To determine the appropriate solar combiner box specifications, the design configuration of the entire system must be analyzed. The number of solar panels (or component strings), electrical parameters (such as open circuit voltage, short circuit current), and input parameters and number of inverters are the core basis for determining the combiner box selection.
PV Combiner Box is a key component in the PV system that connects the PV strings to the inverter. Its main function is to collect the DC power from multiple PV strings and provide overcurrent protection and isolation. Selecting the right size of combiner box is crucial for the safe and efficient operation of the system. This guide will explain in detail how to select the right combiner box based on system parameters.
Let’s dive into it!
Steps To Size A Solar Combiner Box
Following are the steps you need to follow to select the right combiner box.
Step 1: Determine The Maximum System Voltage
First of all, you need to determine the maximum amount of voltage produced by the system. It is calculated in the following way:
-
Locate Parameters: Find the Open-Circuit Voltage (Voc) y el Temperature Coefficient of Voc on the photovoltaic (PV) module’s (solar panel’s) technical datasheet.
PV Module Electrical Data
PV Module Temperature Characteristics
-
Determine Lowest Expected Ambient Temperature: Identify the historically lowest ambient temperature recorded for the installation location. This is critical because PV module voltage increases as temperature decreases.
- Calculate Temperature-Corrected Maximum Module Voltage: Use the following formula to find the maximum voltage a single module can reach at the lowest temperature:
Maximum Module Voltage (Vmax_module) = Voc × [1 + (βVoc / 100) × (Lowest Expected Ambient Temperature – STC Temperature)]
Note: The Standard Test Conditions (STC) temperature is typically 25°C. Ensure the units for βVoc are correctly applied (it’s often given in %/°C and is usually a negative value). If βVoc is given in V/°C, the formula adjusts accordingly: Vmax_module = Voc + [βVoc × (Lowest Expected Ambient Temperature – STC Temperature)].
-
Calculate Maximum String Voltage (Maximum System Voltage): Multiply the maximum voltage of a single module by the number of modules connected in series within one string:
Maximum System Voltage (Vmax_system) = Vmax_module × Number of Modules per String
-
Select Combiner Box Voltage Rating:
-
The DC voltage rating of the combiner box, as well as all components within it (such as fuse holders, circuit breakers, and the disconnect switch), must be higher than or equal to the calculated Maximum System Voltage (Vmax_system).
-
The common standard DC voltage ratings for combiner boxes include 600Vdc, 1000Vdc, and 1500Vdc. Select the standard rating that meets or exceeds your calculated Vmax_system. Ensure compliance with local electrical codes and standards at all times.
-
Step 2: Determine the rating of the string overcurrent protection device (OCPD)
-
Find the parameters: Find the short-circuit current (Isc) y the maximum series fuse rating (Maximum Series Fuse Rating) from the technical data sheet of the PV module.
-
Calculate the minimum rated current of the OCPD required for a single string:
-
According to industry specifications (such as NEC 690.9(A)), the rating of the string protection device (fuse or DC circuit breaker) is usually based on the short-circuit current (Isc) of the module and multiplied by a safety factor. The commonly used calculation method is:
-
Minimum OCPD rating = Isc × 1.56 (This 1.56 factor is based on the NEC’s 1.25 times continuous load and an additional 1.25 times the PV output, that is, 1.25 × 1.25 = 1.56, which is used as the basis for line current carrying capacity and OCPD selection calculation). In some simplified or specific standards, it is also possible to directly use Isc × 1.25 as the minimum rating reference. It is strongly recommended to follow local electrical code requirements.
-
-
-
Select a standard OCPD rating:
-
Select a standard fuse or circuit breaker rating that is higher than or equal to the minimum rating calculated in the previous step.
-
Important constraint: The selected OCPD rating must not exceed the maximum series fuse rating indicated on the module data sheet. If the calculated result exceeds this value, there may be a problem with the system design (for example, improper module selection or string configuration).
-
-
Determine the number of OCPDs required in the combiner box:Equal to the number of PV strings that need to be connected to this combiner box in the system.
Step 3: Determine the total rated current of the combiner box
-
Calculate the total output current of the combiner box:
-
The theoretical maximum continuous output current is approximately: number of strings × maximum power point current (Imp) of a single string.
-
A more conservative or OCPD-based consideration is: number of strings × rated current of the selected single string OCPD (this method takes into account the capacity of the protection device).
-
When selecting, the combiner box will have a maximum rated current that cannot be exceeded. This rating must be higher than or equal to the total current that the combiner box is expected to carry.
-
-
Check the combiner box current rating:
-
Make sure the maximum current rating marked on the selected combiner box meets the requirements. For example, if a combiner box is designed to accommodate 6 strings, and each string uses a 15A fuse, then the total current carrying capacity of the combiner box should be at least 6 x 15A = 90A, or based on Imp calculation and considering a certain margin.
-
Step 4: Select other characteristics of the combiner box
-
Enclosure type and protection level (NEMA/IP Rating): Select the appropriate protection level based on the installation environment (indoor/outdoor, humidity, dust, corrosiveness, etc.), such as NEMA 3R, 4X or the corresponding IP level.
-
DC Disconnect Switch:
-
According to regulations and design requirements, the combiner box usually requires a main DC disconnect switch or main circuit breaker to safely disconnect the connection between the combiner box and the inverter.
-
The rated voltage of this switch must be higher than or equal to the maximum system voltage (Vmax_system).
-
Its rated current must be higher than or equal to the total expected output current of the combiner box (usually calculated based on the total Imp or total OCPD capacity with margin).
-
-
Other components: It may also include surge protection devices (SPDs), monitoring units, etc., to ensure that the ratings of these components also meet the system requirements.
-
Physical size and access method: Make sure that the box has enough space to accommodate all necessary components (fuse holders, circuit breakers, spd, terminal blocks, disconnectors, etc.), and consider the access method and size of the cables.
Summary and verification:
Before finalizing the combiner box model, be sure to check the following points:
-
En DC voltage rating of the combiner box and all internal components ≥ the maximum system voltage (corrected for temperature).
-
The rating of a single string protection device (OCPD) is calculated based on Isc × safety factor, rounded up to the standard value, and ≤ the maximum series fuse rating of the component.
-
En total rated current of the combiner box is ≥ the expected maximum output current (based on total Imp or total OCPD capacity).
-
En voltage and current ratings of the main disconnect switch (if any) meet the requirements.
-
En protection level of the enclosure is adapted to the installation environment.
-
Complying with local electrical codes and standards (such as IEC) is the most important principle.
Things To Consider
-
Need For Monitoring Capabilities
You can monitor the performance of each string or the entire system by selecting a combiner box with integrated monitoring capabilities. Additionally, in some combiner boxes you can remotely control the output of each string. This amazing feature is extremely useful for troubleshooting and controlling massive solar farms.
-
Keep Expansion In Mind
If you plan to expand your solar array in the future, you should consider purchasing a combiner box that has extra input slots than you currently need. It will save your time, money, and effort and you can conveniently connect additional strings in the same combiner box when needed.
-
Budget
Budget is an important consideration while choosing the right combiner box size for your system. Combiner boxes with advanced features like monitoring capabilities and remote handling , are more expensive than simple ones.
Depending on your needs, you can opt for a simpler combiner box if it is a residential project. However, for commercial installations, safety and longevity are more important than a few dollars. To minimize downtime and maintenance costs, choose a combiner box that is rated higher.
LETOP Standard Combiner Box Sizes
LETOP ofrece múltiples series de cajas combinadoras solares, cada una de ellas diseñada específicamente para condiciones de instalación específicas y disposiciones comunes. No importa en qué condiciones se utilice su proyecto solar, será más fácil y eficiente.
-
600V DC Combiner Box – Small Combiner Boxes
600V DC combiner box is LETOP’s economical solution designed for small PV systems. It offers 1-2 string inputs and 1-2 string outputs. It supports 600V DC system voltage. It also uses high-quality components to provide overcurrent and overvoltage protection. This string box is suitable for 5kW-10kW small residential rooftop or small commercial PV systems.
-
1000V DC Combiner Box – Medium Boxes
1000V DC combiner box is specifically designed for professional combining solutions. It supports 1-4 string inputs and 1-3 string outputs. The maximum support for the 1000V DC system voltage is provided. IP65 protection rating and optimized heat dissipation design ensure your system operates reliably in complex environments. You can also choose an optional smart monitoring system. This string box fully meets the requirements for 20kW-50kW large residential rooftop and commercial rooftop PV projects.
-
1500V DC Combiner Box – Larger Combiner Boxes
1500V DC combiner box is a professional solution designed for large ground power stations. The combiner box can support a maximum of 24 string configurations and is suitable for voltages up to 1500V DC. It features industrial-level protection design, including string monitoring, RS485 bus/standard MODBUS-RTU protocol, arc fault detection, and fast shutdown/fireman switch functions. High-quality components and copper bus materials are used to guarantee stable performance. Furthermore, intelligent thermal spacing design maximizes heat dissipation and enhances equipment operational efficiency.
Palabras finales
According to your system requirements, you can choose a suitable-sized combiner box. The size is usually referred to by the number of string inputs the box has. The box also includes fuse/breaker holders, SPD spaces, and slots for other components. All that needs to be taken into account is that the current and voltage ratings of every component must match the current and voltage of your system. Moreover, you can also avail the customization options offered by the professional manufacturer for your unique system installation.
