Class 1, Class 2 and Class 3 remote control, signaling and power-limited circuits are specifically covered in Article 725 and have been in the National Electrical Code for many years. Over the years of working in the field as an electrician, an electrical contractor, longtime member of Code -Making Panel 3 and NEC instructor, I have worked on various aspects of low-voltage power-limited circuits, remote control and signaling systems. But I continue to receive questions about these circuits, and a friend called me about whether it is ever permissible to convert a Class 2 or Class 3 into a Class 1 circuit.
A brief review of the typical uses and definitions for these circuits, as well as an explanation of how to use Class 2 and Class 3 circuits versus Class 1, might be helpful. Article 725 circuits are used as security system circuits, control circuits for motors, lighting dimmer systems, computer network systems, nurse call systems and access control circuits. Article 725’s scope states that remote control, signaling and power-limited circuits that are an integral part of a device or utilization equipment are not covered by this article. Stating that these systems that are integral to a piece of equipment means that the Class 1, Class 2 and Class 3 originate in the equipment and do not leave it. If the system originates in the equipment and then leaves, then Article 725 applies to these circuits.
The definitions of Class 1, 2, and 3 circuits are located in Article 100 of the 2020 NEC.
A Class 1 circuit is defined as “the portion of the wiring system between the load side of the overcurrent device or power-limited supply and the connected equipment.” To totally understand Class 1 circuits, the informational note, located below the definition, refers the user to 725.41 for the voltage and power limitations for Class 1 circuits.
Section 725.41 provides two classifications for Class 1 circuits. The first one is a Class 1 power-limited circuit that is supplied from a source with a rated output of not more than 30V and 1,000 volt-amperes (VA). That circuit would permit 33.3A of current and is certainly considered to be a shock and fire hazard. The second classification is a Class 1 circuit that is only limited to a maximum voltage of 600V, and the power output is not limited at all, so the current could be any one without limitation. The power-limited and nonpower-limited Class 1 circuits are a shock and fire hazard.
We rely very heavily on the voltage rating of the insulation and separation of the Class 2 and Class 3 circuits from power circuits, as indicated in 725.136.
A Class 2 circuit is defined as “the 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 considers safety from a fire initiation standpoint and provides acceptable protection from electric shock.”
Similar to a Class 2 circuit, the definition of a Class 3 circuit is “the portion of the wiring system between the load side of a Class 3 power source and the connected equipment. Due to its power limitations, a Class 3 circuit considers safety from a fire initiation standpoint. Since higher levels of voltage and current than for Class 2 are permitted, additional safeguards are specified to provide protection from an electric shock hazard that could be encountered.”
While Class 2 and Class 3 circuits have a power source maximum nameplate rating of 100 VA, based on tables 11(A) and 11(B) in Chapter 9, a Class 2 circuit is limited to 0.005A at voltages of 60V–150V. This means that a Class 2 circuit usually does not have enough voltage and current to initiate a fire and is not normally a shock hazard in a dry location. A Class 3 circuit can have current as high as 1A at voltages over 100V–150V, so we rely very heavily on the voltage rating of the insulation and separation of the Class 2 and Class 3 circuits from power circuits, as indicated in 725.136.
However, 725.130, Exception No. 2 does permit Class 2 and 3 circuits to be reclassified and installed as Class 1 circuits if the Class 2 and Class 3 markings on the power supplies are eliminated and the entire circuit is installed using wiring methods and materials based on Class 1 circuits. The informational note below Exception No. 2 states that the circuits that have been reclassified are no long Class 2 or Class 3. As long as all components in the reclassified circuits are rated for Class 1, there shouldn’t be an issue using the Class 2 or Class 3 as a Class 1 circuit.
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