Generator Sizing & Step Loading: Avoiding the #1 Emergency Power Mistake
Picture this: 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:
The Relationship:
kW = kVA × Power Factor (PF)
Typical Generator Rating:
500 kW / 625 kVA at 0.8 PF
A 500 kW / 625 kVA generator is limited by both its kW and kVA ratings. If your load has a PF of 0.7, you need: kVA = 500/0.7 = 714 kVA, requiring a larger generator.
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 typically about 6× the running current. A 50 hp, 460 V motor runs at 65 A but starts at about 400 A
- Voltage dip during motor starting must stay within what the loads tolerate (most contactors about 35%, some relays 20%) or contactors drop out and lights 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 a load step larger than the set can accept within the allowed voltage and frequency dip. Confirm each step against the manufacturer's transient-performance data.
Step Loading Sequence Example
For a 400 kW generator serving a 6-storey commercial building:
| Step | Time Delay | Loads Connected | Step kW | Cumulative kW | % of Gen |
|---|---|---|---|---|---|
| 1 | 0 sec (immediate) | Emergency lighting, fire alarm, exit signs | 25 | 25 | 6% |
| 2 | 10 sec | Fire pump (jockey + main) | 75 | 100 | 25% |
| 3 | 20 sec | Elevators (one car at a time) | 45 | 145 | 36% |
| 4 | 30 sec | Stairwell pressurization fans | 60 | 205 | 51% |
| 5 | 45 sec | Critical HVAC (server room cooling) | 80 | 285 | 71% |
| 6 | 60 sec | Selected receptacles, BAS, security | 55 | 340 | 85% |
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.
Six load steps onto a 400 kW generator
The article’s 6-storey commercial example. Blocks show cumulative kW against time after the set comes on line.
Detail A: a load step
Voltage dip and recovery; confirm with the set’s transient data
Parts
Generator: 400 kW; check both kW and kVA: kW = kVA × PF, so a 500 kVA set at 0.8 PF delivers only 400 kW.
Largest motor early: the fire pump (step 2, 75 kW) goes on while the set is lightly loaded, with the most headroom for its starting transient (about 6 × running current).
No step above 80 kW: 20 % of the generator capacity; never apply a step larger than the set can accept within the allowed voltage and frequency dip.
Total: 340 kW, 85 % of the rating; allow 15–25 % for growth and derate for altitude, temperature and harmonic loads (UPS, VFDs, LED drivers).
Emergency power under CEC Section 46 (Rules 46-202, 46-206, 46-108, 46-208); start time, fuel and testing under the OBC and CSA C282, including monthly load tests and an annual 2-hour load test.
- Step 1
- Step 2
- Step 3
- Step 4
- Step 5
- Step 6
- Generator rating
CEC Section 46: Emergency Power Requirements
The Canadian Electrical Code, Section 46, defines the requirements for emergency power systems in buildings:
| Requirement | CEC Reference | Key Detail |
|---|---|---|
| Emergency power source | Rule 46-202 | Rechargeable battery system or generator, with duration tied to building-code requirements |
| Automatic transfer control | Rule 46-206 | Emergency power supply controlled by automatic transfer equipment upon normal supply failure |
| Emergency wiring methods | Rule 46-108 | Life-safety conductors must use approved emergency wiring methods and separation where required |
| Overcurrent coordination | Rule 46-208 | Emergency supply OCP must be coordinated with life-safety feeders and branch circuits |
| Transfer timing / fuel / testing | OBC/NBC + CSA C282 | Confirm start time, fuel supply, room/exterior installation, maintenance, and load testing under the applicable building and generator standards |
| Unit equipment | Rules 46-302 / 46-304 | Applies to emergency lighting unit equipment, not generator transfer-time requirements |
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: Output falls above the rated altitude; apply the manufacturer's derating data for the model
- Temperature derating: Above the rated ambient, apply the manufacturer's temperature derating for the model
- 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 may need oversizing; confirm the factor with the UPS and generator manufacturers
- No load bank testing - a generator that only runs unloaded may still stall under real load. CSA C282 requires monthly tests under load and an annual 2-hour load test
Frequently Asked Questions
What's the difference between kW and kVA?
kW is real power; kVA is apparent power. kVA = kW ÷ power factor (typically 0.8). A 100 kW generator rated at 0.8 PF is 125 kVA. Check both kW and kVA.
What is step loading?
The sequence in which loads are applied after generator startup. Motors draw about 6× full-load current when starting. Proper sequencing prevents voltage/frequency dips. See our load calculation guide.
Does CEC require emergency generators?
CEC Section 46 governs emergency power wiring, control, and overcurrent protection where emergency power is required. Building triggers, duration, generator performance, fuel supply, and testing must also be checked against OBC/NBC and CSA C282. See our permit guide for compliance.
