Only solar modules that use the same technology (e. g. crystalline silicon solar cells) and that present equivalent Voc and VMPP values can be parallel connected; if possible, the same type of module should of course be used. The number of parallel-connected modules is designated as nMP. Inasmuch as solar modules are actually super solar cells with elevated voltages and currents, the observations in Section 4.2.3 also apply to modules that are wired to each other in parallel. Even in cases where numerous modules of the same type are directly parallel connected, shading of individual intact modules does not engender any hazardous reverse current, even if the solar generator is at open circuit.
In the interest of reliable handling of the very rare instances of short circuits in a module, no more than three or four modules should be parallel connected to each other, since the power and thus the hazard entailed by parallel-connected modules are considerably greater than for parallel-connected cells. However, in order to do this the wire size must be large enough and precautions must be taken to prevent reverse current from arising from connected appliances such as battery banks and inverters. For a relatively large number of parallel-connected modules (nMP > 4), a fuse (nominal current around 1.4 to 2 times module short-circuit current ISC-STC) should be series connected for each module in order to allow for reliable shutdown of a module short circuit (see Figure 4.30). However, in view of the elevated cost of such arrangements, they are a rarity in actual PV systems.
If these parallel-connected modules are series connected to other parallel-connected modules (see Section 4.3.4), a single bypass diode should be integrated for the entire parallel connection that can conduct at least 1.25 times the aggregate short-circuit current of all modules, i. e. 1.25 ■ nMP ■ ISC-STC. This is necessary because, if a series of smaller bypass diodes is used, internal current distribution will be imperfectly harmonized, and this can provoke module damage secondary to overheating and failure of the small diodes that are integrated into the modules.