PV Fact Sheet
06 | Combination of PV strings
How to combine PV strings and when is this efficient?
This fact sheet focuses on photovoltaic installations on top of buildings within the European Union. One essential part of such an installation is the combiner box. These boxes are used to combine several strings and to protect against overvoltage and feature many more functions.
This fact sheet answers the question of why it is efficient to combine strings in residential PV installations. In residential installations, the PV modules are often installed on the roof and the string inverters are placed in the basement. So, these installations require sufficient space for the PV cables running through the building.
Installation time, material and space for the cables running through the building can be saved if strings are combined close to the PV modules, such as on the attic.
Example: A building is equipped with PV modules on the east and west side of the roof. There is a 2 MPP string inverter in the basement. To save cables, the installer mounts a combiner box on the attic. He combines 2 strings (each with +/-) from the east side and 2 strings (each with +/-) from the west side. In doing so, he saves a total of 40 metres of cable and corresponding installation time.
When designing the strings with a combiner box, the same rules apply as when designing the strings for an inverter. That means the voltage of the first and the second string (same MPPT) should be as equal as possible. Here it is always important that the start voltage of the inverter is reached.
One question that arises from this configuration is which cross connection is required for the combined strings.
Is it 4 mm² or more? The answer can be found in the following table from EN 50618:2014:
|
Cable dimension |
Current for single cable in free space | Current for single cable on a surface |
Current for two cables contacting |
| 4 mm2 | 55A | 52A | 44A |
| 6 mm2 | 70A | 67A | 57A |
| 10 mm2 | 98A | 93A | 79A |
| 16 mm2 | 132A | 125A | 107A |
Figure 2: Current rating of PV cables at 60°C ambient temperature, source: EN 50618:2014
That means for a combination of 2 or 3 strings (each with max. 15 A) a 4 mm² cable can be used.
Nevertheless, to avoid power losses, it is recommended to use a larger cross-section depending on the length of the cable.
One final question remains: Can the string inverter handle this higher current per input? Let’s assume we are dealing with a string inverter with 24A per input. If 2 strings with 15 A each are combined, the string inverter is not specified for this load. In this case, a Y-cable or a Y-connector can be used to decouple the two strings.
Physically, the current is flowing 50/50 through each cable and this is why the current per input at the inverter is not higher than 15 A subsequently.
Advantages of Weidmüller products
PV Next is the global combiner box portfolio for rooftop installations made by Weidmüller. These products are based on a modular concept. Specifically, Weidmüller offers PV Next combiner boxes for residential and commercial installations, which always offer the option of combining 2 strings into one.
Pascal Niggemann
Head of PV Systems Home & Business,
Weidmüller Interface GmbH & Co. KG, Germany
Pascal.Niggemann@weidmueller.com
www.weidmueller.com/pv-rooftop
Documents / Resources
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Weidmuller PV Next Combiner Box [pdf] Instruction Manual PV Next Combiner Box, Next Combiner Box, Combiner Box, Box |
Solutions for rooftop photovoltaic systems 