You're correct to be looking for guidance in the NEC. Here's the relevant information regarding unterminated conductors in a panelboard or extending from a raceway system

The short answer is yes, unterminated conductors (spare cables) that extend beyond the panelboard or raceway system must be properly terminated and protected, typically in a junction box, or cut back and abandoned following NEC guidelines.

Here's a breakdown of the NEC reasoning and relevant sections: General Requirement for Exposed Conductors

The NEC generally prohibits exposed, unterminated conductors. This is to prevent accidental contact, short circuits, and fire hazards.

110.12 Mechanical Execution of Work

This general section requires work to be installed in a neat and workmanlike manner. Leaving unterminated conductors dangling is generally considered a violation of this.

300.14 Length of Free Conductors at Outlets, Junctions, and Switch Points

This section specifies the minimum length of conductors required at outlets, junctions, and switch points for making connections. It implies that conductors should be properly terminated or protected at such locations.

312.8 Switch and Overcurrent Device Enclosures with Splices, Taps, and Feed-Through Conductors

This section allows for splices, taps, and feed-through conductors within enclosures for switches and overcurrent devices, provided certain conditions are met. However, simply leaving unterminated conductors is generally not allowed.

408.41 Unused Openings

For panelboards, unused openings must be closed (covered). This reduces the risk of accidental contact with live parts. This could apply where conductors leave the panelboard.

Here's why you can't just leave spare conductors hanging, and what the implications are



Safety Hazard

Exposed conductors pose a shock and fire hazard.

NEC Violation

As described above, it violates the general spirit and specific requirements of the NEC.

Workmanship

It's simply not good electrical practice.

Acceptable Methods for Handling Spare Conductors


Terminate in a Junction Box

The most common and generally preferred method is to terminate the conductors in a properly sized junction box. The junction box must be appropriately rated for the environment (e.g., NEMA 1 for dry indoor locations, NEMA 3R for outdoor locations). You'll need wire connectors to terminate the spare conductors.2.

Cutting Back and Abandoning (If Applicable)

If the conductors are truly not needed and unlikely to be needed in the future, and you can safely and properly cut them back to their origin (e.g., inside the panelboard, if allowed by manufacturer instructions), this may be an option. However, you must ensure the cut ends are properly insulated and secured within the panelboard (if that's where you cut them). Again, follow manufacturer's instructions for the panelboard.

Important Considerations



Panelboard Fill

Ensure that adding the spare conductors to a panelboard doesn't exceed the fill capacity (the number of conductors allowed in the panelboard). This is related to heat dissipation and is specified by the panelboard manufacturer.

Derating

If the conductors are bundled together, you may need to derate their ampacity according to NEC 310.15.

Manufacturer's Instructions

Always consult the panelboard and equipment manufacturer's instructions. They may have specific requirements for handling spare conductors.

Example Scenario

You have a 12/2 w/g NM cable running from a 20A circuit breaker in your panelboard to a location, but the device that was to be installed is not currently needed.1. Install a junction box at the end of the NM cable.
2. Connect the black, white, and green conductors inside the junction box using appropriate wire connectors (e.g., wire nuts).3. Secure the NM cable to the junction box using an appropriate cable clamp.
4. Properly close the junction box cover.

In summary, you cannot* simply leave unterminated conductors dangling. You must either terminate them properly in a junction box or, if appropriate, cut them back and abandon them following NEC guidelines and manufacturer's instructions. When in doubt, always consult with a qualified electrician. Flag for review