You've correctly identified a specific point regarding conductor identification in the NEC for DC systems exceeding 60V and using conductors 4 AWG and larger. Let's break down your observations and explore the potential reasoning

Understanding NEC 210.5(C)(2)
Context

This section deals with the identification of grounded and ungrounded conductors in

branch circuits supplied by a DC system exceeding 60 volts. 4 AWG and Larger

The specific allowance you're highlighting applies only to conductors sized 4 AWG or larger.

Limited Identification

The core of the allowance is that only the terminating and splice points of these larger DC conductors need to be identified for polarity.

Permitted Identification Methods

The NEC specifies `MARKING TAPE`, `TAGGING` or `OTHER APPROVED MEANS`.

Silence on Specific Colors

The crucial point you've noticed is the absence of a prohibition on using green (or white/grey) insulated conductors for ungrounded (hot) conductors.

Why This Allowance Exists (Potential Interpretations of Intent)

Several factors likely contribute to this allowance and the seeming exception to standard color-coding practices:1. Practicality and Availability

Large gauge conductors are often more difficult to obtain in a wide range of colors. Requiring specific colors for large conductors can significantly increase cost and lead times. Green is a common insulation color available on large conductors.2.

DC Systems are Distinct

While AC systems rely heavily on color-coding for safety and identification, DC systems have different characteristics. The focus shifts to ensuring proper polarity (positive and negative) is maintained, especially at termination points. DC polarity is crucial for the proper functioning of DC equipment and preventing damage.3.

Focus on Termination Points

The NEC likely recognizes that with large conductors, the critical area for identification is where the wires connect to equipment or are spliced. Clear and robust labeling at these points is deemed sufficient. The assumption is that qualified individuals are working on these installations, and they will understand the polarity designations at the terminations.4.

Cost Benefit Analysis

Requiring specific colors for long runs of large gauge conductors is a significant cost increase, and there is an argument for not mandating it if proper identification at terminations can be achieved with labeling. At this ampacity large wire, color is less critical and can be confusing when the wire might be used for ground if the system is reconfigured.

Important Considerations and Best Practices Clear Documentation

Even though the NEC allows this flexibility, it's essential to have clear and detailed documentation showing the polarity of all conductors in the DC system. This includes wiring diagrams, labels at panels, and any other relevant information.

Consistency

Strive for consistency within an installation. If you use green for the ungrounded conductor in one part of the system, maintain that practice throughout (or have a compelling reason not to).

Training

Ensure that all personnel working on the DC system are thoroughly trained on the specific wiring configurations and identification methods used.

Local Jurisdictional Requirements

Always check with your local electrical inspector or authority having jurisdiction (AHJ). They may have specific interpretations or requirements that go beyond the NEC. Some jurisdictions might not allow this practice, even if the NEC technically permits it.

Safety

Prioritize safety above all else. Even with proper labeling, be extremely cautious when working on energized DC systems.

Labeling

Make sure the labeling is durable, easily readable, and resistant to environmental factors (e.g., moisture, UV exposure).

Conclusion

You are correct that NEC 210.5(C)(2) provides an allowance for using green insulated conductors (and potentially other colors not typically associated with "hot" wires) for ungrounded conductors in large (4 AWG and larger) DC branch circuits exceeding 60V*. The likely intent is a balance between practicality, cost, and the recognition that proper identification at termination points, combined with clear documentation and qualified personnel, can provide adequate safety and functionality.While this provision exists, it's vital to use it judiciously and always prioritize clear identification, thorough documentation, and adherence to local codes and best practices. A conversation with your local electrical inspector before implementing this approach is always recommended. Flag for review