What is the difference between Class 2 and Class 3 circuits?

Both are power limited. Class 2 is limited for both fire and shock. Class 3 permits higher voltage where shock is possible, so it requires additional shock protection.

Can Class 2 and Class 3 wiring share a raceway?

With each other, yes, per NEC 725.139(A). Not with power or Class 1 conductors except under the exceptions in 725.136(B) through (I).

Can CL2 cable replace CL3 cable?

No. Table 725.154 allows higher rated substituting for lower, so CL3 can replace CL2 but not the other way around.

What class of circuit is the wiring between a Class 3 power source and the connected equipment?

The answer is Class 3. Article 725 defines each class by where the wiring sits relative to the power source. A Class 1 circuit is the portion of the wiring system between the load side of the overcurrent device or power limited supply and the connected equipment. A Class 2 circuit is the portion between the load side of a Class 2 power source and the connected equipment. A Class 3 circuit is the portion between the load side of a Class 3 power source and the connected equipment. The class of the source determines the class of the circuit.

What separates Class 2 from Class 3

Both are power limited, and both take their limits from Tables 11(A) and 11(B) in Chapter 9. The difference is the level of hazard the limits address. A Class 2 source is limited so that the output is considered safe from both a fire initiation standpoint and an electric shock standpoint. A Class 3 source is limited for fire initiation but can operate at a higher voltage where shock is a possibility, so it requires additional protection against electric shock. In practical numbers, Class 2 tops out around 30 volts for continuous alternating current, while Class 3 can reach 100 volts.

Section 725.121 lists the acceptable power sources: a listed Class 2 or Class 3 transformer, a listed Class 2 or Class 3 power supply, listed equipment marked to identify the Class 2 or Class 3 power source, a listed audio amplifier, a listed information technology equipment limited power source, or a dry cell battery within voltage and size limits.

Wiring rules that follow from the class

Section 725.136(A) prohibits Class 2 and Class 3 conductors from occupying the same cable, cable tray, enclosure, or raceway as electric light, power, Class 1, non power limited fire alarm, or medium power network broadband circuits, with the exceptions listed in 725.136(B) through (I). Section 725.136(D) is the useful one, allowing them together where the Class 2 or Class 3 conductors are separated by a barrier or where the power limited conductors are installed with a permanent barrier in an enclosure.

Section 725.139(A) permits Class 2 and Class 3 circuits to occupy the same cable, enclosure, or raceway as each other. Section 725.139(B) permits Class 3 circuits to be reclassified and installed as Class 1 if the Class 3 markings are eliminated and the circuit is installed using Class 1 wiring methods, per 725.130.

Section 725.127 requires the power source for a Class 2 or Class 3 circuit to be supplied by a branch circuit whose overcurrent protection does not exceed 20 amperes, unless the source is listed for a higher rating.

Cable types and substitutions

Section 725.179 covers listing requirements for CL2 and CL3 cable, in plenum, riser, and general purpose ratings. Table 725.154 gives the permitted substitutions, and the general rule is that a higher rated cable can replace a lower rated one. CL3P can substitute for CL3R, CL3, CL2P, CL2R, and CL2. CL2 cannot substitute for CL3, because the Class 3 shock protection requirement is higher.

Field takeaway

Class of the source equals class of the circuit. Class 3 is power limited like Class 2 but permitted at higher voltage, so it carries extra shock protection requirements. Keep Class 2 and Class 3 conductors out of power raceways unless a listed barrier separates them, and feed the power source from a 20 ampere or smaller branch circuit.