A premises has branch circuits from both a 120/208 V system and a 277/480 V system. What does 210.5(C) require for the ungrounded conductors?
Not quite. The answer is B: Each ungrounded conductor must be identified by phase or line and by system at every termination, connection and splice point, using a documented method such as color coding, tape or tagging
The NEC does not mandate a color palette for ungrounded conductors. When a premises has branch circuits from more than one nominal voltage system, 210.5(C) instead requires each ungrounded conductor to be identified by phase or line and by system wherever it terminates, connects or splices, using any method (separate color coding, marking tape, tagging) that is documented. Brown/orange/yellow for 277/480 V and black/red/blue for 120/208 V is a widespread industry convention, not an NEC rule. The colors the NEC does fix are green or bare for equipment grounding (250.119) and orange or another effective marking for a delta high leg (110.15).
Reference: NEC 2023 210.5(C)(1) (Identification of Ungrounded Conductors — Multiple Voltage Systems)
Checked against multiple published sources
Two trade-press explanations agree on the requirement; neither is the code text. Sub-item lettering under 210.5(C)(1) differs by edition.
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