by David Wren
Please see the text below as a reminder of the calculation checks, in Regulation 712.431, that are carried out when sizing cables and fusing for a PV Array. Plus, it’s individual Strings, if it is an Array with paralleled panels.
In a PV array with one PV string or two PV strings in parallel, no overcurrent protective device is required (see also Regulation 712.431.101). The continuous current-carrying capacity Iz of the PV string cable shall be greater than or equal to the short-circuit maximum current of the string: ISC MAX of the string ≤ Iz
In a PV array with Ns strings in parallel (with Ns greater than 2), the maximum reverse current flowing in the PV string cable is (Ns – 1) ISC MAX. One of the following measures shall be used:
- Where a PV string overcurrent protective device is not required in accordance with Regulation 712.431.101, the continuous current-carrying capacity Iz of the PV string cables shall be greater than or equal to the maximum reverse current: (Ns – 1) ISC MAX of the string ≤ Iz
- Where a PV string overcurrent protective device is required in accordance with Regulation 712.431.101, the continuous current-carrying capacity Iz of the PV string cables shall be greater than or equal to, the rated current of the string protective device In: In ≤ Iz
When selecting & sizing fusing for use in PV Arrays, the calculation checks are carried out in compliance with the requirements of sections:
- 712.431.101 - In a PV array with Ns strings (above 2 strings) in parallel, protective devices shall be provided to protect each PV string where the following condition is met: 1.35 IMOD_MAX_OCPR < (Ns -1) ISC MAX.
- 712.431.102 - For the protection of the PV strings, the use of protective devices as specified in 712.432, complies with the thermal withstand capability of the PV modules (as referred in BS EN IEC 61730-1:2018). Where protective devices are required their rated current In shall comply with the following conditions: 1.1 ISC MAX of the string ≤ In ≤ IMOD_MAX_OCPR.
Please see below, Figure 712.1 for an example of a single array and Figure 712.2 for an example of multiple arrays.