Pulling It All Together: Strategies for Making Common Connections in PV Power Circuits
Inside this Article
As defined in the National Electrical Code Article 690.2 a photovoltaic power source is “an array or aggregate of arrays that generates dc power at system voltage and current.” Additional definitions clarify that at least three dc circuit types may be present in the PV power source for a grid-tied PV system, defined as photovoltaic source circuit, photovoltaic output circuit and inverter input circuit. While these terms are self-explanatory to many industry veterans, terminology common in the field may vary.
The photovoltaic source circuit, for example, is defined as "circuits between modules and from modules to the common connection point(s) of the dc system." In practice, PV source circuits are commonly referred to as strings or simply as source circuits. PV output circuits are often referred to as array circuits. The “common connection point(s) of the dc system” are multiple and varied, as reflected by the nomenclature— combiner boxes, recombiner or subcombiner boxes and so on. In some cases, connection points occur at dc disconnects that are inverter integrated or even external to the inverter. In this article we discuss the location and form of these common connection points and the intelligent use of the equipment options available to PV system designers and installers.
IDENTIFIED AND LISTED FOR THE APPLICATION
Although equipment for making common connections is often anonymously aggregated under the heading "balance of system components" (BOS), the 2008 NEC requires that source circuit combiners be "identified and listed for the application." So from a Code point of view, a combiner box is treated no differently than a PV module or a grid-direct inverter.
Prior to the 2008 NEC, it was not uncommon for PV installers to build their own PV source circuit combiner boxes. Many of these custom built combiners utilized high quality components that were properly rated for their application. However, while the individual components may have been listed, identified and even used properly (given the environmental conditions, system voltage and current, and so forth), the final product itself was not identified and listed as an assembly. Further, not all site built combiners included properly rated components.
John Berdner, groSolar VP of technology, notes that the use of plastic enclosures for combiner boxes is a common mistake. "Solar systems should be designed for a minimum life of 30 years," Berdner observes, "so enclosures should be metallic or fiberglass. Plastic degrades when exposed to sunlight. It gets brittle and will fail." This is precisely the sort of scenario that NEC 690.4(D) seeks to avoid.
For companies accustomed to building their own combiner boxes, the NEC requirements may seem unnecessary and inconvenient. But having combiner boxes independently tested and listed is in the best long-term interests of the PV industry. For example, AMtec Solar, a leading manufacturer of combiner and recombiner boxes, produces industrial grade equipment that is rated to UL 1741. In fact, the company goes one step further by building to a standard it anticipates UL 1741 will require of all combiner boxes in the future: silkscreened labeling, touch safe fuse holders, oversized output lugs, labeled torque values, busbars supported with large insulators and finger safe covers over all live busses. This approach gives AMtec Solar confidence that its products will last as long as a PV installation itself and be just as safe. As Tom Willis, sales manager at AMtec Solar, points out, "The combiner box is just as important as the inverter; if the combiner box fails, so does the system."
DEFINITIONS: IDENTIFIED AND LISTED
The authors of the NEC have a very specific intent in requiring combiner boxes that are "identified and listed for the application." This is clarified in Article 100, which provides specific definitions. When the term identified is applied to equipment, for example, the equipment is recognized as "suitable for the specific purpose, function, use and environment." Per the fine print note that follows, "investigation by a qualified testing laboratory" is one of the main ways that products are evaluated. This process is expanded upon in the definition of listed. Equipment is considered listed when it is "included in a list published by an organization … that maintains periodic inspection of production of listed equipment … and whose listing states that … the equipment … has been tested and found suitable for a specified purpose."