UL has released a standard for photovoltaic wire. UL 4703 applies to “covers single-conductor, insulated and integrally or non-integrally jacketed, sunlight resistant, photovoltaic wire rated 90°C wet or dry, 600 V, for interconnection wiring of grounded and ungrounded photovoltaic power systems as described in Section 630.31(A) and other applicable parts of the National Electrical Code (NEC), NFPA 70.”
The standard is available for purchase on UL’s website.