9.2 Signaling Systems (0802)

Scope of Article 725

The 2023 NEC split this material into two articles. Through the 2020 edition, Article 725 covered Class 1, Class 2, and Class 3 circuits together. For 2023, Class 1 circuits were removed from Article 725 and relocated to a new Article 724, Class 1 Power-Limited Circuits and Class 1 Power-Limited Remote-Control and Signaling Circuits. Article 725 is now titled Class 2 and Class 3 Power-Limited Circuits and Cables. If your jurisdiction is on the 2020 NEC the rules are largely the same, but the article numbers are not.

These circuits are not part of a power or lighting circuit and operate at voltages and power levels lower than typical branch circuits. They are grouped into three classes, each with a different hazard potential and correspondingly different installation rules.

Class 1 Circuits (Article 724 in the 2023 NEC)

Class 1 circuits are not inherently power-limited, which is why they are wired like power circuits. Two kinds exist:

• Class 1 power-limited circuits — supplied from a source rated not more than 30 volts and 1000 volt-amperes (724.40(A)), with the power source protected by an overcurrent device not exceeding 167% of the volt-ampere rating divided by the rated voltage.
• Class 1 remote-control and signaling circuits — permitted to operate at up to 600 volts, with the equipment supplied limited to 2500 volt-amperes.

Class 1 conductors use Chapter 3 wiring methods, are overcurrent-protected at their ampacity (724.43), and may be installed in the same raceway or enclosure with power conductors only where the equipment powered is functionally associated (724.48). Any circuit whose failure or interruption would introduce a direct fire or life hazard must be classified Class 1.

Class 2 and Class 3 Circuits (725.121)

Class 2 and Class 3 circuits are inherently power-limited — the limitation is built into the power source, so the circuit itself cannot deliver enough energy to start a fire (Class 2) or to present a serious shock (both classes, at different thresholds). 725.121 requires the source to be a listed Class 2 or Class 3 transformer, power supply, or equipment marked as a Class 2 or Class 3 source, or a listed information technology equipment limited-power circuit.

The actual limits are not in a table numbered 725.121 — they are in Chapter 9, Table 11(A) for alternating-current sources and Table 11(B) for direct-current sources. From Table 11(A), a Class 2 AC source is limited to 100 volt-amperes, and Class 2 circuits generally operate at 30 volts or less. Class 3 circuits are permitted at higher voltage — up to 150 volts — which is why Class 3 wiring methods and cable are more robust than Class 2.

Class 1 / Class 2 / Class 3 Parameters

Wiring Methods and Separation (725.136)

725.136 governs the separation of Class 2 and Class 3 conductors from electric light, power, Class 1, non-power-limited fire alarm, and medium-power network-powered broadband conductors. The general rule in 725.136(A) is that Class 2 and Class 3 conductors shall not be placed in any cable, cable tray, compartment, enclosure, manhole, outlet box, device box, raceway, or similar fitting with those other conductors.

The exceptions are the practical part:

• Where the conductors are separated by a barrier, or where the Class 2/3 conductors are in a raceway inside the enclosure.
• In enclosures, where the power conductors are introduced solely to connect the equipment and a minimum 6 mm (0.25 in.) separation is maintained.
• In manholes, where the power conductors are in metal-enclosed cable or a listed nonmetallic-sheathed cable with a firmly fixed nonconductor barrier.
• Where the Class 2 or Class 3 circuit conductors are reclassified and installed as Class 1 under 725.130(B), in which case the whole circuit follows Article 724 and the Class 2/3 markings must be removed.

Separately, Class 2 and Class 3 circuits may generally occupy the same cable, raceway, or enclosure with each other, and may share with power-limited fire alarm circuits where the Class 2/3 conductors are insulated to the fire alarm cable requirements.

Cable Types and Substitution (725.154)

Section 725.154 and Table 725.154 define the cable markings and substitution ladder. Common cable types include:

• CL2, CL3 — general-purpose power-limited cable
• CL2P, CL3P — plenum-rated (for use in plenum spaces per 300.22(C))
• CL2R, CL3R — riser-rated (for vertical runs in risers per 300.19)
• CL2X, CL3X — limited-use (residential, behind finish surfaces)

The substitution rule in Table 725.154 permits a cable with a higher rating to substitute for one with a lower rating. Two independent ladders are stacked in that table: a fire-resistance ladder (plenum > riser > general purpose > limited use) and a class ladder (Class 3 > Class 2, because Class 3 cable is built for the higher voltage). Both must be satisfied. So CL3P may substitute anywhere in the Class 2 or Class 3 family; CL2P may substitute for CL2R and CL2, but not for CL3R, because it fails the class test even though it passes the fire-resistance test. The reverse of either ladder is never permitted.

Cable Substitution Ladder

Article 760 — Fire Protective Signaling (overview)

NEC Article 760 covers fire protective signaling systems, including fire alarm circuits. While detailed coverage appears in Section 9.3, the E2 inspector should note the relationship:

• 760.3 — fire alarm systems must comply with both Art 760 and NFPA 72 (National Fire Alarm and Signaling Code).
• 760.41 / 760.81 — power sources for non-power-limited (NPLFA) and power-limited (PLFA) fire alarm circuits.
• 760.179 — cable types: FPL (general-purpose fire alarm), FPLP (plenum), FPLR (riser).
• Table 760.154 — fire alarm cable substitution ladder, which follows the same upward-substitution principle as 725.154.

Where a fire alarm cable must be identified as fire alarm only, the cable marking begins with FPL and a color trace is not required by the NEC unless specified by NFPA 72.

Inspector Field Checks

1. Verify the listed Class 2 or Class 3 power supply is used and not a non-listed substitute (725.121), and that the source marking matches the class of the installed cable.
2. Confirm Class 2 and Class 3 conductors are separated from light, power, Class 1, and non-power-limited fire alarm conductors per 725.136, or that one of the barrier, raceway, or 0.25 in. separation exceptions is properly applied.
3. Confirm the installed cable is listed and rated for the environment — plenum cable in plenums, riser cable in risers (725.154 + Table 725.154).
4. Confirm substitutions are upward only — a CL3 cable is acceptable where CL2 is required, but not the reverse.
5. Where a circuit is part of a fire alarm system, verify Art 760 cable (FPL family) and NFPA 72 compliance per 760.3.