A.C. current flowing in a conductor creates an alternating magnetic field. This is made stronger if the conductor is surrounded by a ferrous ‘containing iron’ enclosure such as with steel wire armour ‘SWA’ or steel conduit and also if a single conductor enters a ferrous enclosure on its own. This electromagnetic effect produces heat which could seriously damage an installation.
The current flowing within a twin cable or two single conductors ‘Line and Neutral’ feeding the same circuit will be the same, but the magnetic fields generated will exert opposite electromagnetic effects that will cancel each other out and there should be no overheating due to the electromagnetic effects. This is also true of a 3-Phase 3 wire, or 4 wire circuit providing that all of the conductors are contained within the same conduit or SWA.
In the 17th Edition of BS7671, Regulation 521.5.1 says that “Line and Neutral conductors should be within the same ferrous enclosure and a single conductor should not enter a ferrous enclosure individually”.
And also 521.5.2 “Single core SWA cables shall not be used for A.C circuits”.