A:  NEC Article 690.31(C)(2) addresses support and securement requirements which apply to Cable Tray and Cable Rings and Saddles. A maximum spacing of 12” should be used for cable sizes less than 1/0 AWG. A maximum spacing of 18” should be used for cable sizes greater than 1/0 AWG. The same article requires securement intervals no larger than 4.5 feet for USE-2 and PV Wire. – Bill Brooks, P.E.,  White Paper: Cable Management in Solar PV Arrays, Pages 5-6, Jan. 16, 2014