ACPV systems use microinverters, which fully convert DC power to grid-compliant AC power at each PV module. One of the benefits of an ACPV system is that there is no high-voltage DC present. This reduces the need for code considerations and product functionality related to compliance with safety requirements for high-voltage DC circuits. It also introduces more commonly understood, and often seen as safer, AC electrical wiring from the roof to the interconnection point. AC systems are an easy way to comply with NEC rapid-shutdown requirements without requiring additional complex system functionality or placing string inverters on roofs, which is not ideal for reliability. Systems that use field-installed microinverters, and those with certified ACPV modules with integrated microinverters, use AC cabling systems. This AC cabling system, similar to DC cabling, is located underneath the PV array. Here are three important considerations for AC cabling systems.
Product certification for the application: When cabling systems are introduced to the roof, whether they are DC or AC, they must be designed and certified for use in the application. AC cabling systems should have the proper temperature and UV exposure ratings, as well as integrated connector systems that ensure watertight seals. A third-party certification body should evaluate and permit use of the cable, connectors and accessories for use in PV applications according to applicable standards. UL has created a new standard called UL 3003 for Distributed Generation Cable, which allows listing of newer cable assemblies specifically designed for ACPV applications. Historically, ACPV systems use TC-ER as part of the cable assemblies, which carry ratings for “oil resistance,” “wet locations” and “sunlight resistance.” These cable assemblies were evaluated and listed under previously existing UL standards and can be installed to NEC requirements in PV applications.
NEC compliance: NFPA 70: NEC 690.31(D) permits the use of “jacketed multiconductor cable assemblies listed and identified for the application in outdoor locations.” It is important that the cable be secured at intervals not exceeding 1.8 m (6 ft). UL 3003 listing specifically evaluates installation of this cable type in accordance with the NEC, ANSI/NFPA 70.
Wire management for the long haul: To ensure long-term reliability of AC cabling systems in PV systems, it is important to install the AC cable system according to the manufacturer’s recommendations and applicable code requirements. For example, when installing the Enphase Q Cable, follow these best practices:
- Use a good cable cutter
- Cut cable for the job prior to bringing the cable on the roof
- Cut cable mid-way between connectors
- Position cabling prior to managing it with clips to help determine quantity of connectors and accessories as well as optimal junction box location
- Use stainless steel cable clips to manage cable along rail or along back edge of PV module for rail-less racking systems
- Enphase Q cable can be supported with clips that are also used with USE-2 cable or PV cable
- Observe cable bend radius and don’t overstress the cables at the connectors
- When entering junction boxes, use proper glands for the application to provide strain relief and seal from water and dust
- Consider drip loops in cables before entering junction boxes
- Properly install a terminator at the AC cable end that is not landed in the junction box
- Use sealing caps on unused AC connectors to make a water tight connection
This installation tip was provided by Marko Rosenfeldt, senior manager, field applications engineering, Enphase Energy
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