AC Module Installation 101: New Wiring Considerations And Safety Guidelines

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solarindustrymag.com (June 2012) - by Terence Parker - As more AC solar modules are becoming available, it is important for installers to understand how the installation process may differ from their past experiences, and which rules apply for installing these new products. This article provides some guidelines and outlines the steps to install an AC module system. But before we do that, let’s define what constitutes a true AC module in terms of what is commercially available today.

The NFPA 70 National Electrical Code (NEC) defines an AC module as a complete, environmentally protected unit consisting of solar cells, an inverter and other components, exclusive of a tracking system, designed to produce AC power from sunlight. These integrated units are brought to the field as a certified assembly. AC modules are connected in parallel to other AC modules, and the output of the combined modules is then wired as a dedicated AC branch circuit.

There is no DC side to an AC module. Each module contains its own integrated micro-inverter, and DC from the solar module is wired directly to the micro-inverter at the factory. To qualify as an AC module, the integrated unit must be evaluated by a Nationally Recognized Testing Laboratory (NRTL) using product safety standards such as UL1741 and UL1703.

Because all DC is contained within the listed module assembly and the AC module is evaluated as “utility interactive,” there is no need for a DC grounding electrode conductor to establish a DC supply reference. The AC grounding electrode conductor system must already be provided at the building’s AC service entrance.

Under this definition, an AC module is a single product sold by a single company, typically a module manufacturer. Vendors of AC modules offer a single warranty and act as a single point of contact for technical support

Installers need to be aware that an AC module is not the same thing as a micro-inverter bolted to a PV module in the field. If building a PV system with individual components (PV module, DC wiring, inverter, AC wiring), the installer needs to comply with all sections of NEC Article 690. Adding inverters to NRTL-certified PV modules not evaluated for this purpose will void the NRTL’s marking.

Installation steps
NEC Section 690.6 distinguishes AC module installation from the inbstallation of PV systems that have both DC and AC sides. Part (A) of this section of the code states that “the requirements of Article 690 pertaining to PV source circuits shall not apply to AC modules.”

For example, there is no need to be concerned with multiplying shortcircuit DC currents by 125%, or with DC disconnects, DC grounding or DC ground fault detection interrupters (GFDIs). This is because there is no need for protection or installation of DC wiring in the field.

There is also no string sizing based on voltage capacity required with AC modules. The installer only needs to know the wire gauge within the dedicated branch circuit. This limits the current that the wire can carry and limits the number of AC modules that can be connected to it.

The nominal output of AC modules is given in amps. The markings on the AC module and the manufacturer’s instructions will identify the size of the branch circuit overcurrent device and the number of AC modules that can be attached.

The typical AC module installation process includes the following steps:

• Install the AC modules onto the racking system.
• Attach one end of an integrated AC cable to the first module in the system and wire the other end to a transition box.
• Daisy-chain the integrated AC cabling from module to module until all modules have been connected.
• Use an extender cable to connect rows of modules. Use cable management equipment to secure the extension cable off the roof.
• On the last module in the row, insert the end cap onto the connecting plug or receptacle. This cap protects the cabling from dirt and moisture.
• Run the home run wiring from the transition box to the load center.

Residential wiring connection
Figure 1 shows a typical wiring diagram for connecting groups of AC modules. As shown, the only wiring that needs to be done on the roof is to make the parallel AC pluggable connections between the modules and to wire the AC modules’ interconnecting cable system to the dedicated branch circuit wiring provided by an
electrician.

AC modules are typically installed as groups using the amperage rating of the branch circuit conductors and manufacturer’s instructions and markings on the module to determine the number of modules that can be connected to the circuit. They do not use a separate equipment grounding conductor (EGC) run to the frame of the module, because the module frame is grounded to the micro-inverter chassis and bonded to the integrated EGC within the interconnecting cable system.

The entire EGC path (including the grounding between inverter and PV module) is evaluated as part of the listing/certification by the NRTL.

If, however, the module support rails are metal, they will need to be grounded as part of the installation process. This step can be done by connecting external ground wires or by mechanically fastening all metal parts together with hardware evaluated for this purpose, such as with WEEBs (washers with spikes used to bond the frame of a PV module to a pole or mounting structure).

With the WEEBs installed, there’s a ground path from the rails, through the module frames, through the inverter chassis and on to the integral EGC in the AC cable.

It is highly recommended that a transient-voltage surge protector be installed at the AC module branch circuit interconnection point. If lightning is common at the site, an auxiliary ground wire can be run directly to a second premises ground rod. This auxiliary ground needs to be run separately from the AC phase conductors
and EGC. If a separate ground rod is installed, it must be connected to the building’s primary premises grounding electrode per local codes.

A DC GFDI and indicator light are not required for certified/listed AC modules, because there are no fieldwired DC PV source circuits. Because the AC module is “utility interactive,” ground fault detection for the AC circuit is provided by the single listed overcurrent device on the dedicated branch circuit feeding the AC
modules.

Additionally, because the DC voltage within an AC module is less than 80 V, AC modules are not subject to any of the arc-fault detection requirements
detailed in NEC Article 690.

Some AC modules are provided with monitoring devices that log the energy produced by individual modules. By being able to monitor individual modules and not just the system as a whole, homeowners and maintenance personnel can see when problems such as dirt or shading are affecting individual panels and compromising the power
output of the system. They can then take corrective action.

If there is a failure with an AC module, the monitoring device will instantly identify the faulty unit and its location in the array and provide the installer diagnostic information so that he can take appropriate action. Typically, system status may be viewed locally via a direct-connected PC or remotely via any device with Internet access.

In summary, there are many differences between installing AC modules and installing other types of PV systems, so personnel should be aware of the rules that apply. In addition, these differences are important to note for local permitting agencies and electrical inspectors across the country who have become familiar with other types of PV systems.

For more information on AC module systems, installers (and inspectors) should consult NEC Article 690, Section 690.6. In addition, SolarTech has published a Microinverter and AC PV Module Application Guide. To download a copy or provide feedback on the topic, visit www.solartech.org.

Terence Parker is applications engineering manager at SolarBridge Technologies, an Austin, Texas-headquartered provider of solar micro-inverters. He can be contacted at [email protected].