What is a combiner box?
The role of the combiner box is to bring the output of several solar strings together. Daniel Sherwood, director of product management at SolarBOS, explained that each string conductor lands on a fuse terminal and the output of the fused inputs are combined onto a single conductor that connects the box to the inverter. “This is a combiner box at its most basic, but once you have one in your solar project, there are additional features typically integrated into the box,” he said. Disconnect switches, monitoring equipment and remote rapid shutdown devices are examples of additional equipment.
Solar combiner boxes also consolidate incoming power into one main feed that distributes to a solar inverter, added Patrick Kane, product manager at Eaton. This saves labor and material costs through wire reductions. “Solar combiner boxes are engineered to provide overcurrent and overvoltage protection to enhance inverter protection and reliability,” he said.
“If a project only has two or three strings, like a typical home, a solar combiner box isn’t required. Rather, you’ll attach the string directly to an inverter,” Sherwood said. “It is only for larger projects, anywhere from four to 4,000 strings that combiner boxes become necessary.” However, combiner boxes can have advantages in projects of all sizes. In residential applications, combiner boxes can bring a small number of strings to a central location for easy installation, disconnect and maintenance. In commercial applications, differently sized combiner boxes are often used to capture power from unorthodox layouts of varying building types. For utility-scale projects, combiner boxes allow site designers to maximize power and reduce material and labor costs by distributing the combined connections.
The combiner box should reside between the solar modules and inverter. When optimally positioned in the array, it can limit power loss. Position can also be important to price. “Location is highly important because a combiner in a non-optimal location may potentially increase DC BOS costs from losses in voltage and power,” Kane explained. “It only constitutes a few cents per watt, but it’s important to get right,” Sherwood agreed.
Little maintenance is required for combiner boxes. “The environment and frequency of use should determine the levels of maintenance,” Kane explained. “It is a good idea to inspect them periodically for leaks or loose connections, but if a combiner box is installed properly it should continue to function for the lifetime of the solar project,” Sherwood added.
The quality of the combiner box is the most important consideration when selecting one, especially since it’s the first piece of equipment connected to the output of the solar modules. “Combiner boxes are not expensive compared to other equipment in a solar project, but a faulty AC combiner box can fail in a dramatic way, involving shooting flames and smoke,” Sherwood warned. “All should be third-party certified to conform to UL1741, the relevant standard for this type of equipment,” Sherwood said. Also be sure to pick a combiner box that meets the technical requirements for your project.