Photovoltaic (PV) combiners accumulate voltage from a series string of PV panels, as well as from multiple strings, and create a summed DC output with overcurrent protection. PV combiners are also known as solar combiners.
The charge collected by PV panels in an array must be unified before it can be harnessed. As a result, PV combiners are commonly found in two locations:
String combiners collect current from a series-circuit string, which is a group of wired PV panels.
Array combiners create a cumulative output from multiple series strings. They can handle higher loads than string combiners. This combiner contains series fuses and parallels these inputs, reducing the output wiring to the charge controller or inverter. These are also called recombiners.
A combiner provides overcurrent protection, reduces system installation time and material costs, and makes identifying and repairing malfunctioning strings or panels more convenient. Many also transition from expensive USE-2 or PV wires to less costly and more manageable types, such as THWN-2.
Only small- and medium-sized grid-tied PV systems can safely eliminate a PV combiner. PV arrays which generate less than 5 kW and have only one or two panel strings do not require current overload protection. Instead, transitions between wire types take place in a pass-through junction, and these circuits can then terminate at the inverter or controller. Some inverters are capable of combining circuits before converting the current into AC, and these are known as input converters.
Customized PV combiners are frequently produced because of the variability in PV system design; until recently all combiners were completed in this manner.
Effective solar combiners commonly include the following.
DC disconnect: This switch discontinues power transfer between busbars, but does not eliminate charge from the system. Manufacturers have begun integrating this component into the combiner, but some building codes require DC disconnects to be placed near AC equipment or within the structure. Regardless, the DC disconnect must be placed in an easily accessible location. Manual and remotely actuated disconnects are available.
Enclosure: This part protects the electrical connections from contaminants and must have a service life at least as long as the components within. Combiners used outdoors must withstand environmental variables such as moisture, heat, cold, ice, sunlight, dirt, wind, insects, and animals. These housings are classified by NEMA ratings, which indicate the enclosure's resistance to such outdoor conditions.
Type 3 enclosures protect against rain, sleet, snow, and windblown debris, and are undamaged by accumulated ice.
Type 3R protects components from rain, sleet, snow and are undamaged by accumulated ice.
Type 3S enclosures protect equipment from rain, sleet, snow, and windblown debris. External mechanisms, such as a disconnect, remain operable even after accumulating ice.
Type 4 guards against rain, sleet, snow, windblown debris, and splashing and hose-directed water. These enclosures are not damaged by ice formation.
Grounding terminal: By connecting the ground to the enclosure or other article, stray currents are safely routed away from combiner components.
Knockouts: These pre-cut perforations provide installers with conduit access, and hole plugs close any gaps between the enclosure and wire.
Lightning arrestor: While this can be implemented at other points in the PV system, manufacturers are routinely integrating this component into solar combiners. By decreasing the clamping voltage to a safe level, system components will not be damaged by stray currents from lightning strikes.
Overcurrent protection device (OCPD): Most combiners rely on fuses since capacities often exceed the voltage capabilities of circuit breakers. However fuse holders cannot be opened under load, so a DC disconnect must be included in the PV system.
Power distribution blocks: The conductors meet at designated busbars located within the combiner. These can be single, double, or triple pole, and typically route multiple inputs into a cohesive output.
Positioning the combiner near the array is optimal as it reduces wiring efforts and circuit resistance, but difficulties often arise since the combiner should be located where it will encounter the least amount of environmental adversity. It must also be easily accessed for conduit installation and system maintenance.
Ideally, combiners will face north and be light in color to reduce the amount of sunlight they receive and absorb. Though the enclosures themselves are well-suited for high temperatures, fuses and other internal equipment are more sensitive.
Combiners should also be oriented so wires are routed below the terminals and through knockouts which are directed downwards; these measures prevent the ingress of water. Combiners are frequently mounted on racks which support PV panels, which is acceptable provided the device obeys the aforementioned principles and is not affected by peripheral heat from the roof, ground, or PV panel. When possible, it is recommended to run PV circuits inside a structure and place the combiner box at an accessible indoor location.
Dead front: A polycarbonate or acrylic shield allows access to the fuses or breakers, but prevents user contact with terminals and live wires.
Lockable: An integral or aftermarket lock prevents unauthorized access to the combiner. Some combiners utilize screws or security screws.
Segmented disconnections: Combiners with multiple disconnect mechanisms allow selected circuits to be interrupted for maintenance.
Smart: PV combiners with a data connection facilitate real-time monitoring of the system.
Ventilated: An active or passive exhaust system enhances the exchange of air between the enclosure interior and exterior, which is useful in humid climates.
To find the maximum combiner voltage rating for a given system, first determine the system voltage of a panel string by multiplying the open-circuit voltage of a single panel by the number of panels in the string, times a temperature correction factor. While this coefficient is supplied by panel documentation, according to NEC Table 690.7 the worst-case correction factor is 1.25.
Maximum Output Current
Determine the series fuse rating for a PV panel, and multiply by the number of series strings. This product is the maximum current and the minimum acceptable rating value for the combiner.
Maximum Fuse Size
The limited ampacity of each fuse, typically one per input circuit.
The limited ampacity of the electrical disconnect.
Input: Quantity and Wire Gauge
Manufacturers will identify the number of knockouts on each PV box, which determines the number of circuits which the box can safely manage. Also identified will be a range of acceptable wire gauges.
Output: Quantity and Wire Gauge
The number of output circuits which have been consolidated from the input circuits. The output will be a larger diameter and higher voltage wire gauge. This serves to make an installation less costly and easier.
This is the volume of the PV combiner enclosure. Large enclosures make installation of conduits and regular maintenance easier. An increased interior air volume and enclosure surface area assists in the temperature regulation of electronic components.
The range of suitable temperatures for combiner circuits. Manufacturers usually specific this value in documentation, with 50° C and 60° C maximum temperatures being most common.
State and local laws in the United States often require structures to follow the National Fire Prevention Association's established National Electrical Code outlined in NFPA 70. In regards to PV systems, article 690 specifically covers overcurrent protection devices. Canadian Standards Association Code C22.1 Section 50 regulates PV systems for provinces and territories. British Standard 7671 actually exceeds the electrical system requirements set forth by Building Regulations Part P.