Class 2, communications and fire alarm circuits

Not every circuit an electrician installs carries power at 120 V or more. Thermostats, doorbells, security sensors, building automation, network cabling, Power over Ethernet devices and fire alarm systems all use low-voltage or power-limited circuits. Because their energy is limited, the code relaxes some requirements, allowing lighter cables and simpler wiring methods, but it adds others: these circuits must be kept apart from power circuits, their cables must resist the spread of fire, and fire alarm circuits must keep working when the building is burning. This lesson explains the classes of power-limited circuits, communications and data cabling, Power over Ethernet, and the special rules for fire alarm systems.

In the National Electrical Code, remote-control, signaling and power-limited circuits are covered in Chapter 7 (Articles 722, 724 and 725 for Class 1, 2 and 3 circuits), fire alarm systems in Article 760 together with NFPA 72, and communications systems in Chapter 8.

Classes of circuits

Remote-control, signaling and power-limited circuits are grouped by the energy their source can deliver:

• Class 1 circuits can carry substantial power (power-limited Class 1 sources up to 30 V and 1,000 VA, or non-power-limited up to 600 V) and are wired much like power circuits. Motor control circuits are a common example.
• Class 2 circuits are limited, by their power source, to levels considered safe from both fire and electric shock: typically not more than 30 V AC or 60 V DC and not more than 100 VA for most sources. Thermostat wiring, doorbells, most building automation, low-voltage lighting and many sensors are Class 2.
• Class 3 circuits are limited to levels safe from fire but permitting higher voltages, so they are more of a shock hazard than Class 2.

The classification depends on the source, not the wire: a circuit is Class 2 only if it is supplied by a listed Class 2 power source, such as a Class 2 transformer, power supply or control panel output, marked as such.

Separation from power circuits

The chief hazard for a low-voltage circuit is contact with power conductors: if a 120 V conductor faulted onto a thermostat cable, the thin wire and the devices on it could carry line voltage throughout a building, where no one expects it. The code therefore requires Class 2 and Class 3 conductors, and communications cables, to be separated from power and Class 1 conductors:

• Not in the same raceway, cable, box or enclosure, unless separated by a fixed barrier or the power conductors are only there to connect to equipment that uses both (and are kept apart within it).
• Separated by a minimum distance when run openly, unless one set is in raceway or metal-sheathed cable.
• Not strapped or tied to power conductors or raceways, or supported by them.

Communications and data cabling

Network cables (twisted-pair categories such as Cat 6 and Cat 6A), coaxial cables and optical fiber cables are installed by electricians as well as by specialized cabling technicians. The code is concerned mainly with fire spread and separation; performance standards such as ANSI/TIA-568 and ISO/IEC 11801 govern how well the cabling carries data (DCE-320).

Cables run through buildings can spread fire and smoke, so the code requires cables to be listed for the space they occupy, with tests of flame spread and smoke production:

Where communications cables enter a building from outside, they must be grounded or bonded at the entrance and protected against lightning and power-line contact by listed primary protectors, bonded to the building's grounding electrode system (Lesson 9).

Abandoned cable

Old data and low-voltage cables accumulate above ceilings as systems are replaced, adding fuel and smoke. The code requires accessible abandoned cables to be removed, or tagged for future use if they are kept.

Power over Ethernet

Power over Ethernet (PoE) sends DC power along the same twisted-pair cable that carries data, powering wireless access points, cameras, phones, sensors and even LED luminaires. The IEEE 802.3 standards define power classes: up to about 15 W at the source (802.3af), 30 W (802.3at) and 60 W or 90 W (802.3bt, using all four pairs). PoE circuits are typically Class 2 (at about 50 V to 57 V DC), so they can be installed with data cabling methods, but the current in large cable bundles generates heat.

The NEC (725.144) limits the current per conductor in bundles of Class 2 and Class 3 data cables carrying power, using Table 725.144, and permits higher currents for cables marked with a limited-power (LP) rating.

Fire alarm circuits

Fire alarm systems detect fires, warn occupants and summon help, and they must work during the fire itself. Their wiring therefore has requirements beyond those of other low-voltage systems:

• Fire alarm circuits are supervised: the control panel continuously monitors each circuit for opens, shorts and ground faults, reporting a trouble signal so faults are fixed before a fire.
• Circuits are identified (for example by red cable jackets or marked junction boxes) and kept separate from other systems.
• Power to the panel comes from a dedicated branch circuit, with the breaker identified and locked so it cannot be switched off accidentally, plus standby batteries.
• Survivability: in many buildings, circuits that must operate during a fire, such as those to notification appliances in high-rise buildings, must be protected by fire-rated construction or be listed circuit integrity (CI) cable.
• Installation and verification follow fire alarm standards, and systems must be inspected and tested by qualified persons before occupancy and regularly afterwards.

In the US, fire alarm systems are required by building and fire codes and designed, installed, tested and maintained to NFPA 72; NEC Article 760 covers their wiring, including power-limited fire alarm (PLFA) cables such as FPLP, FPLR and FPL.

Other low-voltage systems

The same principles of a limited source, separation and fire-rated cabling apply to other systems an electrician meets: intrusion alarms, access control, nurse call, sound and public address systems, and low-voltage lighting. Audio amplifiers can produce Class 2 or Class 3 outputs depending on their power, and their speaker wiring must match the class marked on the amplifier. Low-voltage landscape lighting uses listed Class 2 or similar power units, often with GFCI protection on the supply receptacle outdoors.

Building automation and control systems increasingly combine power and data: a single cable can carry Ethernet and power to a sensor, a damper actuator or a luminaire. This blurs the line between the electrician's and the cabling technician's work, so electricians need to know both the code rules for the circuit class and the performance requirements of the cabling standards. Labeling each cable at both ends, recording it in a schedule and leaving service loops make these systems far easier to maintain.