Generator & Automatic Transfer Switch Installation
Install a standby generator with ATS — generator sizing, transfer switch rating, and NEC 700/701/702 compliance.
Scenario Overview
Install a 100 kW natural gas standby generator with an automatic transfer switch (ATS) for a small commercial building, covering emergency, legally required standby, and optional standby loads.
Given Information
- •Building: small commercial office/retail, 400A main service
- •Emergency loads (NEC 700): exit lighting, fire alarm = 5 kW
- •Legally required standby (NEC 701): emergency lighting = 15 kW
- •Optional standby (NEC 702): HVAC, select receptacles = 60 kW
- •Generator: 100 kW, 480V 3Φ, natural gas
- •ATS: 200A, 3-pole, service entrance rated
Calculation Steps
Classify Loads by NEC Category
NEC 700 (Emergency): exit signs, egress lighting, fire alarm — 5 kW. Must have power within 10 seconds.
NEC 701 (Legally Required Standby): emergency lighting, elevators — 15 kW. Must have power within 60 seconds.
NEC 702 (Optional Standby): HVAC, receptacles, normal lighting — 60 kW. No time requirement.
Total standby: 80 kW.
Result: 80 kW total standby (5 + 15 + 60 kW)
Generator Sizing
Total standby: 80 kW.
Motor starting factor: largest HVAC compressor start may add 30% momentary.
100 kW generator at 80% loading = 80 kW continuous. Adequate.
Verify: 100 kW / (480 × √3) = 120.3A generator output.
Result: 100 kW generator (80% loaded)
ATS Selection
Total standby current: 80,000 / (480 × √3) = 96.3A.
ATS: 200A, 3-pole, service entrance rated per NEC 700.
Closed-transition (make-before-break) type for minimal power interruption.
Must include 5-minute cooldown timer and exerciser.
Result: 200A, 3-pole closed-transition ATS
Wiring Separation Requirements
NEC 700.10: Emergency circuit wiring must be completely independent from all other wiring.
Emergency (NEC 700) circuits: separate raceway, separate panels.
Cannot share raceway or enclosures with any non-emergency circuits.
Legally required standby (701) can share raceway with emergency but NOT with normal or optional.
Result: Separate raceways and panels for each NEC category
Testing Requirements
NEC 700.3(E): Monthly testing under load for minimum 30 minutes.
Written records required of all tests and maintenance.
Authority Having Jurisdiction (AHJ) may require witnessed testing.
Generator must reach operating temperature during test.
Result: Monthly 30-minute load test with documentation
Final Answer
100 kW natural gas generator with 200A closed-transition ATS. Three load categories kept on separate panels and raceways: Emergency (5 kW, NEC 700), Legally Required Standby (15 kW, NEC 701), Optional Standby (60 kW, NEC 702). Monthly 30-minute load testing required.
Key Takeaways
- •NEC 700 (Emergency) has the strictest requirements: 10-second transfer, separate wiring, monthly testing
- •Wiring for different NEC categories (700/701/702) cannot be mixed in the same raceway
- •Generator sizing should account for motor starting surge — typically 20-30% above continuous load
- •ATS must be service entrance rated when serving as main bonding point for standby system
Calculators Used
NEC References
- NEC 700 — Emergency Systems
- NEC 701 — Legally Required Standby
- NEC 702 — Optional Standby
- NEC 700.10 — Wiring Separation
- NFPA 110 — Emergency Power Systems