Understanding NEC Article 710: Stand-Alone Systems

NEC Article 710 provides requirements for stand-alone electric power production systems that operate independently, without connection to an electric utility or other power production and distribution networks. These systems are ideal for remote locations and specialized applications requiring isolated power sources.

Scope of Article 710

This article applies to:

• Island-mode systems, including isolated microgrids.
• Power production sources like solar PV, wind, fuel cells, and engine generators.
• Systems supplying electricity to facilities such as remote cabins, signs, or irrigation systems​.

Note: If a stand-alone system connects with an alternating current (AC) system, Article 705 requirements for interconnected systems also apply.

Key Requirements for Stand-Alone Systems

1. Equipment Approval (710.6)
   • All power production equipment must be:
     • Listed, or
     • Field-evaluated for compliance, with appropriate labeling​.
2. Identification of Power Sources (710.10)
   • A permanent label or directory must be installed to identify the location of each power source disconnecting means.
   • Labels must comply with Article 705.10 when multiple sources supply the same building​.
3. Voltage and Frequency Control (710.15(F))
   • Stand-alone systems must provide stable voltage and frequency outputs compatible with connected loads​.

Wiring and Sizing Requirements

1. Supply Capacity (710.15(A))
   • System capacity must meet or exceed the largest single load connected.
   • Variable sources (e.g., PV, wind) can combine with firm sources (e.g., generators) to meet load requirements​.
2. Single 120-Volt Supply (710.15(C))
   • Permitted when no 240-volt outlets or multiwire branch circuits are present.
   • Equipment must be labeled:
     “WARNING: SINGLE 120-VOLT SUPPLY. DO NOT CONNECT MULTIWIRE BRANCH CIRCUITS!”​.
3. Energy Storage (710.15(E))
   • Not required but often included for reliability. Systems with energy storage must meet specific operational and sizing requirements​.

Special System Configurations

1. Three-Phase Supply (710.15(D))
   • Stand-alone systems can provide three-phase, 3-wire, or 4-wire service.
   • Balancing of loads is required to ensure compatibility with the supply capacity​.
2. Stand-Alone Inverter Input Circuit Current (710.12)
   • Maximum current must match the inverter’s rated input current at its lowest input voltage​.

Applications

NEC Article 710 supports the safe installation of stand-alone systems for:

• Remote Locations: Cabins, off-grid homes, and irrigation systems.
• Specialty Loads: Signs, pumps, and isolated utility functions.
• Isolated Microgrids: Independent systems for resilience in remote or critical areas.

Conclusion

NEC Article 710 ensures the safety and functionality of stand-alone systems by addressing equipment approval, wiring, and voltage control requirements. These standards enable the reliable use of isolated power systems in diverse applications.