When a PV system contains multiple solar strings, the DC outputs of the strings need to be centralized before connecting to the inverter. A Caja combinadora de CC provides a centralized connection point for these strings and can be configured with the required protection and isolation devices according to the system design.
Connecting multiple solar strings is not simply connecting the positive and negative cables together. Each string must have correct polarity, compatible electrical characteristics, and appropriate overcurrent protection. The combined output must also comply with the voltage and current limits of the downstream equipment.
This article will introduce how to connect multiple solar strings in a combiner box, and what needs to be checked before PV system commissioning.
How Do Multiple Strings Work in a PV System?
A solar string consists of multiple PV modules connected in series. Multiple strings can then be connected in parallel through a DC combiner box.
Before using a combiner box, it is important to understand the difference between series connection and parallel connection.
Series Connection: Increasing Voltage
In a series connection, the positive terminal of one solar module is connected to the negative terminal of the next module, thereby forming a PV string.
The voltage of each module gradually accumulates, while under the same operating conditions, the string current remains basically consistent with the current of a single module.
For example, if 10 solar modules each have an operating voltage of 40V, the operating voltage of the string is approximately 400V.
Parallel Connection: Aggregating Current
In a parallel connection, multiple PV strings are connected to common positive and negative circuits respectively.
The voltage across parallel strings remains basically consistent with the voltage of a single string, while the total current is equal to the sum of the currents of each string.
For example, if 4 identical PV strings each have an operating current of 12A, the total operating current after parallel connection is approximately 48A.
The DC combiner box provides connection terminals for these strings, centralizes the outputs of each string, and delivers the combined DC power to the inverter.
How to Connect Multiple Solar Strings in a Combiner Box?
Taking a 4-in-1-out combiner box as an example, it can intuitively illustrate how multiple PV strings are combined.
Step 1: Identify the Positive and Negative Cables
Each PV string has two output conductors:
Positive (+)
Negative (−)
Label each string separately, for example String 1, String 2, String 3, and String 4.
Labeling the conductors helps avoid cross-wiring during installation.
Step 2: Check String Voltage and Polarity
Before connecting the cables, the open-circuit voltage (Voc) of each string should be measured.
Compare the measurement results with the expected design values and the maximum DC input voltage of the inverter.
The following items need to be checked:
- Whether the positive and negative poles are correct
- Whether the string open-circuit voltage meets the requirements
- Whether the voltages between strings of the same configuration are consistent
- Whether the rated parameters of the combiner box and inverter are met
- The maximum string voltage calculation must consider the rise in module open-circuit voltage under low-temperature environments
Step 3: Connect Each String to the Corresponding Input Circuit
Lead the positive and negative conductors of each PV string into the corresponding input circuit respectively.
For a 4-string combiner box, the connection method is as follows:
| PV String | Positive Input | Negative Input |
| String 1 | PV1+ | PV1− |
| String 2 | PV2+ | PV2− |
| String 3 | PV3+ | PV3− |
| String 4 | PV4+ | PV4− |
Ensure that the positive and negative conductors of each string always correspond and are not cross-connected.
At the same time, check whether the conductors are suitable for the input terminals, and complete the connection according to the tightening torque specified by the manufacturer.
Step 4: Install DC String Fuses
When the system requires independent string overcurrent protection, each string should be connected to the designated fusible circuit.
Fuse selection needs to consider the following factors:
- Maximum series fuse rating of the PV module
- String short-circuit current
- Maximum DC voltage of the system
- Cable current-carrying capacity
- Reverse current that may be generated by parallel strings
- Applicable electrical standards
The fuse rating must not exceed the maximum series fuse rating specified by the PV module manufacturer.
The number and installation location of fuses depend on the system grounding method. Some designs require protection on both non-grounded conductors, while other configurations use different protection methods.
PV-specific DC fuses suitable for the corresponding voltage and current ratings should be used.
Step 5: Connect the Positive and Negative Busbars
After each string passes through the designated protection circuit, its conductors are collected to the common busbars.
The positive conductors of each string are connected to the common positive busbar, and the negative conductors are connected to the common negative busbar.
This connection method can aggregate the currents of all parallel strings while maintaining correct DC polarity.
The current-carrying capacity of the busbar must meet the calculated design current requirements, and its terminals must also match the selected conductor specifications.
Step 6: Connect the SPD and DC Isolating Switch/Circuit Breaker
Install the DC SPD according to the wiring diagram provided by the manufacturer and the system grounding method.
The SPD is connected to the designated circuit position to limit transient overvoltages. Its grounding method and conductor connection must comply with the manufacturer’s installation instructions.
El DC isolating switch o cortacircuitos is connected in the combined output circuit according to the combiner box design.
The actual connection sequence varies among different models of combiner boxes, and installation should be carried out according to the product wiring diagram.
Step 7: Connect the Combined Output to the Inverter
Connect the common positive and negative output circuits to the corresponding DC input terminals of the inverter respectively.
Before commissioning, the following need to be confirmed:
- Output polarity
- Open-circuit voltage after combining
- Maximum operating voltage under expected temperature conditions
- Operating current and design current after combining
- Ratings of the output cables and disconnect switch
- Maximum DC input voltage and current of the inverter
- Compatibility with the designated MPPT input
Unless the inverter and system design explicitly permit it, do not directly combine strings assigned to different MPPT inputs.
Common Mistakes When Connecting Multiple Solar Strings
Incorrect wiring or component selection may affect the performance and electrical safety of the PV system.
The following issues should be focused on during installation.
Reversed Positive and Negative Poles
If the positive and negative conductors are connected incorrectly, equipment damage may be caused, or the system may fail to operate normally.
Before connecting the inverter, the polarity of each string input and the combined output should be checked separately.
Using DC Fuses with Incorrect Ratings
Fuses with inappropriate current or voltage ratings may fail to provide the required protection.
The PV module specifications, system voltage, calculated current, and rated parameters provided by the fuse manufacturer should always be checked.
Exceeding the Busbar Current-Carrying Capacity
After increasing the number of connected strings, the total current after combining also increases.
If the rated current-carrying capacity of the busbar is insufficient to meet the calculated design current, excessive heating may occur. Before increasing the number of strings, the rated parameters of the busbar should be checked.
Connecting Incompatible PV Strings
If different strings have significantly different voltage characteristics, they should not be directly connected in parallel without confirming compatibility.
Different numbers of modules or operating conditions may cause mismatched operating points, thereby reducing system performance.
Exceeding Inverter Input Limits
A combiner box can aggregate the current of multiple strings, but this does not mean that each inverter input can withstand the combined output.
Before completing the connection, the inverter’s maximum DC voltage, maximum input current, maximum short-circuit current limits, and MPPT configuration should be checked.
Conclusión
Connecting multiple solar strings in a caja combinadora requires collecting the outputs of each PV string through designated input circuits, configuring appropriate overcurrent protection, and aggregating the positive and negative conductors through common busbars.
The most important considerations include string compatibility, correct polarity, busbar current-carrying capacity, total current after combining, and inverter MPPT configuration. A properly designed DC combiner box can provide a centralized and standardized connection point for multiple PV strings, while integrating the protection and isolation components required by the system design.
