Solar Power Combiner Boxes

What is a Solar Combiner Box?

 In larger solar photovoltaic (PV) systems, multiple solar panels are connected in series in a string to increase the voltage before going to the inverter. Multiple strings of the solar panels are also combined together in parallel to produce higher output currents. A solar power combiner box is a device that combines the output of multiple strings of solar panels for connection to the inverter. This reduces the number of wires that need to go to the inverter. The solar combiner box houses the input over current protection fuses for several strings and the combined junction of the inputs into a larger capacity single main output set of wires.

 Most Common Types of Solar Combiner Boxes

  • Fused Solar Combiner Boxes
  • Pre-wired Solar Combiner Boxes
  • Non-Fused Solar Combiner Boxes
  • Dual Fused Solar Combiner Boxes
  • Solar Combiner Boxes with Circuit Breakers
  • Multi String Solar Combiner Boxes

There are several key components that make up a solar combiner box. Let's start by talking about the enclosure types used for solar combiners. Most solar string combiners are designed for outdoor use. They most often have NEMA 3R, 4 or 4X ratings, with NEMA 3R being the minimum requirement for outdoor use. A better choice would be a solar combiner with a NEMA 4 or NEMA 4X rating. These boxes are available in many different types of material such as non-metallic plastic, fiberglass and painted steel In higher humidity areas, or areas with a large variance in humidity, you should add a breather vent. The enclosure is the highest cost single item in the string combiner design. Mounting your combiner on a north facing wall in the shade will reduce heat build up within the box. This is recommended to extend the life of the components inside the unit. Size can also have an impact. Comparing two combiner designs with the same internal components, the combiner in the larger enclosure allows the larger air volume and surface area inside the enclosure to assist in better cooling of internal components. Smaller also has a benefit in providing a more compact installation but a breather vent should be installed on smaller combiner boxes. Output wire bend radius is also important in solar combiner box design. Both the NEC (National Electric Code) and UL (Underwriters laboratory) have requirements for wire bend radius spacing inside a solar string combiner. There are both opposite wall and adjacent wall bend radius requirements that are determined by the terminal or lug size where your output wire is connecting. Bending conductors in the field is time consuming and can be costly, using a combiner box that provides a straight line to the output connection without a bend is preferred by most installers.

A solar combiner box (or string combiner) is simply a electrical component for combining and housing the solar power cables coming from your solar panels. Combiner boxes are usually made of sheet metal, plastic or fiberglass. If you are going to use one then it will usually be mounted on the roof where the wiring exits from the solar panels. Solar panels are usually wired together in a series string of anywhere between 6 to 10 panels. The exact number of panels wired in the string is usually dependent upon the requirements of your inverter. The main function of the box is very straight forward. Multiple strings of solar panels are combined in the box and then a single set of wires (positive, negative and ground) come out of the combiner box to the inverter.

A Solar Combiner Box will usually have the following components:

Main Enclosure
The solar combiner box will have either a metal, plastic or fiberglass enclosure that holds the components and wiring. The enclosure protects the components and wires from the sun and moisture. Most combiner enclosures have a NEMA rating of 3R, 4 or 4X. Enclosures used for solar combiner boxes should be UL listed. Most combiner box enclosures will either include a latch or side hole for locking the box. One feature that you should look for in the combiner box is what is called a dead front.

Over-current Protection (Fuses or Circuit Breakers)
All combiner boxes will have either fuses or circuit breakers. The number depends upon how many strings your box can accommodate. These protection devices prevent too much current being sent to the inverter if there is a short in the system. Fuses are more common than circuit breakers in combiner boxes. This is because most strings in a solar system can get up to 600 volts and over. Most DC circuit breakers are only rated up to 150 volts. Fuse holders used in solar combiner boxes are rated for 600-1000 volts. A normal setup will have one fuse holder for each string coming from your system.

Junction Blocks
Junction blocks used in combiner boxes are electrical connectors used to attach solar cables in the same circuit without having to cut or splice the solar cables wires. This type of junction blocks provide an easy way to combine all the solar input strings input into just one set of output cables requiring higher-current capacity.

Earth Ground Terminals
Electrical power distribution systems are often connected to ground to limit the voltage that can appear on distribution circuits. A distribution system insulated from ground may attain a high potential due to transient voltages caused by arcing, static electricity, or accidental contact with higher potential circuits. A ground connection of the system dissipates such potentials and limits the rise in voltage of the grounded system.