Only double-insulated cables and wires that are UV resistant and rated for high operating temperatures should be used for outdoor wiring or wiring behind modules/laminates. The insulation should be halogen free, self-extinguishing and barely combustible. Inasmuch as solar cell temperature can exceed ambient temperature by up to 50 oC behind poorly ventilated modules or laminates that are mounted directly on thermal insulation, cables and wires with a rated operating temperature of at least 85 oC (but preferably upwards of 100 o C) should be used wherever possible.
Wire colour coding (e.g. red for positive, blue for negative, black for all other wires) greatly eases the system realization and servicing process. Special PV cables and wires are available from many vendors.
The string wiring gauge should be at least 2.5 mm2, although with modules comprising larger cells, 4–6 mm2 gauges can be used in order to minimize power loss.
String wiring using a product such as double-insulated Huber þ Suhner Radox 2.5 mm2 125 PV wires, which are rated for an operating temperature of 125 oC, has been shown to be highly effective. Up to 16 mm2 solar wires are also available for PV plants with two or more non-fused parallel strings. Shielded cable made of this same material is available for DC wiring for gauges of up to 2 · 10 mm2 and 4 · 10 mm2, as well as common shielding measuring up to 13 mm2 that allows for
optimal PV installation lightning protection . Shielded individual conductors with bilateral, grounded and double-insulated shielding can also be used in such settings. Wires should always be: (a) connected to terminals or screws using wire pins and cable lugs; and (b) led into housings and module junction boxes and housings from below so as to prevent the wires from
bringing in any water.
Source: Book on PV system design and practice