er’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 of the building. In large commercial applications, there can be hundreds of circuits located within the array that require dozens of boxes to be strategically located and specified to minimize conductor size and overall length.
Power electronics — The output of the PV array will be connected to some form of power electronics. In systems that employ battery backup, both utility-interactive and stand-alone, the array will connect to a charge controller before going into the batteries. These battery-based systems will also use one or more inverters to then invert the energy stored in the batteries to AC for use by the loads. For grid-direct PV systems, the array will go directly into an inverter that will invert the DC energy from the array into AC for interconnection with the electric utility. An example wiring diagram for grid-direct systems is shown in Fig. 3 (click here to see Fig. 3).
Charge controllers — Charge controllers are only necessary for systems that use batteries. The primary purpose of a charge controller is to keep the battery bank from becoming overcharged, which could damage the batteries. Today’s charge controllers also have multiple auxiliary features that can be used to do things like engage relays and power up loads when the battery bank reaches meets certain conditions. One of the most common applications is a relay that sends power to a vent fan when the batteries are charging so the buildup of hydrogen gas does not occur.
In stand-alone systems, when the battery bank has been fully recharged, it is the charge controller’s job to isolate the PV array from the battery bank by opening the circuit and stopping all current flow from the PV array. For PV systems that use battery storage as a form of backup to the electric utility, the charge controllers are now manufactured with the ability to “talk” to the inverters connected to the other side of the battery bank and make sure the PV array is able to send as much energy back into the electric utility when possible.
Inverters — The inverters used