ok, i think i know what you are after. you are using two single pole ssr together so that when they are of there is no power ot ether side of the element? correct?
you can use 14 gauge if you fuse the wire at 15 amps, but that don't give you any head room. if you don't fuse it then you need 10 awg . if i was you i would use 12 awg and fuse it at 20 amps. also check to make sure that the terminal strip is rated for 20 ~ 30 amps
other than that you look to be in good shape