You're right, the NEC doesn't explicitly require labeling the source of power to a panel or disconnect in all situations. However, there are some sections that strongly imply it's necessary in specific circumstances, and more importantly, it's considered a very good practice for safety.Here's a breakdown: No General Blanket Requirement

You won't find a single sentence in the NEC that says "Every panel and disconnect shall be labeled with the source of its power."

Implied Requirements and Specific Cases



Article 110.22 Identification of Disconnecting Means

This is a crucial section. It requires that each disconnecting means be legibly marked to indicate its purpose unless the location and arrangement makes the purpose evident. While this focuses on the load side of the disconnect, a clear understanding of the source is vital to correctly interpret the disconnect's purpose. Imagine a panel labeled "Lighting Panel #3". Knowing it's fed from "Generator Main Breaker" versus "Utility Transformer T2" provides critical safety information.

690.56(B) PV System Disconnects

For PV systems, there are explicit labeling requirements for disconnects, including their source of power.

700 (Emergency Systems), 701 (Legally Required Standby Systems), 702 (Optional Standby Systems)

These articles dealing with emergency and standby power often require labeling to clearly identify the normal and alternate power sources. This is especially important for transfer switches.

Health Care Facilities (517)

Hospitals have very strict labeling requirements for power systems, including clear identification of sources, especially related to emergency and essential electrical systems.

Factory Assembled Control Equipment (409)

409.110 requires that the panel be marked with the "short-circuit current rating," which implies you know where the power is coming from to calculate that value.

Good Engineering Practice and Safety



Troubleshooting and Maintenance

Imagine a scenario where a technician needs to de-energize a panel. Without knowing the source, they could inadvertently shut down the wrong equipment or create a more hazardous situation. Proper source labeling drastically simplifies troubleshooting and reduces the risk of accidents.

Arc Flash Hazard

Understanding the source of power is essential for performing an arc flash risk assessment and selecting the appropriate PPE. The available fault current (which determines arc flash potential) depends on the source impedance (transformer size, utility capacity, generator capacity, etc.).

Lockout/Tagout (LOTO)

Clear source identification is critical for effective LOTO procedures.

Future Modifications

Over time, electrical systems get modified. Clear labeling helps future electricians understand the original design and make safe and informed changes.

Why the NEC Isn't More Explicit



Variability of Systems

The NEC aims to be a broadly applicable standard. Specifically prescribing the exact source labeling for every situation would be difficult because electrical system configurations vary so widely.

Enforcement Challenges

It's challenging to define precisely what constitutes "adequate" source labeling for every situation, which would make consistent enforcement difficult.

Recommendations


Err on the side of labeling

Even though it's not always explicitly required, label the source of power for panels and disconnects. It's a best practice.2.

Be specific

Don't just say "Transformer." Say "Transformer T1, 480-120/208V" or "Generator Main Breaker, ATS-1." The more information, the better.
3.

Consider the context

Think about who will be working on the equipment in the future and what information they will need to do their job safely.4.

Coordination Studies: After any modification of the electrical system perform a new coordination study with the new situation of the power system and new labels for each equipment.

In summary: While the NEC doesn't always mandate* labeling the power source, it's a vital safety practice that should be implemented. Focus on 110.22 and remember that providing source information contributes to a safer and more maintainable electrical system. Local codes may have more stringent requirements, so always check with your local authorities. Flag for review