Conductor Identification, NEC 310-12

(a) Grounded Conductors. Insulated conductors of No. 6 or smaller, intended for use as grounded conductors of circuits, shall have an outer identification of a white or natural gray color. Multiconductor flat cable No. 4 or larger shall be permitted to employ an external ridge on the grounded conductor. 

Exception No. 1: Fixture wires as outlined in Article 402.
Exception No. 2: Mineral-insulated, metal-sheathed cable.
Exception No. 3: A conductor identified as required by Section 210-5(a) for branch circuits.
Exception No. 4: Where the conditions of maintenance and supervision ensure that only qualified persons will service the installation, grounded conductors in multiconductor cables shall be permitted to be permanently identified at their terminations at the time of installation by a distinctive white marking or other equally effective means. 

For identification requirements for conductors larger than No. 6, see Section 200-6. 

(b) Equipment Grounding Conductors. Bare, covered, or insulated grounding conductors shall be permitted. Individually covered or insulated grounding conductors shall have a continuous outer finish that is either green, or green with one or more yellow stripes. 

Exception No. 1: An insulated or covered conductor larger than No. 6 shall, at the time of installation, be permitted to be permanently identified as a grounding conductor at each end and at every point where the conductor is accessible. Identification shall be accomplished by one of the following means: 

a. Stripping the insulation or covering from the entire exposed length;
b. Coloring the exposed insulation or covering green; or
c. Marking the exposed insulation or covering with green colored tape or green colored adhesive labels. 

Exception No. 2: Where the conditions of maintenance and supervision ensure that only qualified persons will service the installation, an insulated conductor in a multiconductor cable shall, at the time of installation, be permitted to be permanently identified as a grounding conductor at each end and at every point where the conductor is accessible by one of the following means: 

a. Stripping the insulation from the entire exposed length;
b. Coloring the exposed insulation green; or
c. Marking the exposed insulation with green-colored tape or green-colored adhesive labels. 

(c) Ungrounded Conductors. Conductors that are intended for use as ungrounded conductors, whether used as single conductors or in multiconductor cables, shall be finished to be clearly distinguishable from grounded and grounding conductors. Ungrounded conductors shall be distinguished by colors other than white, natural gray, or green; or by a combination of color plus distinguishing marking. Distinguishing markings shall also be in a color other than white, natural gray, or green, and shall consist of a stripe or stripes or a regularly spaced series of identical marks. Distinguishing markings shall not conflict in any manner with the surface markings required by Section 310-11(b)(1). 

Exception: As permitted by Section 200-7.