A 400 volt, three phase final sub circuit in a non-domestic installation consists of three X-90, single core, thermoplastic sheathed copper conductors supplying a 50 Ampere resistive load. The circuit is enclosed in heavy duty rigid thermoplastic conduit with no other circuits. The conduit is buried in the ground having an ambient soil temperature of 25C and has a top cover of 0.5 meter. Circuit protection is by way of HRC fuses and the distance from the switchboard to the load terminals of the equipment is 41 metres.

Calculate the minimum permissible cable size if the voltage drop in this cable segment is not to exceed 2.5%.