The voltage of a PV combiner box is determined by the maximum open-circuit voltage (Voc) of each string, multiplied by the number of panels per string, and adjusted for the lowest expected ambient temperature.

Step 1: Determine String Configuration

A PV string is a series connection of solar panels. To calculate the voltage for a combiner box, first decide how many panels will be in each string. The open-circuit voltage (Voc) of a string is the sum of the Voc of each panel in series. For example, if each panel has a Voc of 40V and there are 20 panels in a string, the string Voc is 40V × 20 = 800V .

Step 2: Consider Temperature Effects

Voc increases in cold temperatures. To ensure safety, calculate the maximum Voc at the lowest expected ambient temperature using the panel's temperature coefficient. This ensures the combiner box and protection devices can handle voltage spikes during cold weather .

Step 3: Check Inverter Limits

The string Voc must not exceed the inverter's maximum DC input voltage. For instance, if the inverter's maximum DC voltage is 1000V, the string configuration must keep the maximum Voc below this limit .

Step 4: Combine Strings in Parallel

The combiner box connects multiple strings in parallel. In a parallel connection, the voltage remains equal to the string Voc, while the currents add up. Therefore, the combiner box voltage rating should match the maximum string Voc, not the sum of all string voltages .

Step 5: Include Safety Margins

Always include a safety margin above the calculated maximum Voc to account for unexpected conditions, voltage spikes, or future system expansion. This ensures compliance with IEC standards and long-term reliability .

Summary Formula

Maximum Combiner Box Voltage (Vmax) = Voc per panel × Number of panels per string × Temperature correction factor

By following these steps, you can accurately size the PV combiner box voltage rating, ensuring safe and efficient operation of your solar power system.