Low voltage and communications circuits
Scope
Low-voltage and communications circuits are covered by Article 725 (Class 1/2/3), Article 760 (fire alarm), Article 770 (optical fiber), Article 800 (communications), Article 820 (CATV), Article 830 (network-powered broadband), and Article 840 (premises-powered broadband). The exam tests circuit class, power source, cable substitution, separation, and grounding.
Core rules
- Class 1 circuits have power and energy levels above Class 2/3 limits and are wired more like power circuits; Class 2/3 are limited-energy and depend on a listed power source.
- Do not reclassify a circuit in the field. The power supply listing and Chapter 9 tables define the class.
- Cable substitution is one-way: a higher-rated cable may replace a lower-rated one where the Article permits, but not the reverse. Plenum and riser ratings still control.
- Communications cables must be separated from power conductors unless the Article permits close proximity; ask whether the circuit is Class 2, communications, or power.
- Article 800 grounding rules are separate from Article 250. Bond the communications grounding conductor to the building grounding electrode system.
Worked example
A 24 V camera draws 12 W at 150 ft from the source, using 18 AWG copper. Current = 12 W / 24 V = 0.5 A. 18 AWG resistance is about 6.4 ohms per 1,000 ft; the loop is 300 ft, so R = 6.4 x 0.3 = 1.92 ohms. Voltage drop = 0.5 A x 1.92 ohms = 0.96 V, about 4%. The camera sees about 23 V, usually acceptable, but check its listing.
Traps
Treating all 24 V circuits as Class 2; using a Class 2 cable for a Class 1 circuit; ignoring plenum/riser ratings; grounding a communications shield at both ends without following the Article; and forgetting voltage drop on long, low-voltage runs.