18.1 Class 1, 2, and 3 Circuits (Art 725)
Key Takeaways
- Article 725 covers remote-control, signaling, and power-limited circuits and splits them into Class 1, Class 2, and Class 3 based on voltage, power, and wiring rules
- Class 1 circuits follow Chapter 3-style wiring methods; Class 2 and Class 3 are power-limited and use listed Class 2/Class 3 sources and cable types
- Chapter 9 Tables 11(A) and 11(B) define the Class 2 and Class 3 power-source limitation values used throughout Article 725
- 725.136 generally keeps Class 2/Class 3 conductors out of the same enclosure, raceway, or cable with power, light, Class 1, and non–power-limited fire alarm conductors unless a Code-listed separation method applies
- On Arkansas Prov Journeyman, Low Voltage is only 2 items—still treat Class identity, power limits, and separation as high-value open-book lookups
18.1 Class 1, 2, and 3 Circuits (Art 725)
Quick Answer: Open Article 725 when the stem involves remote-control, signaling, or power-limited circuits. Identify the class first. Class 1 uses Chapter 3–style wiring and higher energy. Class 2 and Class 3 are power-limited from listed sources, use Class 2/Class 3 cable types, and must obey 725.136 separation from power, light, Class 1, and non–power-limited fire alarm conductors unless a listed barrier or exception applies.
On the Arkansas Journeyman Electrician exam (Prov / ADLL, NEC 2023, 60 questions, 3 hours, 70% pass), Low Voltage Circuits, Including Alarms and Communications is only 2 items (~3.3%). That small count is not permission to skip the chapter—those two stems are usually pure Code lookups: class identity, power-source limits, cable markings, or separation. Journeyman fieldwork still depends on Article 725 every week for thermostat, control, doorbell, access-control, and signaling installs, so Prov expects solid fundamentals even with a light outline weight.
What Article 725 covers (and what it does not)
Article 725 applies to remote-control, signaling, and power-limited circuits that are not an integral part of a device or utilization equipment. Classic examples: thermostat cable, remote-control relays, signaling loops, and many listed Class 2 power supplies feeding low-energy loads.
Article 725 is not the home for fire alarm circuits (those live in Article 760) or for telephone/CATV/network communications circuits (Chapter 8, especially Article 800 and neighbors). If the stem says “fire alarm,” jump to 760. If it says “communications,” “primary protector,” or “coaxial,” jump to Chapter 8. Mixing articles is the most common LV miss on timed exams.
Class 1 vs Class 2 vs Class 3 — identity first
Class 1 circuits
Class 1 circuits include:
- Power-limited Class 1 circuits limited to 30 volts and 1000 volt-amperes, and
- Remote-control and signaling Class 1 circuits that may operate at higher voltages (up to 600 volts) when installed under Class 1 wiring rules.
Class 1 wiring methods generally follow Chapter 3 methods—raceways, cables, and boxes as for power circuits—because Class 1 energy levels can be hazardous. Treat Class 1 like “small power” for wiring-method purposes, not like doorbell cable.
Class 2 and Class 3 circuits
Class 2 and Class 3 circuits are power-limited. Their safety model is limited energy from a listed Class 2 or Class 3 power source, not heavy Chapter 3 raceway rules alone. Class 2 limits are lower (safer for accessible wiring); Class 3 limits are higher than Class 2 but still power-limited compared with ordinary branch circuits.
Exam habit: Read the stem for “Class 2,” “Class 3,” “power-limited,” “CL2/CL3 cable,” or “listed Class 2 transformer/power supply.” Those words lock you into Article 725 Parts for Class 2/3, not ordinary 120 V branch-circuit rules.
Power limits — Chapter 9 Tables 11(A) and 11(B)
NEC Chapter 9, Tables 11(A) and 11(B) list the voltage and volt-ampere (or current) limitations that define Class 2 and Class 3 power sources for AC and DC. You do not need to memorize every cell, but you must know:
- Class 2/Class 3 status comes from the listed power source meeting Table 11 limits (or equivalent listing), not from “it feels low voltage.”
- Overcurrent protection and source construction for those circuits follow Article 725 and the listing—do not casually fuse a Class 2 circuit like a 20 A lighting branch.
- If a supply exceeds Class 2/3 limits, it is not a Class 2/3 circuit no matter how small the load looks.
Prov trap: A 24 V control circuit is not automatically Class 2. Many 24 V controls are Class 2 when fed from a listed Class 2 transformer; the same voltage from a non-limited source can be Class 1. The source listing and limits decide the class.
Wiring methods and cable types
Class 1 methods
Install Class 1 conductors using Chapter 3 wiring methods suitable for the location and voltage. Conductor ampacity, raceway fill, and box rules track the power-wiring world more closely than Class 2 cable practices.
Class 2 / Class 3 cables
Class 2 and Class 3 circuits typically use listed cables such as CL2, CL3, and their riser/plenum/limited-use variants (CL2R, CL3R, CL2P, CL3P, CL2X, CL3X, and related markings). Substitution hierarchies in Article 725 allow higher-rated cables (for example, plenum-rated) to substitute downward where permitted—open the substitution table in 725 rather than guessing from memory.
Other permitted methods (raceways, certain cable types) appear in the Class 2/Class 3 parts of Article 725. On exam day, if the stem asks “which cable type is permitted in a plenum,” match the P (plenum) marking or an allowed substitute from the Code table.
Abandoned cable
Where Article 725 requires removal of accessible abandoned Class 2/Class 3 cable (or tagging when left for future use under the section’s rules), treat abandoned low-voltage cable as a real installation issue—not optional cleanup. Prov may phrase this as a life-safety/maintenance recognition item rather than a calculation.
Separation from power conductors — 725.136
725.136 is the separation core for Class 2 and Class 3. In plain language: Class 2 and Class 3 circuit conductors generally shall not be placed in any cable, enclosure, outlet box, raceway, or similar fitting with conductors of electric light, power, Class 1, and non–power-limited fire alarm circuits unless separated by a barrier or another method the section expressly permits.
Practical field translations that show up on exams:
| Situation | Typical Code move |
|---|---|
| Thermostat cable and 120 V conductors in the same box | Separate by barrier, or keep Class 2 out of the power box unless an allowed arrangement applies |
| Class 2 and power in the same raceway | Generally prohibited without continuous barrier / permitted separation |
| Class 2 cable run near power cable in a tray or open space | Maintain required separation or barriers per 725.136 and related sections |
| Same jacket multipurpose cable | Only if listed and constructed for the combination—do not invent “shared sheath” installs |
Why it matters: A fault or contact that introduces power-circuit voltage onto a Class 2 conductor defeats the limited-energy safety model. Separation protects people and equipment; it is not a cosmetic preference.
Also watch support and securing rules for Class 2/3 cables and any raceway fill when Class 2/3 conductors are installed in raceway under permitted conditions. Do not assume “low voltage means no Code.”
Class 1 sharing and control circuits — exam nuances
Class 1 conductors may share raceways with power conductors of the same circuit under Article 725 conditions more readily than Class 2 can, because Class 1 already uses power-like methods. Still open the Class 1 part of 725 for the exact allowance—never apply Class 1 sharing logic to Class 2 cable.
Motor control circuits and remote-control circuits may be Class 1 or Class 2 depending on the supply and design. If a stem describes a control circuit derived from a listed Class 2 source, stay in Class 2 rules. If it describes a control circuit run with Chapter 3 methods from a non-limited supply, think Class 1.
Worked open-book scenarios
Scenario A — 24 V thermostat cable. A listed Class 2 transformer feeds thermostat cable marked CL2. Separation from line-voltage conductors in shared boxes/raceways follows 725.136. Do not land the Class 2 conductors on the same terminal strip as 120 V without barriers and listing that permit the arrangement.
Scenario B — “Is 30 V always Class 2?” No. Power-limited Class 1 can be 30 V / 1000 VA. Class 2 requires the Table 11 / listing limits for Class 2. Voltage alone is not the class.
Scenario C — Plenum ceiling. Use CL2P / CL3P (or allowed substitutes) where plenum cable is required. Riser (R) and general-purpose markings are not automatic plenum equivalents unless the substitution table says so.
Scenario D — Mixing fire alarm and Class 2. Power-limited fire alarm is Article 760, not 725. Separation rules are cousins, but open the correct article.
Timed Prov tips for the 2 LV items
Tab Article 725 contents, 725.136, cable substitution tables, and Chapter 9 Tables 11(A)/(B). On any LV stem, ask in order: (1) Is this 725, 760, or Chapter 8? (2) Which class or power-limited type? (3) Source listing/limits? (4) Separation required? That four-step filter converts a low-weight outline area into reliable points.
Which NEC article primarily covers remote-control, signaling, and power-limited Class 1, Class 2, and Class 3 circuits?
What generally determines whether a circuit qualifies as Class 2 or Class 3 under Article 725?
Under 725.136, Class 2 conductors are generally kept separate from which of the following unless a barrier or other permitted method is used?
A 24-volt control circuit is installed from a non–power-limited supply using Chapter 3 wiring methods. Which classification is the best first description?