How do you identify ungrounded conductors in branch circuits supplied by DC systems? The 2014 NEC added nearly half a page of text to address this question [210.5(2)]. The requirements apply to DC systems operating at over 50V.
Identify all ungrounded conductors by polarity at all termination, connection, and splice points. If it’s 4AWG or larger, accomplish this with marking tape, tagging, or other approved means.
If it’s 6AWG or smaller, the requirements are more detailed. These requirements constitute about two-thirds of Sec. 210.5(2). And they differ, depending on polarity. The differences are the colors, polarity signs, and word markings. The means of identification are identical, except for those parameters. The requirements below apply to positive polarity wiring, but a few changes can transform them into the negative polarity wiring requirements.
• Continuous red outer finish (black for negative).
• Continuous red (black for negative) outer stripe along the length of the conductor; the insulation cannot be green, white, gray, or black (for negative polarity wiring, change black here to red).
• Imprinted plus signs or the word POSITIVE or POS durably marked no more than 12 in. apart on insulation some color other than green, white, gray, or black (for negative polarity wiring, plus signs become minus signs and POS becomes NEG, change black here to red).