12.5 Class 1, Class 2 & Class 3 Remote-Control, Signaling & Power-Limited Circuits

12.5 Class 1, Class 2 & Class 3 Remote-Control, Signaling & Power-Limited Circuits

Thermostats, doorbells, motor control stations, building automation, and many lighting controls run on remote-control, signaling, and power-limited circuits. ICC 703's Kentucky outline lists Control Devices (4% of the exam), and these circuits also appear in Prov's Motors and Controls and Special Equipment areas. Because the power source limits the energy available, the NEC relaxes wiring methods for these circuits but adds strict rules for separating them from power wiring.

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1. Where the Rules Live in the 2023 NEC

The 2023 NEC split the old Article 725 into three articles:

Fire alarm circuits stay in Article 760, and communications circuits stay in Chapter 8. The 2023 NEC also added Article 726 for Class 4 fault-managed power systems. If your exam allows the 2017 or 2020 NEC (as Prov does), the same rules are found in the older Article 725, so look for familiar section titles rather than exact numbers.

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2. Class 1 Circuits (Article 724)

• Power-limited source: A Class 1 power-limited circuit is supplied by a source with a rated output of not more than 30 volts and 1,000 volt-amperes.
• Source protection: When the source is not a transformer, its overcurrent protection may not exceed 167% of the source's volt-ampere rating divided by its rated voltage. Transformers that supply these circuits follow Article 450.
• Conductors: 18 AWG and 16 AWG conductors are permitted when they are installed in a raceway, an approved enclosure, or a listed cable. They must be protected at no more than 7 amperes (18 AWG) and 10 amperes (16 AWG). Conductors 14 AWG and larger are protected according to their ampacity.
• Shared raceways: Class 1 conductors may share a raceway, cable, or enclosure with power-supply conductors only when they are functionally associated with the equipment the power conductors supply, or in factory- or field-assembled control centers and other specific cases the code lists.

Worked example: A listed 24-volt, 1,000-VA Class 1 power-limited supply that is not a transformer may be protected at no more than $\frac{1{,}000}{24} \times 1.67 = 69.6$ amperes. The 18 AWG control conductors it feeds still need 7-ampere protection.

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3. Class 2 and Class 3 Circuits (Article 725)

The Article 100 definitions explain the difference:

• A Class 2 circuit is limited in power so that it is safe from a fire-initiation standpoint and provides acceptable protection from electric shock. Most thermostat, doorbell, and sensor circuits are Class 2, typically at 24 volts ac or less.
• A Class 3 circuit is also limited for fire safety, but it permits higher voltage than Class 2 (up to 150 volts), so extra safeguards protect against electric shock. Some intercom, sound, and older signaling systems are Class 3.

Both are generally limited to 100 volt-amperes. The exact voltage, current, and power limits come from Chapter 9, Tables 11(A) (ac) and 11(B) (dc), which separate inherently limited sources from sources that rely on overcurrent protection.

Power sources (725.60)

A Class 2 or Class 3 circuit must be supplied by one of these:

1. A listed Class 2 or Class 3 transformer.
2. A listed Class 2 or Class 3 power supply.
3. Other listed equipment marked to identify its Class 2 or Class 3 output, such as a furnace control board or alarm panel.
4. Listed information technology equipment with limited-power circuits.
5. A dry-cell battery of 30 volts or less, which is considered an inherently limited Class 2 source when its capacity is no more than that of series-connected No. 6 carbon-zinc cells.

The primary side of a Class 2 transformer is a power circuit. A doorbell transformer mounted on a junction box has 120-volt branch-circuit wiring inside the box and Class 2 wiring on the outside secondary terminals.

Cable types

Class 2 and Class 3 cables are listed by fire performance:

Cables with higher fire ratings can substitute for lower ones. For example, CL2P can replace CL2R or CL2, and Class 3 cable can replace the matching Class 2 cable. Communications cables (CMP, CMR, CM) and PLTC can also substitute in many applications.

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4. Separation From Power Conductors (725.136)

Separation rules keep a fault in a power circuit from energizing low-voltage wiring that people handle freely.

• General rule (725.136(A)): Class 2 and Class 3 conductors must not be placed in any cable, cable tray, compartment, enclosure, outlet box, device box, raceway, or similar fitting with conductors of electric light, power, Class 1, or non-power-limited fire alarm circuits, unless one of the following permissions applies.
• Barriers and raceways (725.136(B) and (C)): A barrier in a shared enclosure or cable tray, or a separate raceway inside the enclosure, can separate the circuits.
• Inside equipment (725.136(D)): Where Class 2 or Class 3 conductors enter an enclosure only to connect equipment, the power conductors must be routed to keep at least 1/4 inch (6 mm) away from them. Alternatively, where the power circuits operate at 150 volts or less to ground, the Class 2 or 3 conductors may be in Class 3 cable whose conductors beyond the jacket are separated by 1/4 inch or a nonconductive sleeve.
• Other applications (725.136(I)): Elsewhere, such as cables run alongside open power wiring, Class 2 and Class 3 conductors must be at least 2 inches (50 mm) from power conductors, unless one set is in a raceway or in metal-sheathed, metal-clad, nonmetallic-sheathed, or Type UF cable, or a continuous, firmly fixed nonconductor separates them.

Under 725.139, two or more Class 2 circuits may share a cable, raceway, or enclosure, and Class 3 circuits may share with each other. Class 2 conductors may share with Class 3 conductors when the Class 2 conductors have insulation at least equal to that required for Class 3 circuits.

HVAC example: A furnace has 120-volt supply conductors and a 24-volt Class 2 thermostat cable. The thermostat cable may not run in the same raceway as the 120-volt conductors. Inside the furnace cabinet, where it enters only to connect to the control board, the 120-volt conductors must be routed at least 1/4 inch from it.

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5. Support, Access, and Abandoned Cable

• Support: Cables must be supported by the building structure so normal building use will not damage them. The ceiling grid and ceiling support wires may not be the sole support (300.11(B)), although independent support wires installed for the purpose and secured at both ends are permitted.
• Raceways as support (300.11(C)): Do not strap, tape, or tie Class 2 or Class 3 cables to the outside of a raceway for support. The exceptions are a raceway identified as a means of support, and a raceway containing the power supply for electrically controlled equipment, which may support Class 2 cables that serve only that equipment's control circuits.
• Access (722.21): Cables must not block access to equipment behind panels designed to be removed, such as suspended ceiling panels.
• Abandoned cable (722.25): The accessible portion of abandoned Class 2, Class 3, and power-limited tray cable must be removed. Cable tagged for future use is not considered abandoned.
• Plenums (300.22): Cables in ducts and plenums, and in other spaces used for environmental air above ceilings, must be the plenum type (such as CL2P) or installed in a wiring method permitted for that space.

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6. Kentucky Licensing Note

KRS 227A.030(10) says Kentucky's electrical licensing law does not apply to low-voltage, power-limited installations for the control or coordination of interconnected devices that are separated from the power source by a Class 2 or Class 3 transformer, when installed by certain licensees. These include master or journeyman HVAC technicians employed by licensed HVAC contractors, fire protection sprinkler contractors, master or journeyman plumbers, boiler mechanics, and electricians employed by electrical contractors. The 120-volt or 240-volt circuit that supplies the transformer is still electrical work under KRS Chapter 227A.