It's not NEC, but common practice. For us in SF it is required to phase all circuits. This is an example of our local code.
210-5(c). Add the following new section:
(c)Ungrounded Conductors. Underground conductor insulation shall be color coded as follows:
(1)One-hundred-twenty/two-hundred-forty-volt 3-wire circuits - “A” phase black, “B” phase red; 120/208 volt 4-wire 3-phase wye circuits - “A” phase black, “B” phase red, “C” phase blue; 120/240 volt 3-phase delta circuits - “A” phase black, “B” (high leg) phase purple, “C” phase red; 277/480 volt 4-wire 3-phase wye circuits - “A” phase brown, “B” phase orange, “C” phase yellow.
(2)Conductors for switch legs may be of a different color than the phase conductor other than green, white or gray when suitably identified at pull, junction and outlet boxes with marking tape, tagging or other equally effective means.
(3)Conductor insulation shall contain the applicable color pigment for circuit wire #14 AWG through #10 AWG. Ungrounded conductors #8 AWG and larger and ungrounded conductors of any size in cable assemblies may be pigmented colors other than green, white or gray, provided they are suitably identified at pull, junction and outlet boxes with marking tape, tagging or other equally effective means.
