The overcurrent protection (OCP) for Class 2 and 3 circuits is inherent because it is equipped in their power supply. Separate OCP when used for these circuits only applies to the supply side of the power source. Section 725.51 requires such protection when installed on the supply side to be limited to 20 amps or less. Because their power source has an inherent limited power output, Class 2 and 3 circuits are considered power-limited circuits. Therefore, the requirements of Chapters 1 through 4 are modified as they apply to OCP, minimum conductor sizes, insulation requirements, derating factors and wiring methods.
Article 725 does not mention anything about derating Class 2 or 3 conductors. Neither does Article 310 provide ampacities for the sizes used to cable these circuits. Because the power supply for these circuits are inherently current limiting, if short circuits should occur, there is no hazard, and there is really no need to list ampacity ratings for these conductors.
Article 725 permits the use of small conductors because the risk of overloading is very unlikely due to their power supply being current limiting. Low current levels that are typically available in Class 2 and 3 circuits qualify them, in most cases, not to be considered as having harmful temperature ratings. Section 725.28 states all cables must be used within their ratings and installed accordingly, especially when they can produce heating problems. And typically, when reclassified, these conductors fall under the installation rules for Class 1 circuits.
Selecting wiring methods
There are three choices for wiring methods to enclose Class 2 and 3 circuits. Class 1 wiring methods can be used whether such circuits are reclassified or not. Cable types specifically listed for the circuit type can be used, or substitute listed type cables can be employed based on Class 2 or 3 circuit types. Class 2 or 3 circuits installed entirely or partially with a Class 1 wiring method must remain a Class 2 or 3 circuit and be classified as such. However, when Class 2 or 3 circuits are reclassified per 725.8 or 725.52(A), Ex. 2, they become Class 1 circuitry.
Even though such circuits continue to be supplied by Class 2 or 3 power sources, they must comply with all the requirements of Class 1 circuitry, including appropriate OCP for the circuit. Remember, reclassification of one or more of these circuits will permit the use of the separation requirements of Class 1 circuitry instead of the more stringent rules of separation required for Class 2 or 3 circuits.
One must very carefully apply the rules of 725.52(A), Ex. 2 because this section contains permissive language as outlined in 90.5(B), while 725.8 requires reclassification only if the conditions of the section are applicable. Again, apply these two sections very carefully in order to prevent installation errors. The best choice for selecting Class 2 or 3 wiring methods can be found in 725.52(B). This section coupled with other referenced sections permits the use of special wiring types that are not normally permitted for any other wiring uses.
Separation of circuits
According to inspectors in the field, there has always been a problem separating Class 2 or 3 circuits from power and lighting circuits as 725.55 requires. When installing these circuits or a combination of both, separation may not be necessary; they can be mixed without applying the restrictions of 725.56.
To ensure the low energy output of these circuits is maintained, they must be separated from power circuits. In order for Class 2 or 3 circuits to occupy the same enclosure with non-power-limited circuits, separation must be accomplished by using a raceway or by installing a barrier. At least a 2-inch physical separation is needed.
Grounding Class 2 and 3 circuits
Class 2 and 3 power systems and circuits do not have to be grounded unless derived from systems where the voltage to ground is more than 150 volts; derived from ungrounded systems; or run outside, overhead or on the outside of buildings. If the power source is ungrounded, then the equipment, raceways, boxes and other metallic enclosures can be installed ungrounded per 250.20(A). Class 3 circuits typically have higher voltage levels than Class 2 circuits and, therefore, are more likely to fall under the grounding rules of 250.20(B). Class 3 circuits have the capability to limit fire hazards, while Class 2 circuits limit both fire and shock hazards, so the uses of Class 2 are more desirable. EC
STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the NEC and OSHA,as well as other standards. Contact him at 817.581.2206.