Class 1, Class 2, and Class 3 Remote-Control and Signal Circuits (NEC Article 725)
What this domain tests
Article 725 separates remote-control and signal circuits into Class 1, Class 2, and Class 3. The exam tests classification, wiring method, separation, and overcurrent protection. Class 1 circuits are not power-limited in the same way as Class 2 and Class 3; they generally use Chapter 3 wiring methods and Class 1 circuit conductors. Class 2 and Class 3 circuits are power-limited by Chapter 9 Table 11(A) or 11(B). Class 3 permits higher energy than Class 2 but is still limited.
Core rules
- Determine class from source and load: use the power source listing and Chapter 9 tables. A Class 2 power supply is listed as such; do not reclassify by wire size.
- Class 2 and Class 3 cable must be listed for the location: CL2 or CL3 general, CL2R or CL3R riser, CL2P or CL3P plenum.
- Separation: Class 2 and Class 3 conductors generally cannot be in the same raceway, outlet box, or cable as power conductors unless separated by a barrier or permitted by 725.136. Class 1 circuits follow Chapter 3 methods.
- Class 1 circuits need overcurrent protection per 725.43 unless listed as power-limited.
Worked example
A 24 VAC transformer rated 40 VA supplies a thermostat. Current is 40 divided by 24, or 1.67 A. Check Chapter 9 Table 11(A): at 24 V the Class 2 limit is above 1.67 A, so the circuit is Class 2. Use CL2 cable, keep it separated from 120 V power, and do not run it in the power conduit without a barrier.
Traps
- Calling any 24 V circuit Class 2 without checking the source.
- Using CL2 cable in a plenum when CL2P is required.
- Mixing Class 2 with power in a box without a barrier.
- Forgetting Class 1 overcurrent protection.