good primer on the major components and the roles they play in the entire system.
Modules — The heart of the PV system is the module, an individual unit that contains a number of PV cells, typically encapsulated behind a sheet of glass with an aluminum frame protecting the edges. A number of PV modules are wired together to produce a desired amount of voltage and current that matches the requirements of the power electronics connected on the other end of the PV array. In grid-direct PV systems, a number of modules are typically connected in series to create a string of modules that can operate at elevated voltages (i.e., up to 600VDC in some situations). Battery-based systems, either stand-alone or utility-interactive, will generally operate at lower voltage levels but can still exceed 150VDC within the string. These PV strings are then connected in parallel to create the PV array.
PV modules are offered in a variety of technologies; however, crystalline silicone modules are the most commonly used. Thin film modules, such as amorphous crystalline or cadmium telluride, are another category of PV modules you may come across.
Racking — Traditional PV modules require a support structure to hold the array in place. There are modules known as building integrated photovoltaics (BIPV) that actually replace the outer shell of a building, but these systems are not as common as PV modules mounted to the exterior of a building or ones mounted at ground level. PV modules can be installed in a variety of locations, and there are racking systems for every conceivable location. The racking systems generally fall into one of three categories: roof mount, ground mount, or top-of-pole mount. Within each of these general classifications, there are multiple subcategories.
Regardless of the mounting classification, racking systems are commonly made of extruded aluminum rails that support the PV modules at four points along the module’s aluminum frame. The exact form the rack’s extrusion takes on is a function of the racking manufacturer’s engineering and final location of the array. Common applications include flush-to-roof racking systems that hold the array parallel to and in close proximity (i.e., within 6 inches) to the roof surface, as shown in the Photo. Another racking type is a ballasted rack, which does not penetrate the roof membrane or ground, and uses weights, such as concrete ballast blocks, to hold the array in place. PV arrays can also be mounted close to the ground on piers that support the racking system, which directly support the modules.
Combiner and junction boxes — Today, PV modules are nearly universally manufactured with conductors pre-installed on the back of the unit with quick-connect plugs on the end. These plugs allow the PV installer to quickly make the series connections without additional tools. This wiring method is allowed per the NEC (Sec. 690.31). Once the conductors leave the vicinity of the array, either through a building, along the exterior of the building, or through a trench in ground-mounted arrays, these source circuit conductors are most often transitioned into a different wire type, such as THWN-2. This transition happens in either a PV combiner box or a junction box. In a combiner, the source circuits are placed in parallel and very often include overcurrent protection. A junction box is used solely as a location to make the transition from the outdoor-rated conductors to the THWN-2 conductors.
The use of a combiner or junction box is a design decision that is made based on the specifics of the PV array location, proximity to the power electronics, and the preferences of the designer and installer. The NEC has requirements for placing disconnecting means in proximity to fused combiner boxes, so this can affect the combiner versus junction box design decision.
For smaller residential installations, there may only be one or two PV source circuits to deal with, and the box chosen is selected for its ability to transition into a raceway and route the conductors down off the roof through the interior 