Class 1

A Class 1 remote control or signaling circuit cannot operate above 600 V and could be powered from sources with no power or current limitations. These circuits must be wired with 600 V insulated conductors regardless of their operating voltage. A Class 1 power-limited circuit is that portion of wiring between a power-limited supply and the connected equipment. Voltage cannot exceed 30 V (AC or DC) and power is limited to 1,000 VA. Although the voltage is low, the same 600 volt conductors must be used. The Class 1 limited-power circuit is used when the power required exceeds that of a Class 2 circuit as defined below.

Class 2

A Class 2 circuit is defined as the wiring between a Class 2 power source and connected equipment. By limiting the power to 100 VA and the voltage to 30 V, safety is ensured against shock or fire initiation. This is the type of wiring commonly used with intercoms, security systems, thermostats and other low-energy devices. Interestingly, local area network (LAN) cables such as twisted-pair or coaxial cabling are considered Class 2 circuits. The code states that Class 2 circuits must be wired with cables rated as Class 2 (CL2) with a voltage rating of 150 V, or with Communications cable (CM) with a voltage rating of 300 V, or with Power-Limited Tray Cable (PLTC) with a voltage rating of 300 V. Do not be fooled by the voltage rating; Class 2 circuits must still operate at voltages not exceeding 30 V and must be connected to power sources that are clearly marked as Class 2 power sources. Control circuits providing 120 V control are not Class 2 circuits. The advantage of using Class 2 circuits versus Class 1 circuits is that there are fewer restrictions in wiring, but there are still restrictions.

Class 3

Class 3 circuits exceed the 30 V limitation of a Class 2 circuit so they are power-limited to only 0.5 VA. However, the voltage can be as high as 150 V. These circuits are used in similar applications as Class 2 devices but when higher voltages are required. Cabling can be CM, PLTC or Class 3 (CL3) with a voltage rating of 300 V. Class 3 circuits are not as popular as Class 2 circuits.

Other Considerations

Since LAN cabling is considered a Class 2 circuit, there would be an advantage to having the power to the Industrial Ethernet hub or switch be treated as a Class 2 circuit as well. In this way wiring constraints for both power and signal would be the same. With power requirements well below the 100 VA limit, it would seem possible to use Class 2 power sources. However, if direct mains powering is preferred, Class 1 wiring restrictions apply.