Electrical Installation Standards (NFPA 70, National Electrical Code)
The Electrical Installation Standards domain tests your ability to apply NFPA 70 to what the trade calls "low voltage": control, signaling, communications, and fire-alarm circuits. Question writers rarely ask about 24-volt hardware by name; they ask which article governs it, which wiring method is legal, and whether you may share a raceway with power conductors.
Start with the article, not the voltage
- Article 725 - Class 1, Class 2, and Class 3 remote-control and signaling circuits.
- Article 760 - fire alarm systems; Article 800 - communications; Article 820 - CATV and radio distribution; Article 830 - network-powered broadband; Article 770 - optical fiber.
- Chapter 3 methods still apply: 725.3 pulls in box fill (314.16), support, and raceway rules.
Class is set by the source
A circuit is Class 2 or Class 3 because it is fed by a listed Class 2 or Class 3 power source (725.121), not because the conductors are thin or the load is small. Conductor size then comes from Chapter 9, Table 11(A) for AC and Table 11(B) for DC. Class 2 and 3 conductors generally may not occupy the same raceway, box, or enclosure as power conductors absent a barrier or the separation allowed by 725.55. A CL3 cable may serve a Class 2 circuit; the reverse is not true.
Worked example
A 24 VDC actuator draws 1.5 A over 250 ft of 18 AWG solid copper. Chapter 9, Table 8 gives 6.385 ohms per 1000 ft. Round trip = 500 ft → 3.19 ohms. Drop = 1.5 A × 3.19 = 4.79 V, leaving 19.2 V - a 20 percent loss, well beyond the 3–5 percent the NEC informational notes suggest for branch circuits and typically outside the device's rating. Substituting 14 AWG (2.525 ohms per 1000 ft) cuts the drop to 1.89 V.
Traps
Assuming "low voltage" exempts you from permits, box fill, or support rules; treating a CL3 marking as irrelevant; confusing Class 1 (wired as power) with power-limited wiring.