Inground Conduit Box for Driveway Snow Melting
If in only 1 pipe is there a derate for the number of current carrying  conductors? In the U.S. I would be using thhn/thwn wire, but with 25 amp actual draw I would need to upsize the wire to #8 because 125% of 25 is 31.25a . 1-1/2 rigid pvc would allow 18 #8 , i would not try to pull multi conductor cables through conduit it costs more and is not needed since you are running conduit. Thhn/thwn wire is rated for wet locations and would cost less than NMWU. And as Isherwood mentioned since the splices are going to be below grade they need to be listed for wet / direct burial. 
On a side note are you going to put insulation under the mats? It will really help with power consumption that is going to be an expensive way to melt the ice and snow.