What is a solar combiner box and why is it used in photovoltaic designs?

What is a solar combiner box?

Combiner box definition: A combiner box is an electrical enclosure that combines, houses, and organizes solar strings. The purpose of a combiner box is to take several solar strings and bring them together to create a single source of power before it goes into the inverter. A solar string refers to a series connection of solar panels. When optimally positioned within the solar PV system, the combiner box will help limit energy losses.

Combiner boxes are required when there are more than three solar strings that need to be connected to the inverter. When working with less than three solar strings, they can be connected directly to the inverter without additional devices. Utility-scale PV plants largely benefit from combiner boxes because they allow PV plant designers to reduce material and labor costs by distributing combined connections without affecting maximum power output.

Combiner boxes require little maintenance as long as they are installed correctly and of high quality. The box should meet the technical requirements for your solar project and conform to UL1741, which is the relevant standard for this type of equipment. Though it is unlikely to need maintenance, it is still recommended to inspect combiner boxes regularly to ensure they are working correctly. A poor quality or faulty combiner box can fail, potentially catching fire and causing major damage to your PV system. When using both DC and AC components in solar combiner installations, it is essential to separate them for safety and compliance with code standards (e.g., NEC 690 in the U.S.). 

How does a solar combiner box work?

A solar combiner box is an essential component in large solar arrays, connecting multiple solar strings to a single inverter. Its primary function is to house the input overcurrent protection fuse assemblies for solar arrays using multiple strings. Surge protection devices (SPDs) prevent excess power from entering the inverter, triggering a shutdown switch during voltage surges.

The combiner box can connect either AC or DC strings but cannot handle both DC and AC types in a single device. It is crucial to separate them for safety and compliance. For AC strings, the combiner box typically features a grid-form contact arc extinguishing structure, while for DC strings, it uses a magnetic blowing form for arc extinguishing. This distinction ensures the safe and efficient operation of the solar array.

A solar combiner box is similar to a junction box, acting as an electrical enclosure that brings together multiple wires and cables through various ports. Despite their compact and straightforward design, solar combiner boxes offer significant advantages by integrating all the wires in one place. They reduce the number of connections to the fuse terminals, which can help engineers during installation and maintenance as they can easily identify each wire.

Why do you need a solar combiner box?

Complex solar arrays need to be carefully organized in order to help engineers install and maintain the system quickly and reduce energy loss. Combiner boxes are an affordable solution for connecting solar strings to an inverter. Without a solar combiner box, engineers must wire multiple strings directly to the inverter, which can be confusing, messy, and potentially unsafe.

Combiner boxes also prevent too much power from entering the inverter. They can be used to quickly shut the system down in the event of a voltage surge, acting as a rapid remote shutdown switch. This also helps save money as it can reduce the need for maintenance and reduces the risk of damaging the inverter.

What are the parts of a solar combiner box?

Solar combiner boxes are relatively simple devices with several key components, including: 

• DC molded case circuit breakers (MCCB) - MCCBs are found in high-power PV systems and help protect circuits under currents between 63A and 630A. Although it is important to note that specific current rating of the MCCB in a combiner box would be based on the system’s design and the size of the array, as different systems require different circuit protection ratings.

• PV String fuses - PV String fuses are designed to interrupt electricity flow during incidents. They help protect the system from overcurrents that could cause damage. Fuses are used to protect individual strings from faults (e.g., short circuits) and should be rated according to the string’s current.

• DC surge protection devices (SPD) - SPDs conduct power surges into the ground to protect the system from damaging transient overvoltages.

• DC disconnect switch or DC circuit breaker - The disconnect switch or circuit breaker will cut power during a surge or short-circuit to protect the rest of the system.

• Busbar - The busbar is typically used to combine incoming negative or ground leads from solar panels. It is a conductive metal strip with multiple connection points that connect incoming wires to a single unit. 

• Terminal Strip - The terminal strip functions like the busbar but is made up of ABS composite instead of metal.

• Enclosure - The combiner box is contained in a box made of PV or ABS material. It should be dustproof, windproof, waterproof and treated to handle ultraviolet light.

The solar combiner box may also have additional components, such as monitoring devices. 

Types of Combiner Boxes

Combiner boxes come in various types, each designed to cater to specific solar panel installation requirements. Understanding the different types can help you choose the right one for your PV system:

• Standard Combiner Boxes: These are the most common type, designed to combine multiple DC inputs from a solar array into a single output. They are ideal for standard solar installations where simplicity and efficiency are key.

• Disconnect Combiner Boxes: These boxes offer the added functionality of disconnecting the solar array at a single point. This feature provides an extra layer of safety, allowing for quick and easy shutdowns during emergencies or maintenance.

• AFCI Combiner Boxes: Arc Fault Circuit Interrupter (AFCI) combiner boxes are designed to detect arc fault signals and interrupt the circuit before faults can develop into fires or short circuits. This makes them an excellent choice for installations where fire safety is a primary concern. In some areas AFCI combiner boxes are a requirement according to code standards in certain jurisdictions, although this is more relevant for residential installations as opposed to utility-scale solar plants.

• AC Combiner Boxes: These boxes are equipped with an integrated AC Molded Case Circuit Breaker (MCCB) for AC input. They are generally used for applications like aggregating AC circuits from inverters in larger systems or handling multiple inverters. They also include Type 2 AC Surge Protection Devices (SPD) for overvoltage protection on inverters, making them suitable for systems that require robust AC protection.

By selecting the appropriate type of combiner box, you can enhance the safety, efficiency, and reliability of your solar panel installation.

Safety Considerations

Safety is paramount in solar panel installations, and combiner boxes play a crucial role in safeguarding the entire system. Here are some key safety considerations to keep in mind:

• Surge Protection: Ensure your combiner box is equipped with Surge Protection Devices (SPDs) to shield the system from power surges and lightning strikes. This helps prevent damage to the solar panels and other components.

• Overcurrent Protection: Combiner boxes should include overcurrent protection devices, such as fuses or circuit breakers, to prevent electrical overloads that could lead to fires. These devices interrupt the flow of electricity when current levels exceed safe limits.

• DC Isolation: To prevent electrical shock and ensure safe maintenance, combiner boxes should have DC isolation features. This allows for safe disconnection of the DC circuits during repairs or inspections.

• Quality Control: Always choose a reputable brand and ensure that the combiner box meets industry standards and regulations. High-quality combiner boxes are less likely to fail and can provide reliable protection for your PV system.

By prioritizing these safety features, you can ensure that your solar panel system operates safely and efficiently.

Installation and Maintenance

Proper installation and maintenance of combiner boxes are essential for the safe and efficient operation of your solar panel system. Here are some key considerations:

• Installation Time: Combiner boxes should be installed in a manner that minimizes installation time and reduces the risk of errors. Efficient installation practices can save time and reduce labor costs.

• Wiring: Correct wiring is crucial to prevent electrical shock and ensure safe operation. Make sure all connections are secure and follow the manufacturer’s guidelines to avoid potential hazards.

• Breakers: Ensure that the combiner box includes breakers that are compatible with your solar panel system and meet industry standards. Properly rated breakers help protect the system from overcurrents and short circuits.

• String Monitoring: Combiner boxes with string monitoring features allow you to track the performance of individual strings. This helps in identifying issues quickly and ensures that the solar panel system is operating at peak efficiency.

• Maintenance: Regular maintenance is vital to keep the combiner box and the entire solar panel system functioning safely and efficiently. Periodic inspections can help identify and address potential issues before they become major problems.

By following these installation and maintenance guidelines, you can ensure the long-term reliability and safety of your solar panel system.

How to place your combiner box with RatedPower software

Correctly positioning your combiner box will help to reduce energy loss. RatedPower’s software will identify which structures share a combiner box and look to see which structure is positioned best to reduce the length of wire required to link up with the inverter. Once the optimal structure is chosen, the string box is located at the edge of the most centered structure with respect to its cluster.

You can learn more about how RatedPower considers combiner boxes and how RatedPower minimizes cable length at every stage to help PV plants achieve their maximum output in our guide to cabling topology.