Three Categories of Backup Power
Emergency Systems (NEC 700): Supply power to loads essential for life safety — egress lighting, exit signs, fire alarm systems, fire pumps, smoke control. Must restore power within 10 seconds. Wiring must be completely independent from all other wiring — separate raceways, boxes, and panels.
Legally Required Standby (NEC 701): Loads required by municipal building codes or regulations but not essential for life safety — heating/cooling for nursing facilities, sewage pumps, process ventilation. Must restore power within 60 seconds. May share raceways with emergency circuits.
Optional Standby (NEC 702): Loads that the owner chooses to back up for convenience or property protection — refrigeration, data systems, production equipment. No specific restore time. Normal wiring methods permitted.
Generator Sizing
Size emergency generators to carry the total connected emergency and standby load. Include: all emergency loads at 100%, all legally required standby loads at 100%, and motor starting kVA for the largest motor (or group starting simultaneously).
Generator sizing formula: kW_gen = Total_connected_kW / (generator PF). Add 25% for motor starting if the largest motor is >25% of generator capacity. Standard generator sizes: 20, 30, 50, 80, 100, 150, 200, 250, 300, 400, 500, 750, 1000, 1500, 2000 kW.
Fuel requirements: NEC 700.12(B)(2) requires on-site fuel for minimum 2 hours at full demand. NFPA 110 (referenced by many AHJs) requires fuel for the duration of the expected outage — often 24-72 hours for hospitals and critical facilities.
Automatic Transfer Switch (ATS)
NEC 700.5 requires automatic transfer for emergency systems — manual transfer is not permitted. The ATS must monitor normal source voltage and transfer to the generator within 10 seconds of detecting a failure. Retransfer to normal source typically uses a time delay (5-30 minutes) to ensure stable utility power.
NEC 700.5(D) requires load shedding capability if the generator cannot carry all loads simultaneously. Emergency loads have highest priority, followed by legally required standby, then optional standby.
Open-transition vs closed-transition: Open-transition ATS briefly disconnects both sources during transfer (10-100ms outage). Closed-transition (make-before-break) parallels sources momentarily for seamless transfer — required for UPS-backed loads and sensitive equipment.
Wiring Requirements
Emergency circuits (NEC 700.10): Must be kept entirely independent of all other wiring. Separate raceways, separate junction boxes, separate panels. Emergency panels must be in separate enclosures from normal distribution. This separation ensures that a fault on normal wiring cannot disable emergency lighting.
Fire protection: NEC 700.10(D) requires emergency circuit wiring to be protected against fire damage using one of: 2-hour fire rating, embedded in 2 inches of concrete, listed cable with 2-hour fire rating, or installation within 2-hour fire-rated assemblies.
Color coding: NEC does not mandate specific colors for emergency wiring, but common practice uses separate colors (e.g., red conduit or red wire marking) to distinguish emergency from normal circuits for maintenance awareness.
Testing and Maintenance
NEC 700.3(B): Emergency systems must be tested every 30 days for minimum 30 minutes under load. NFPA 110 requires additional monthly inspections and annual 4-hour load bank tests for Level 1 systems (hospitals, high-rise buildings).
Maintenance: Generators require regular oil changes (every 250-500 hours), coolant testing, battery maintenance, and annual fuel polishing for diesel systems. Neglected generators commonly fail during actual emergencies.
Record keeping: All tests, maintenance, and repairs must be documented per NEC 700.3(D). Records must be maintained and available for inspection by the AHJ.