackout. The ATS transfers life-safety loads. Then the HVAC kicks in — and the generator stalls. 200 patients are in the dark. The cause? The engineer sized the generator for running load but ignored step loading. Here's how to avoid this catastrophic mistake.

kW vs. kVA: The First Trap

Generators are rated in both kW (real power) and kVA (apparent power). These are NOT the same thing. The relationship is governed by power factor:

What is Step Loading?

Step loading is the practice of sequencing emergency loads onto the generator in planned stages rather than connecting everything simultaneously. This is critical because:

• Motor starting current is 5–7× the running current. A 50HP motor runs at 65A but starts at 400A
• Voltage dip during motor starting must stay within 15–20% or motors won't start and lights will visibly flicker
• Frequency dip from sudden load application can cause the generator to stall if it exceeds the governor's recovery capability

The #1 rule of generator sizing: Never apply more than 50% of the generator's rated capacity in a single step. Most manufacturers recommend individual steps not exceeding 30% of rated kW.

Step Loading Sequence Example

For a 400 kW generator serving a 6-storey commercial building:

Notice that no single step exceeds 80 kW (20% of generator capacity), and the largest motor load (fire pump at Step 2) is applied when the generator is lightly loaded, giving it maximum headroom for the starting transient.

CEC Section 46: Emergency Power Requirements

The Canadian Electrical Code, Section 46, defines the requirements for emergency power systems in buildings:

Generator Sizing: The Complete Checklist

• Running load: Sum of all continuous loads at their demand values (kW and kVA)
• Starting load: Largest motor starting kVA in each step + running kVA of everything already connected
• Altitude derating: Generators lose ~3.5% capacity per 1,000 ft above 500 ft elevation
• Temperature derating: Above 40°C ambient, capacity drops ~2% per additional °C
• Future growth: Typically 15–25% spare capacity for additional loads
• Harmonic loads: UPS systems, VFDs, and LED drivers create harmonics that derate the alternator

Common Sizing Mistakes

• Sizing for running load only — ignoring motor starting transients is the most common cause of generator stalling on transfer
• No step loading sequence — connecting all loads simultaneously through a single ATS with no time delays
• Confusing kW and kVA — a 500 kVA generator at 0.8 PF only delivers 400 kW. Specifying "500" without units leads to undersizing
• Undersizing for UPS loads — UPS input current is non-linear. The generator alternator must be oversized by 1.5–2× for harmonic loads
• No load bank testing — a generator that passes a monthly no-load test may still stall under real load. Annual load bank testing is essential