Power-limited circuits are electrical circuits whose voltage, current, and power output are restricted to levels considered safe from fire and electric shock hazards. The National Electrical Code (NEC) governs these circuits primarily under Article 725, classifying them as Class 1, Class 2, or Class 3 based on their maximum allowable power output. The core truth about all power-limited circuits is that their safety comes from limiting energy at the source, not just from protective devices downstream.
What Makes a Circuit “Power Limited”
A power-limited circuit caps the amount of energy available so that even under fault conditions (a short circuit, a pinched wire, contact with skin), the power isn’t enough to start a fire or deliver a dangerous shock. This is fundamentally different from standard branch circuits in your home, which carry enough energy to be lethal and rely on breakers or fuses to cut power after a fault occurs. Power-limited circuits are designed so the energy itself stays below dangerous thresholds at all times.
The NEC sets specific limits depending on the circuit class. Class 2 circuits, the most common type, are limited to 100 volt-amperes (VA) for circuits operating at 30 volts or less, and to just 5 milliamps of current for circuits operating above 30 volts. Class 3 circuits handle higher power demands but are still capped at 100 VA for circuits above 30 volts. Class 1 power-limited circuits fall under a separate set of rules with their own limits.
Class 2 vs. Class 3 Circuits
Class 2 circuits are the workhorses of low-voltage building systems. They power thermostats, intercoms, automated lighting controls, doorbells, and residential control systems where power demands are minimal. Because energy levels are so low, Class 2 circuits are considered safe from both fire and shock hazards, which is why the NEC allows significantly relaxed wiring methods for them. Class 2 cables must have a voltage rating of at least 150 volts.
Class 3 circuits step up to more demanding applications like HVAC controls, security systems, commercial lighting controls, and certain industrial settings. They’re still power-limited, but because they can operate at higher voltage or current levels than Class 2, they’re considered safe from fire ignition but not necessarily from electric shock. Class 3 cables must carry a voltage rating of at least 300 volts, double that of Class 2 cables.
Fire alarm and security signaling systems often use power-limited circuits as well, though they fall under their own cable designations (FPL and PLFA cables) designed to remain operational during fire conditions.
Inherently Limited vs. Non-Inherently Limited Sources
The NEC recognizes two ways a power source can achieve its power limits, and the distinction matters for how the circuit is designed and protected.
An inherently limited power source restricts output current through its own internal design, typically using high internal impedance or temperature-sensitive components that automatically reduce output when the circuit is overloaded. When the overload clears, the source resumes normal operation on its own. No fuse blows, nothing needs to be replaced or reset.
A non-inherently limited power source relies on an external or internal fuse or circuit breaker to cut power during an overload. When the protective device trips, either the fuse must be replaced or the breaker manually reset before the circuit works again. These sources can deliver more power under normal conditions, but they depend on that secondary protection to stay within safe limits during a fault.
For Class 2 power supplies specifically, the maximum nameplate rating is 100 watts (or five times the output voltage for outputs below 20 volts DC). The maximum output current is 5 amps for output voltages up to 20 volts DC, dropping to 100 divided by the output voltage for anything higher. Non-inherently limited sources must also prove that even with a single failure of their voltage regulator or current limiter, the output current stays below the maximums specified in NEC Chapter 9, Table 11(B).
Separation From Power Conductors
One of the most important installation rules for power-limited circuits is that Class 2 and Class 3 conductors cannot share the same cable or raceway with standard power and lighting conductors unless a physical barrier separates them. This rule, found in NEC Section 725.136, exists because mixing low-energy and high-energy wiring could allow a fault on the power side to energize the power-limited conductors, defeating their entire safety purpose.
In practice, this separation can be achieved by running power-limited cables in their own conduit, maintaining physical distance, or using specially constructed cables. One solution recognized by UL is a metal-clad cable where the control conductors sit inside a flexible PVC tube (at least 30 mils thick) before being bundled with power conductors under a common metallic armor. The tube provides the required separation within a single cable assembly.
Recent Code Changes
The 2023 NEC reorganized how power-limited circuits are structured in the code. Article 725, which previously covered Class 1, 2, and 3 circuits together, was split up for clarity. Class 1 circuits moved to a new Article 724, while Article 725 became dedicated exclusively to Class 2 and Class 3 power-limited circuits.
Cable listing requirements for all power-limited cables were consolidated into a new Article 722. And notably, the 2023 NEC introduced Class 4 circuits under new Article 726. Class 4 systems, also called Fault Managed Power Systems (FMPS), take a different approach: rather than limiting power at the source the way Class 2 and 3 circuits do, they continuously monitor for faults between the transmitter and receiver and shut down fast enough to prevent shock or fire. This allows them to deliver significantly more power while still being classified as power-limited with respect to safety risk.
The old Article 720, which covered circuits operating below 50 volts, was deleted entirely to avoid confusion with modern low-voltage systems already addressed elsewhere in the code.
Key Truths to Remember
- Safety comes from the source. Power-limited circuits are safe because the power supply itself is restricted, not because of downstream protection alone.
- Class 2 circuits are safe from both fire and shock. Class 3 circuits are safe from fire but may still pose a shock risk.
- 100 VA is the ceiling for both Class 2 and Class 3 power sources at voltages above 30V.
- Mixing with power wiring requires separation. Power-limited conductors lose their safety advantage if they share space with unprotected high-energy conductors.
- Listed power sources are required. A Class 2 or Class 3 circuit only qualifies as power-limited if its power source is listed and marked for that class.
- Relaxed wiring rules are earned. The reduced installation requirements for power-limited circuits (smaller cables, no conduit in many cases, simpler support methods) exist specifically because the energy levels are inherently safe.