Does NEC allows !!

[Anwar Pasha]

Is it permissable by code to install in a three phase ( using single core cables) system to install the earth and the neutral conductors in a different conduit of the same duct bank?

 

[chris kennedy]

Is it permissable by code to install in a three phase ( using single core cables) system to install the earth and the neutral conductors in a different conduit of the same duct bank?

Are you also isolating each phase?

300.5(I) Conductors of the Same Circuit.

All conductors of the same circuit and, where used, the grounded conductor and all equipment grounding conductors shall be installed in the same raceway or cable or shall be installed in close proximity in the same trench.

Exception No. 1: Conductors in parallel in raceways or cables shall be permitted, but each raceway or cable shall contain all conductors of the same circuit including equipment grounding conductors.

Exception No. 2: Isolated phase, polarity, grounded conductor, and equipment grounding and bonding conductor installations shall be permitted in nonmetallic raceways or cables with a nonmetallic covering or nonmagnetic sheath in close proximity where conductors are paralleled as permitted in 310.4, and where the conditions of 300.20(B) are met.

 

[dbuckley]

a three phase ( using single core cables) system to install the ... neutral conductors in a different conduit

Quite apart from if code allows (which if I've understood your question correctly, it doesn't), if the neutral is in a different metallic conduit from the phase conductors and there is a metallic path between the seperate conduits then there will be an inductive heating effect, which will (at best) cause heating and power loss due to heating, and at worst will melt the insulation of the cores causing failure, and/or possibly fire.

NEC requires that phase and neutral cables are in the same conduit for this reason.

 

[Volta]

Note that the text Chris posted requires the use of non-metallic conduits for that and points you to 300.20 which may require some modification to any ferrous boxes used on the end(s).

In my opinion they could have worded 300.20(B) a little better, since in this case there will not be single conductors, only two groups of less than the entire circuit. Same problem, but not so clearly disallowed.