Class 1, Class 2, and Remote-Control Circuits
Purpose
This module explains how NEC Article 725 classifies remote-control, signaling, and power-limited circuits, what wiring rules apply to each class, and how those rules affect 24 VDC PLC I/O wiring and conductor separation inside a machine control panel.
Why Article 725 matters in machine panels
Industrial control panels routinely contain both power wiring (line voltage, motor branch circuits) and control wiring (24 VDC PLC I/O, 120 VAC relay coils, field device signals). Article 725 determines:
- Whether a control circuit is power-limited (Class 2 or 3) or unrestricted (Class 1)
- What conductor sizes, insulation ratings, and wiring methods are acceptable for each class
- Where and how control conductors must be separated from power conductors
Getting the classification wrong leads either to over-wiring a low-energy circuit (using full power-circuit conduit and 14 AWG wire for a 24 V PLC loop) or under-wiring an unrestricted control circuit (routing Class 1 conductors without conduit protection).
Class 1 circuits
Class 1 circuits have no power limitation imposed by NEC Article 725. They follow the same wiring rules as power circuits.
Key characteristics:
- No limit on voltage or power imposed by the circuit classification itself
- Must use wiring methods allowed for power circuits — generally conductors in conduit, cable tray, or similar methods
- Minimum conductor size: 14 AWG for conductors not in a raceway, 18 AWG for conductors in a raceway (Art 725.49)
- Protection against physical damage, voltage, and current follows Art 240 and Art 310
When you encounter Class 1 in a machine panel:
A 120 VAC control circuit running from a control transformer through relay coils to field devices is almost always a Class 1 circuit unless it is supplied from a listed Class 2 or Class 3 power supply. It must be wired with conductors and methods adequate for the voltage and current involved.
Art 725.46 permits Class 1 conductors to occupy the same cable, enclosure, or raceway as power conductors subject to Art 725.48.
Class 2 circuits
Class 2 circuits are power-limited. The supply must be a listed Class 2 power supply that limits output power to ≤ 100 VA and voltage to ≤ 30 V (or ≤ 150 V in some configurations — see Table 11(A) and 11(B) in the NEC).
Key characteristics:
- Supply must be listed and marked “Class 2”
- Relaxed wiring methods permitted: smaller conductors, lower-rated insulation, unenclosed wiring in some applications
- No minimum conductor size prescribed by Art 725 for Class 2 conductors within the panel (the listing of the power supply controls)
- Conductors must be separated from power and Class 1 conductors (see separation rules below)
In practice — 24 VDC PLC I/O:
Most 24 VDC PLC power supplies are listed Class 2 supplies. The I/O field wiring from the PLC output cards to field devices (sensors, solenoids) supplied from a listed Class 2 supply qualifies as a Class 2 circuit. This is the most common Class 2 scenario in machine panels.
Class 3 circuits
Class 3 circuits are also power-limited but at higher voltage levels than Class 2 — up to 150 V, with power limits per Table 11(A)/(B). They require more physical protection than Class 2.
Key characteristics:
- Supply must be listed and marked “Class 3”
- Voltage ≤ 150 V, power limited by the listed supply
- More physical protection required than Class 2 (conductors must be protected from physical damage)
- Less common in machine panels than Class 2
Classification decision flowchart
flowchart TD
START([Control circuit to classify]) --> Q1{Supplied from a\nlisted Class 2 power supply?}
Q1 -- Yes --> CL2[Class 2 Circuit\nArt 725.121\nRelaxed wiring methods]
Q1 -- No --> Q2{Supplied from a\nlisted Class 3 power supply?}
Q2 -- Yes --> CL3[Class 3 Circuit\nArt 725.121\nPhysical protection required]
Q2 -- No --> CL1[Class 1 Circuit\nArt 725.41\nPower-circuit wiring rules apply]
CL2 --> SEP[Apply separation rules\nArt 725.136]
CL3 --> SEP
CL1 --> SEP
The key determination is the supply listing, not the voltage level alone. A 24 VDC circuit supplied from an unlisted or non-Class-2-marked power supply is a Class 1 circuit and must follow power-circuit wiring rules even though the voltage is low.
Separation rules — Art 725.136
Art 725.136 governs how Class 2 and Class 3 conductors must be separated from power and Class 1 conductors.
The default rule: Class 2 and Class 3 conductors must not be placed in the same cable, enclosure, or raceway with power or Class 1 conductors.
Permitted exceptions (Art 725.136(B)–(D)):
| Condition | Separation required |
|---|---|
| Class 2 and Class 1 in same enclosure, separated by a barrier | Permitted if barrier maintains separation |
| Class 2 conductors in a cable that is listed for the purpose | May share enclosure with power conductors if listed |
| Class 2 conductors entering an enclosure containing power conductors | Permitted — the enclosure entry point is not the wiring method |
Practical application in a machine panel:
The most common compliant approach is to route 24 VDC Class 2 control wiring in a separate wire duct from 120 VAC and line-voltage conductors. Many panel builders use separate duct channels: one for power wiring, one for Class 2 control wiring. A physical barrier (metal divider, separate duct, or listed combination assembly) satisfies the separation requirement.
Where a barrier is impractical — for example, where field wires from a sensor enter the panel and must pass through the same opening as power conductors — the entry point itself is not a violation. Separation is required inside the enclosure once conductors are routed to their destinations.
NFPA 79 color coding and conductor identification
NFPA 79 (Electrical Standard for Industrial Machinery) requires conductor color coding that aligns with, and supplements, NEC requirements.
| Circuit type | NFPA 79 required color |
|---|---|
| 24 VDC control (Class 2, positive) | Blue |
| 120 VAC control (Class 1) | Red |
| AC power conductors — Line | Black (or per phase marking) |
| Grounded conductor (neutral) | White or gray |
| Equipment grounding conductor | Green (or green/yellow stripe) |
Using NFPA 79 color coding inside the panel also serves as documentation of the circuit classification: blue conductors are immediately identifiable as low-voltage Class 2 wiring, red conductors as 120 VAC Class 1 control.
Some panel builders use white or gray for 24 VDC return (negative) — this does not conflict with NEC as long as the insulation is identified at terminations as a DC negative, not an AC grounded conductor.
Common mistakes
| Mistake | Consequence | Correct approach |
|---|---|---|
| Treating any 24 VDC wiring as Class 2 without checking power supply listing | Class 1 rules not applied where required | Verify power supply is listed and marked “Class 2” |
| Routing Class 2 and 120 VAC control wiring in the same wire duct | Art 725.136 violation | Use separate ducts or a listed combination assembly with barrier |
| Using undersized conductors on Class 1 circuits because voltage is low | Overcurrent protection inadequate for conductor | Class 1 = power-circuit rules; use 14 AWG minimum in raceway |
| Ignoring the separation requirement at panel entry points | Misjudging what counts as a wiring method | Entry point is not the wiring method; separate inside the enclosure |
| Assuming Class 2 wiring needs no conduit inside the panel | Physical damage not considered | Class 2 inside a panel does not require conduit but conductors must be protected from damage |
Practical takeaway
The classification of a control circuit as Class 1, 2, or 3 depends entirely on the supply listing, not on voltage level. A 24 VDC circuit from a listed Class 2 supply qualifies for relaxed wiring methods and must be separated from power conductors. A 24 VDC circuit from an unlisted supply is a Class 1 circuit and must follow power-circuit wiring rules.
In machine panels, maintaining physical separation between Class 2 and power wiring — through separate wire ducts or barriers — is the simplest way to achieve compliance with Art 725.136 and alignment with NFPA 79 color-coding requirements.
This site is a personal-use paraphrase and navigation reference for industrial automation standards. It is not a substitute for authoritative standards documents, professional engineering judgment, or legal review. All content is sourced from a local RAG corpus and has not been independently verified against current published editions.
Items marked TO VERIFY have limited or unconfirmed local coverage. Items marked NOT IN CORPUS are not covered in the local repository. Do not rely on this site for compliance determinations, safety-critical design decisions, or legal interpretation.