PV Combiner Box Buyer's Guide
Combiner boxes are an integral part of many PV installations, serving as the “meeting place” where the wiring from array series strings come together in parallel connections.
In all but the smallest PV systems, modules are wired together in series strings, where the positive leads of one module are connected to the negative leads of the next module. This results in cumulative voltage output, with current (amps) staying the same. The box where the output wires from multiple series strings are joined is the combiner box. Rated for outdoor use, it contains overcurrent protection devices (OCPDs) and the necessary bus bars and terminals for combining the inputs.
Many installers used to build their own combiner boxes for lack of availability, but now there are plenty of commercial products to choose from. Some manufacturers will custom-build combiner boxes to meet specific system requirements. Article 690.4(D) of the 2008 National Electrical Code includes PV combiner boxes as equipment that must be identified for appropriate use and listed by an approved testing laboratory, such as Underwriters Laboratories (UL) or ETL labs. The days of “homemade” combiner boxes are quickly disappearing.
If you live in a locale where the 2008 NEC is in effect, your electrical inspector will likely require one that is listed by an approved testing laboratory, as mandated by NEC Article 690.4(D).
Every inverter and charge controller, whether in a utility-interactive or stand-alone system, has a DC input voltage window that must be adhered to. In batteryless grid-tied systems, inverters require relatively high DC input voltages, from 150 to 600 V (typically 7 to 13 modules wired in series).
Small grid-tied PV systems (less than 5 kW) often have only one or two series strings of modules. With few wires and no need for series fusing, a PV combiner box isn’t necessary: Strings can be terminated directly in the inverter (see the “Why Series Fusing?” sidebar). However, larger residential systems (between 5 and 10 kW) often have three or more series strings, and will likely need fusing. Systems above 5 kW also typically have more wires to deal with, which can mean more installation time, larger conduit, and increased wire costs if all the wires need to be run to the inverter. A combiner box offers a place to house series fuses and parallel the series string inputs, reducing the number of array output wires needed to run to the inverter location.
Some grid-direct inverter manufacturers include series fusing within the inverter or its attached disconnect box, which may eliminate the need for a separate combiner box. However, using a PV combiner box located close to the PV array gives you easy access to the wires for each series string. This can be handy for troubleshooting a malfunctioning PV array without having to run back and forth to the inverter area.
Let’s look at a grid-tied PV system with three series strings of 10 modules each. Three series strings mean three sets of wires—three positives and three negatives, with one ground wire—coming from our PV array. Even though some inverters have multiple DC inputs, they may not include series fusing capability. Then it’s likely that series fusing will be needed. Even if all the necessary input fusing is provided within the inverter, a combiner can be used to eliminate running seven wires all the way from the PV array to the inverter—possibly reducing wiring costs and installation time.
Battery-based systems (off-grid and grid-tied with battery backup) have lower DC system voltages than grid-direct systems (although higher-voltage charge controllers are expected to hit the market soon). Historically, this voltage ranged from 12 to 48 V nominal to match battery voltages, but step-up/down MPPT charge controllers now allow arrays with higher voltages to charge lower-voltage battery banks, making higher array voltages more common (with open-circuit voltages usually limited to 150 V). Even with the use of step-up/down charge controllers, typically only one to six modules are placed in a series string, which means combining more strings in parallel to get the desired power output, which necessitates a combiner box.
In addition, charge controllers typically have only a single set of input wire terminals. For off-grid systems, it’s often wise to have enough room in the combiner for adding more strings of modules in the future. If the combiner and its output wires are sized accordingly, and the charge controller has been sized to handle the additional amperage, wiring more PV modules into the system can be as easy as terminating the new wires in the PV combiner box.
The “Specifications” table on the following pages summarizes the main features common to most PV combiner boxes. We included products that are currently available to consumers in the U.S. market.
Maximum DC Voltage (Max. VDC)
Each combiner is rated to accommodate up to a specific DC voltage. Residential grid-direct systems generally accommodate DC voltages up to 600 V, while most battery-based PV system DC voltages are lower, typically maxing out at 150 V.
Maximum Number of PV Input Circuits
Combiner boxes come with a certain number of terminals where PV source wires can be attached and an equivalent number of OCPD spaces. Residential PV combiners usually need no more than 12 inputs. Grid-direct systems require fewer inputs, while battery-based systems can require more. Additionally, battery-based PV systems commonly have multiple charge controllers, and each may have its own combiner box.
Maximum OCPD Rating
The overcurrent protection devices (OCPDs—fuses or circuit breakers) chosen for the particular combiner cannot exceed this rating. PV module nameplates list the required series fuse rating.
Combiner boxes come with OCPDs, usually either a form of “touch-safe” fuse holders or DC-rated circuit breakers. Since grid-tied systems can generate up to 600 VDC—well above the 150 VDC rating of most available circuit breakers—fuses are almost always necessary. OCPDs found in combiner boxes are generally limited to battery-based systems. Most fuse holders are not rated to be opened under load, so they cannot be used as a DC disconnect. This is an especially important consideration for high-voltage DC systems: Trying to open a fuse holder without first opening the circuit (which can be safely done via the main DC disconnect), will likely draw an electrical arc that can cause a fire and burn and/or shock you.
Wire Range: Size of Input & Output Terminals
Wire terminals are rated to fit a range of wire sizes, in both bus bars and the OCPDs. Because the output circuit is carrying the combined ampacity of all the incoming series strings and also may have to be oversized due to voltage drop for longer wire runs in lower-voltage systems, it may be necessary to connect a relatively large-diameter wire to the output terminals.
Maximum Number of Output Circuits.
Sometimes it is appropriate to use multiple inverters or charge controllers with a single array—like two of four strings into one inverter and the other two into a second inverter. For this, you’ll need either multiple combiners, or one combiner that can accommodate two separate output circuits. Of all the combiners listed, only a handful offer two output circuits.
Maximum Continuous Output Current
Each combiner has a rating for the amount of output current that it can safely handle. Depending on the size and number of OCPDs in use, it may be possible to overload the combiner’s bus bars. For example, in a combiner with sixteen 20 A circuit breakers, 320 A could flow into the output circuit, overloading the bus bar if it is rated at 100 A. To ensure this does not happen, use a combiner with a high enough output rating.
Data Monitoring Capabilities
While individual string monitoring is generally used in commercial, rather than residential-scale, systems, some homeowners are interested in tracking the performance of each string of PV modules to ensure the system is working optimally. Combiner boxes with data monitoring capability are often referred to as “smart” or “intelligent” combiners, and allow easy, quick installation of data monitoring systems.
Most combiner boxes are installed near their PV arrays (that is, in the weather), and require appropriate outdoor ratings. Installations are expected to last at least the lifetime warranty of PV modules (about 25 years), so installing durable, long-lived equipment that will stand up to the environment in which it is placed is critical.
Every approved electrical enclosure has a National Electrical Manufacturers Association (NEMA) rating associated with it. The ratings on PV combiners include NEMA 3 and 3R (rated for outdoor use in limited orientations); and NEMA 4 and 4X (includes additional protection so boxes can be mounted in any orientation, from vertical to horizontal). Wiring must enter the box through a cord grip that will properly seal the penetration and uphold the weatherproof rating of that combiner box.
The NEC requires that all electrical equipment have a valid safety-test listing to ensure safety for the general public. In the United States, most of this testing is performed by UL, an independent nonprofit, although there are other independent laboratories that will test to UL standards. One such lab is Intertek, which offers the ETL-listed mark. All of the PV combiner boxes in the table have a current or pending listing.
Lena Wilensky is co-owner of Nunatak Alternative Energy Solutions in Crested Butte, Colorado. She manages to take time out from playing in the mountains to design and install PV and remote power systems, and instruct PV classes for Solar Energy International.
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