Examples for the thermal ratings of circuit breakers in parallel operation of PV plant.
PV plant with 6 Solis-1P8K-5G inverters.
The required technical specifications can be found in the datasheet of the Solis-1P8K-5G inverter：
• Maximum output current = 34.7A
• Its maximum fuse protection = 50A
The choice of cable as well as wiring method, ambient temperature and other potential conditions limit the maximum fuse protection of the cable.
In our example, we assume that the selected cable (6mm²) has ideal routing and can withstand a nominal current of 35A.
The maximum possible nominal current for the cable used and the maximum possible fuse protection of the Solis-1P8K-5G limit the maximum possible nominal current for the circuit breakers.
Selecting the Correct Circuit Breaker
Using the same example system and assuming the load that has no motors, transformers etc., based on the calculated current of 34.7A, we choose a 40A circuit breaker with a thermal tripping characteristic of B and no gap between the circuit breakers. We then verify whether our selected value is appropriate by checking the thermal adaptability of the circuit breaker:
The load factor meets the specifications of the datasheet:
•Reduction at permanent load> 1 h = 0.9
（Permanent loads of more than 1 hour are possible in a solar PV plant.）
•Reduction factor when 6 circuit breakers are directly arranged next to each other = 0.75
（If one circuit breaker is used or the distance between each other is enough, the coefficient is equal to 1.）
•The increase in the nominal current in the distribution board when the ambient temperature is 40°C = 1.0
The nominal load current of the circuit breaker is calculated as:
Ibn = 40 A x 0.9 x 0.75 x 1.0 = 27A
Since the maximum current carrying capacity for fault-free operation is lower than the maximum output current of the inverter used, the selected circuit breaker cannot be used in this example. The circuit breaker will trip during rated operation.
Use a 50A circuit breaker. There is enough space (＞10mm) for heat dissipation between the circuit breakers, and the maximum current carrying capacity is 40.5A
(Ibn = 50A x 0.9x0.9 = 40.5A), The circuit breaker will not trip under rated operation.
Use a 63A circuit breaker. Maximum current carrying capacity is 42.5A
(Ibn = 63A x 0.75x 0.9x 1 = 42.5A), The circuit breaker will not trip under rated operation.