You do the calculations just like box fill calculations using Table 314.16(B) for the volume of the conductors. A device with a volume of 7.1 would be way too small for that application.I have a project that I have to splice 3 hot, 2 neutral and 1 ground #10 wire in 3/4 T conduit body. It has 7.1 cu in. marking. How many wires can I splice in 3/4 conduit body?