11.3 General Low Voltage Requirements (Article 725)
Key Takeaways
- Article 725 covers Class 1, Class 2, and Class 3 circuits that are not part of appliance wiring or Article 760/800 circuits.
- Class 1 circuits (725.41) come in two flavors: power-limited (≤ 30 V, ≤ 1,000 VA) and remote-control/signaling (≤ 600 V, no power limit) — both are wired like power circuits.
- Class 2 circuits are inherently power-limited (typically ≤ 100 VA / 24 V) and are the backbone of doorbells, thermostats, and low-voltage lighting control; common cable is CL2/CL3.
- Class 3 circuits carry higher voltage than Class 2 but with rated cable; they are used for sound/security systems where shock hazard exists but fire hazard is limited.
- Per 725.136, Class 2 and Class 3 circuits must be separated from power and Class 1 circuits by barriers or by 2 in. spacing unless specifically permitted to share a raceway.
Scope of Article 725
Article 725 covers circuits that are not used for appliance wiring (Article 422), fire alarm (Article 760), or communications (Article 800/805). The circuits it governs are remote-control, signaling, and power-limited circuits — the wiring behind doorbells, thermostats, security sensors, HVAC control, and sound systems. The exam divides these into three classes by voltage and power limits.
Class 1, 2, 3 Definitions — 725.41 and Article 100
| Class | Voltage limit | Power limit | Typical use | Wiring treatment |
|---|---|---|---|---|
| Class 1 — Power-Limited [725.41(A)] | ≤ 30 V | ≤ 1,000 VA | Low-voltage motors, remote-controlled window blinds | Like power circuits |
| Class 1 — Remote-Control/Signaling [725.41(B)] | ≤ 600 V | None | HVAC control, relay logic | Like power circuits |
| Class 2 | Per Chapter 9 Table 11(A)/11(B) — typically ≤ 30 V AC or 60 V DC and ≤ 100 VA | Inherently limited by the power supply | Doorbells, thermostats, networking, low-voltage lighting controls | Lighter cable (CL2, CL3) permitted |
| Class 3 | Higher than Class 2 voltage but still power-limited (≤ 100 V, with current limits per Table 11) | Inherently limited | Sound systems, security circuits where shock hazard exists | Rated cable required (CL3, CL3R, CL3P) |
Class 1 — 725.41(A) and (B)
Class 1 circuits are not considered safe from fire or shock initiation on their own. They are installed using the same wiring methods as power circuits (Chapter 3 wiring methods like EMT, MC, FMC, NM). 725.41(A) power-limited circuits are limited to 30 V maximum and 1,000 VA output. 725.41(B) remote-control and signaling circuits may operate up to 600 V with no power-output limit, and are treated as power circuits for installation purposes. Overcurrent protection for Class 1 circuits follows 725.23 (limited to conductor ampacity per Table 310.16; for 18 AWG no more than 7 A, for 16 AWG no more than 10 A).
Class 2 — The Doorbell Standard
A Class 2 circuit is the portion of the wiring system on the load side of a Class 2 power supply — a listed supply whose output is inherently limited so that it is safe from a fire-initiation standpoint and presents an acceptable shock hazard. The classic limit is 100 VA at 24 V (a 24 V / 4.16 A doorbell transformer is a Class 2 supply). Common cable types are CL2 and CL3 (general purpose), with riser (CL2R/CL3R) and plenum (CL2P/CL3P) variants for vertical and air-handling runs.
Class 3 — Higher Voltage, Still Power-Limited
A Class 3 circuit permits higher voltage (up to 100 V) than Class 2 but with current limits that keep it power-limited. It is used where the higher voltage would present a shock hazard (sound systems, security alarm loops). The cable must be CL3-rated or better — Class 2 cable cannot substitute for a Class 3 circuit.
Permitted Wiring Methods and Cable Types
| Circuit | Permitted cables | Raceway |
|---|---|---|
| Class 1 | Same as power wiring (NM, MC, AC, FMC, EMT, RMC) | Chapter 3 methods required |
| Class 2 / Class 3 | CL2, CL3, CL2P, CL3P, CL2R, CL3R, PLTC | May use Chapter 3 raceways; PLTC cable for industrial |
| Power-limited tray cable (PLTC) | PLTC, PLTC-ER for exposed runs | Tray, raceway, or cable tray in industrial occupancies |
725.121 lists the power sources permitted for Class 2 and Class 3 circuits. 725.130 restricts installation: Class 2 and Class 3 circuits shall not be placed in the same raceway, cable, enclosure, or fitting with power or Class 1 conductors unless separated by a barrier.
Separation from Power Circuits — 725.136
725.136 is the central separation rule. Key provisions:
- 725.136(A) — Class 2 and Class 3 conductors shall not be placed in the same raceway, cable, enclosure, or fitting with conductors of electric power, lighting, or Class 1 circuits.
- Exceptions: (1) separated by a listed barrier or divider; (2) power conductors introduced only to supply Class 2/3 equipment and separated by at least ¼ in. (6 mm) from the low-voltage conductors; (3) in a manhole with fixed spacing.
- 725.136(B) — in cable trays, Class 2/3 cables must be separated from power cables by a solid fixed barrier.
- 725.136(C) — underground installations must maintain a minimum 12 in. spacing where Class 2/3 and power conductors share a trench.
Worked Example: Doorbell Wiring
A 16 V / 10 VA doorbell transformer is a Class 2 power supply — its output is below the 30 V and 100 VA limits. The button wiring from the transformer to the chime is therefore a Class 2 circuit and may be run as CL2 cable (a small, two-conductor jacketed cable). It cannot be run in the same NM-B sheath as the 120 V lighting circuit feeding the transformer. The 120 V primary side is a standard power circuit; the 16 V secondary side is Class 2.
Worked Example: HVAC Thermostat Control
A modern HVAC control circuit carrying 24 V AC at <100 VA from a transformer inside the air handler to the wall thermostat is Class 2. The cable must be CL2 or CL3. If the installer routes the thermostat cable through the same conduit as the 240 V condenser conductors, the installation violates 725.136(A) unless a listed barrier separates the two. The correct approach is to run the Class 2 cable in its own raceway or use a separate sleeve.
Exam Traps
- Treating a Class 1 remote-control circuit as "low voltage" and wiring it with thermostat wire — wrong. Class 1 is wired like power.
- Running Class 2 and power conductors in the same NM cable — violates 725.136(A) unless a barrier is present.
- Using CL2 cable for a Class 3 sound system — wrong; Class 3 requires CL3-rated cable.
- Assuming any 24 V circuit is Class 2 — only true if the power supply is listed as Class 2 (≤ 100 VA). A 24 V / 10 A supply is not Class 2.
A 24 V / 4 A doorbell transformer supplies a chime with button wiring run in a small jacketed cable. What class is the secondary circuit, and what cable type is required?
An installer routes a 24 V thermostat cable (Class 2) in the same conduit as the 240 V conductors feeding an outdoor condenser. What does 725.136 require?