Class 2 and Class 3 Circuits and Power-Limited Systems
What makes a circuit Class 2 or Class 3
Article 725 defines Class 2 and Class 3 circuits by the power source and its limits. A listed Class 2 or Class 3 power source establishes the circuit class. The circuit cannot be reclassified by adding a fuse, breaker, or other overcurrent device. Class 3 circuits generally permit higher voltage and energy than Class 2, but both remain power-limited and both require separation from power circuits unless an allowed method is used.
Installation rules
- Conductors must be installed using a method permitted for the circuit class.
- Class 2 and Class 3 cables must be separated from power conductors by the required spacing, barrier, or raceway method.
- A cable or conductor used in a plenum must have the appropriate plenum rating.
- Circuit identification and marking must follow the article and the equipment listing.
- The power source must be listed for the application; a generic transformer is not automatically acceptable.
Worked example
A 24-volt door strike draws 4 amperes from a listed Class 2 power supply rated 100 volt-amperes. The circuit is Class 2 because the source is listed and within the Class 2 limits. If a technician replaces the supply with an unlisted 24-volt transformer and adds a 5-amp fuse, the circuit does not become a compliant Class 2 circuit. The correct fix is to use a listed Class 2 source and keep the Class 2 separation.
Traps that cost marks
- Believing a fuse can turn an unlimited source into Class 2.
- Running Class 2 wiring with line voltage without an allowed barrier or method.
- Using a plenum-rated cable of the wrong circuit class.
- Assuming all 24-volt systems are Class 2; the source listing controls.