Classifying and Using Class 1, 2, and 3 Circuits
Class 1, 2, and 3 circuits are classified as remote-control, signaling, and power-limited circuits in the National Electrical Code (NEC). The NEC defines such circuits as that portion of the wiring system between the load side of the overcurrent protection device (OCPD) or the power-limited supply and all connected equipment.
These circuits are characterized by their usage and electrical power limitation, which differentiates them from light and power circuits. These circuits are also classified in accordance with their respective voltage and power limitations.
Class 1 circuits. The NEC divides Class 1 circuits into two types: power-limited and remote-control and signaling circuits. Power-limited Class 1 circuits are limited to 30V and 1000VA. Class 1 remote-control and signaling circuits are limited to 600V, but there are limitations on the power output of the source.
Class 1 power-limited circuits have a current limiter on the power source that supplies them. This limiter is an OCPD that restricts the amount of supply current on the circuit in the event of an overload, short circuit, or ground-fault. A transformer or other type of power supply supplies power to Class 1 circuits.
Generally, Class 1 remote-control and signaling circuits must meet most of the same wiring requirements for power and light circuits. We commonly use Class 1 remote-control circuits in motor controllers (which operate mechanical processes), elevators, conveyors, and in equipment controlled from one or more remote locations. Class 1 signaling circuits are used in nurses' call systems in hospitals, electric clocks, bank alarm systems, and factory call systems.
Conductors of different circuits. Class 1 circuits can occupy the same cable, enclosure, or raceway without regard to whether the individual Class 1 circuits are AC or DC, provided all the Class 1 conductors are insulated for the maximum voltage of any conductor in the cable, enclosure, or raceway. The NEC allows Class 1 circuits and power supply circuits to occupy the same cable, enclosure, or raceway in situations where the equipment power system is functionally associated.
One example is when the power supply conductors and control circuit conductors are run in the same conduit to control and operate the same piece of equipment, such as a motor controller.
Exception 1 to Sec. 725-26(b) clarifies you can mix these circuits when installed in factory- or field-assembled control centers. Exception 2 to Sec. 725-26(b) allows mixing for underground conductors in a manhole if you comply with all of the following conditions: (1) The power-supply or Class 1 circuit conductors are in a metal-enclosed cable or Type UF cable; (2) The conductors are permanently separated from the power-supply conductors by a continuous firmly fixed nonconductor, such as flexible tubing, in addition to the insulation on the wire; and (3) The conductors are permanently and effectively separated from the power supply conductors and securely fastened to racks, insulators, or other approved supporting means.
Derating requirements. Where only Class 1 circuit conductors are in a raceway, you can determine the number of conductors by the provisions outlined in Sec. 300-17. The derating factors given in Sec. 310-15(b)(2)(a) and the accompanying table apply only if such conductors carry continuous loads in excess of 10% of the ampacity of each control conductor routed through the raceway system.
You must use Sec. 300-17 to determine the number of power-supply conductors and Class 1 circuit conductors you can pull through a raceway [based upon the rules of Secs. 725-28(a), (b), and (c)]. The derating factors given in Art. 310, Table 310-15(b)(2)(a) apply to: (1) All conductors where the Class 1 circuit conductors carry continuous loads in excess of 10% of the ampacity of each conductor and where the total number of conductors is four or more; (2) The power-supply conductors only, where the Class 1 circuit conductors do not carry continuous loads in excess of 10% of the ampacity of each conductor, and where the number of power-supply conductors is four or more.
If you install Class 1 circuit conductors in cable tray systems, they must comply with the rules and regulations of Secs. 318-9 through 318-11 of Art. 318.
You can use No. 18 and No. 16 conductors for Class 1 circuits, if they supply loads that do not exceed the ampacities given in Sec. 402-5, and if you install them in a raceway, approved enclosure, or listed cable. Conductors larger than No. 16 are not allowed to supply loads greater than the ampaciti es given in Sec. 310-15. Flexible cords must comply with the design and installation requirements of Art. 400.
Insulation on conductors for Class 1 circuits are required to be suitable for 600V. Conductors larger than No. 16 must comply with the requirements of Art. 310. Conductors in sizes No. 18 and 16 shall be Type FFH-2, KF-2, KFF-2, PAF, PAFF, PF, PFF, PGF, PGFF, PTF, PTFF, RFH-2, RFHH-2, RFHH-3, SF-2, SFF-2, TF, TFF, TFFN, TFN, ZF, or ZFF. However, you can use conductors with other types and thicknesses of insulation if listed for Class 1 circuit use [See Sec. 725-27(a) and (b)].
Class 2 and 3 circuits. The NEC defines Class 2 and Class 3 circuits and Tables 11(a) and (b) in Chapter 9 provides the power limitations for the power sources: one for AC current and one for DC current. In general, a Class 2 circuit (operating at 24V with a power supply durably marked "Class 2" and not exceeding 100VA) is the type most commonly used.
The NEC defines a Class 2 circuit as that portion of the wiring system between the load side of a Class 2 power source and the connected equipment. Due to its power limitations, a Class 2 circuit is considered safe from a fire initiation standpoint and provides acceptable protection from electrical shock.
The Code defines a Class 3 circuit as that portion of the wiring system between the load side of a Class 3 power source and the connected equipment. Since the Class 3 circuits has higher levels of current than for Class 2 circuits, it specifies additional safeguards to provide protection from an electric shock you might encounter on the job site.
Power for Class 2 and Class 3 circuits is limited either inherently (in which no overcurrent protection is required) or by a combination of a power source and overcurrent protection.
The maximum circuit voltage is 150VAC or VDC for a Class 2 inherently limited power source and 100VAC or VDC for a Class 3 inherently limited power source. The maximum circuit voltage is 30VAC and 60VDC for a Class 2 power source limited by overcurrent protection, and 150VAC or VDC for a Class 3 power source limited by overcurrent protection.
For example, heating system thermostats are commonly Class 2 systems and the majority of small bell, buzzer, and annunciator systems are Class 2 circuits. Class 2 also includes small intercom phone systems in which a battery and the ringing circuit supply the voice circuit.
Where a Class 2 circuit is routed a distance where voltage drop becomes a problem (by not providing a voltage that will energize the equipment), Class 3 circuits are sometimes used to provide the necessary voltage and current. Class 3 circuitry and accessories can be designed to alleviate the problem of excessive voltage drop.
Class 2 and 3 systems do not require the same wiring methods as power, light, and Class 1 systems. There are cases when a 2-in. separation is required between these systems.