Understanding NEC Article 702: Optional Standby Systems
NEC Article 702 provides standards for the installation and operation of optional standby systems, which supply power to loads not required by law but deemed essential for operational continuity or convenience during power outages. These systems range from permanently installed generators to portable power supplies.
Scope of Article 702
This article applies to:
- Permanently installed optional standby systems, including their prime movers and transfer equipment.
- Portable generators connected to premises wiring.
Informational Note:
Optional standby systems are commonly installed in residences, farms, and commercial or industrial buildings to supply loads like heating, refrigeration, data processing, or critical industrial processes during outages.
Key Requirements for Optional Standby Systems
- Capacity and Ratings (702.4)
- Manual or Nonautomatic Load Connection: Systems must have sufficient capacity to supply all connected loads that the user intends to operate simultaneously.
- Automatic Load Connection: Systems must be capable of supporting the full automatic load or be managed by an energy management system to avoid overloading.
- Transfer Equipment (702.5)
- Interconnection or transfer equipment is required to prevent the inadvertent connection of normal and standby sources.
- Transfer switches must be listed and designed to avoid unintentional backfeeding.
- Temporary portable generator connections without transfer switches are allowed under qualified supervision with safeguards against backfeeding.
- Signaling and Marking (702.6–702.7)
- Audible and visual indicators are required for malfunction or when the standby source is carrying a load (except for portable systems).
- Signage must identify the type and location of on-site standby power sources, including detailed warnings about grounding and bonding configurations.
- Wiring Methods (702.10)
- Wiring for optional standby systems can share raceways, cables, and enclosures with general wiring.
- This is more flexible compared to emergency or legally required standby systems.
- Grounding and Bonding (702.11)
- Systems using portable generators as separately derived systems must be grounded per Article 250.30.
- Non-separately derived portable systems must bond the equipment grounding conductor to the system grounding electrode.
Portable Generators and Power Inlets
- Outdoor Generators (702.12)
- Outdoor portable generators rated over 15 kW require a readily accessible disconnect located near the building or structure.
- Generators rated 15 kW or less may use cord-and-plug connections without additional disconnects if located outside.
- Power Inlets
- Inlets rated at 100 amperes or more must be interlocked with a disconnecting means to prevent safety hazards when disconnected under load.
Applications
NEC Article 702 supports a wide range of optional standby system applications:
- Residential Settings: Backup power for heating, refrigeration, and security systems.
- Commercial and Industrial Facilities: Preventing process interruptions or product losses.
- Data Centers and IT Systems: Maintaining operational continuity for critical servers and communication systems.
Conclusion
NEC Article 702 ensures the safe and effective operation of optional standby systems, emphasizing capacity, transfer equipment, grounding, and marking requirements. By adhering to these standards, users can reliably maintain power during outages without compromising safety.
