Voltage Classification: Rated for Confusion

1. Does the NEC define “low voltage”?

Not directly. The NEC classifies voltage levels as high, medium, and low—but these refer to system voltage ranges, not cable types or applications. “Low voltage” is often used informally to describe circuits under 50V, but the NEC does not treat it as a standalone technical category.

2. What are the NEC’s formal voltage classifications?

The NEC uses three general voltage categories:

• High Voltage: This refers to systems operating over 600V, typically found in utility or industrial environments.
• Medium Voltage: This covers systems from 601V to 35,000V, commonly used in distribution networks and substations.
• Low Voltage: This includes systems from 0 to 600V, encompassing most building wiring and structured cabling circuits.

Structured cabling typically operates below 50V, which technically falls under the NEC’s broad “low voltage” category—leading to ambiguity in terminology.

3. Why is “low voltage” confusing in structured cabling?

The NEC’s “low voltage” category includes everything from 12V doorbells to 480V motors. Structured cabling usually involves circuits under 50V—like PoE, data, and security—but calling them “low voltage” without context can mislead installers, inspectors, and specifiers who associate the term with power wiring or industrial systems.

4. What are examples of cable types in each NEC voltage category?

Here’s how typical cable types map to NEC voltage classifications:

• High Voltage (Over 600V): MV-105 cable, shielded 15kV feeder cable, and industrial tray cable used for motors and switchgear.
• Medium Voltage (601V–35kV): URD cable, EPR-insulated feeder cable, and primary distribution cable for substations.
• Low Voltage (0–600V): THHN/THWN building wire, MC cable, fire alarm cable (FPL, FPLP), security cable (CL2, CL3), Category cable (Cat5e, Cat6, Cat6A), coaxial cable (RG6, RG11), speaker wire, thermostat wire, nurse call cable, and PoE cabling (often Class 2 or Class 3 circuits).

Most structured cabling falls under the “low voltage” umbrella but operates at 12–57V DC, often governed by NEC Article 725 or 800—not Article 300.

5. Is structured cabling governed by the same rules as 120V power wiring?

No. Structured cabling is typically governed by the following NEC articles:

• Article 725: Covers Class 2 and Class 3 circuits used in power-limited applications.
• Article 800: Governs communications cabling for voice, data, and signaling systems.
• Article 840: Applies to broadband communications systems and their associated cabling.

These articles include specific rules for routing, separation, and fire rating that differ from power wiring under Article 300.

6. Should I avoid using the term “low voltage” in documentation?

Yes—unless you define it clearly. Use terms like “limited energy,” “Class 2 circuit,” or “communications cabling” to avoid ambiguity. Always anchor your language to NEC articles and voltage thresholds to ensure clarity across disciplines.

7. What’s the best way to label structured cabling in specifications and guides?

Use terminology tied to application and NEC code:

• “Class 2 power-limited circuit” for PoE, security, and similar low-energy applications.
• “Communications cable per NEC Article 800” for data and voice cabling.
• “Fire alarm circuit per NEC Article 760” for signaling and emergency systems.

Avoid generic labels like “low voltage” unless scoped by voltage, power, and function.