Following the 2002 NEC 300.5(I) Ex. 2, the NEC Handbook adds the folowing:
Isolated phase installations contain only one phase per raceway or cable. The spacing between isolated phase raceways and cables should be as small as possible and the length of the run limited to avoid increased circuit impedance and the resulting increase in voltage drop inherent in an installation involving ac circuits. Isolated phase installations may be used in ac circuits to limit available fault current at downstream equipment.
Isolated phase installations present an inherent hazard of overheating, a risk that must be understood and carefully controlled. This hazard results from induced currents in metal surrounding a raceway that contains only one phase conductor. {See 300.20(A) and 300.20(B) for more information on induced currents in raceways.} The surrounding metal acts as a shorted transformer turn. In underground installations, a single conductor is unlikely to be installed in a metal raceway or, if it were, it is unlikely to present a fire hazard. This is not true, however, for aboveground raceways, and it is the reason isolated phase installations have limited application for aboveground installations.
[This message has been edited by ElectricAL (edited 08-29-2002).]
