A class 1 circuit is the portion of the wiring system between the load side of the overcurrent protection device (OCPD) or the power-limited supply and the connected load. For example, Class 1 power-limited circuits are supplied by a power supply with an output that does not exceed 30 volts and 1,000 volt-amps. Also, Class 1 remote control circuits are limited up to 600 volts with no other power limitations.

However, Article 725 distinguishes between the two types of circuits with the rules for installation being basically the same. Article 725.21 points out that a Class 1 power-limited circuit is not restricted to specific uses, but a Class 1 non-power-limited remote control circuit is restricted to remote control operations. Article 725 shows the proper uses of these two circuits.

Class 1 remote control circuits are very common in commercial and industrial settings where motors and compressors are heavily employed. In many situations, the power for a motor control circuit is tapped from the branch circuit supplying the controller and motor. There are many installations where a high-impedance transformer, OCPD or both are used to produce or protect this type of circuit.

Class 1 circuits can be 600 volts or less, but 120 volts are more common and are usually preferred by OSHA inspectors for safety reasons. Article 430 of the National Electrical Code (NEC) has less restrictive requirements when providing overcurrent protection for these conductors than provisions of Article 725 permit. However, 725.3(F) of the NEC recognizes the existence of these special rules, noting Articles 430 and 310 of the NEC recognize smaller conductor sizes for control circuits.

For example, if a 100-amp branch-circuit supplies power to a controller and motor, and if 14 AWG copper conductors are tapped from such a circuit and remain in the controller enclosure as permitted in Table 430.72 (B) of the NEC, the conductors are considered protected. However, when control circuits are not tapped from the motor’s branch-circuit OCPD, the rules of Article 725 are to be applied.

According to the Fine Print Note (FPN) in 725.1, the Code language clarifies that the requirements of chapters 1 through 4 of the NEC—with regard to minimum conductor sizes, derating factors of 310.10, overcurrent protection, insulation requirements, wiring methods and grounding techniques—are to be considered when designing and installing such circuitry. For example, a circuit controlling a coil in a starter not tapped from the branch-circuit supplying the motor may be equipped with a 14 AWG copper conductor protected by a 15-amp OCPD, which 725.23 of the NEC allows. Note chapters 1 through 4 of the NEC also apply to this installation.

Tables 310.5 and 310.16 of the NEC outline the minimum size of conductors and insulation requirements for Class 1 circuits. These two tables list 14 AWG as the smallest conductor permitted for general wiring. But according to 725.27, the NEC permits the use of 18 and 16 AWG size conductors while 110.5 requires them to be copper unless specifically stated otherwise in other areas of the NEC. Sometimes, it is hard for many electricians to accept the fact that the NEC contains rules that permit smaller conductors to be used for Class 1 control circuits because they are so accustomed to using 14, 12 and 10 AWG size conductors for controlling purposes. The use of 18 and 16 AWG size conductors is perfectly legal, as long as the load on such size conductors is limited to their rated ampacities, which are listed in Table 402.5. However, electricians must remember when using these conductors that 18 AWG is limited to 6 amps, and 16 AWG is limited to 8 amps.

However, 725.27(B) actually modifies the above restrictions by allowing certain types of fixture wires to be used for Class 1 circuits. There again, electricians must understand the NEC permits these other types if they are listed for such use. Fixture wire types TFN and TFFN often are used because the insulation is equivalent to THHN thermoplastic. Table 402.3 describes these types of conductors as having an outside nylon jacket similar to THNN that protects their insulation during installation.

The NEC has many requirements for Class 1 circuits. You must know them all to be able to appropriately diagnose any situation.