To avoid excessive voltage drop and also to ensure equal division of current, it is essential that different phase conductors be located close together and that each phase conductor, grounded conductor, and the grounding conductor (if used) be grouped together in each raceway or cable. However, isolated phase installations are permitted underground where the phase conductors are run in nonmetallic raceways that are in close proximity.
The impedance of a circuit in an aluminum raceway or aluminum-sheathed cable differs from the impedance of the same circuit in a steel raceway or steel-sheathed cable; therefore, separate raceways and cables must have the same physical characteristics. Also, the same number of conductors must be used in each raceway or cable. See 300.20 regarding induced currents in metal enclosures or raceways.
All conductors of the same phase or neutral are required by 310.10(H)(2) to be of the same conductor material. For example, if 12 conductors are paralleled for a 3-phase, 4-wire, 480Y/277-volt ac circuit, 4 conductors could be installed in each of three raceways. The Code does not intend that all 12 conductors be copper or aluminum but does intend that the individual conductors in parallel for each phase, grounded conductor, and neutral be the same material, insulation type, length, and so on. Also, the three raceways are intended to have the same physical characteristics (e.g., three rigid aluminum conduits, three steel IMC conduits, three EMTs, or three nonmetallic conduits), not a mixture (e.g., two rigid aluminum conduits and one rigid steel conduit).
The presence of the word polarity throughout the section specifically allows the inclusion of dc circuits.