Low-Voltage and Fire Alarm Circuits (NEC 725 and 760): Cable Ratings, Separation and Support for Apprentices

Tsai Electro is an independent learning site. Not affiliated with TDLR, PSI or NFPA. We explain the ideas in our own words and do not reproduce NEC text. Check every rule and table value in your own 2026 NEC. In Texas, fire alarm work has its own licensing and rules through the State Fire Marshal’s Office, and some electrical work may be exempt only under specific supervision and responsibility conditions. Confirm the licensing, permitted work and supervision before you work on a fire alarm system.

Where these circuits live in the NEC

Chapter 7 of the 2026 NEC holds the rules for limited-energy systems such as Class 2, Class 3 and fire alarm circuits. The 2026 edition reorganized it: general rules, power sources, cables and raceways now have their own articles, and the circuit-class articles point back to them. Chapter 8 now focuses on communications systems outside a building or entering it, and most requirements for communications wiring inside the building are now in Chapter 7. Before the exam, practice finding three rules in your own 2026 book: where Class 2 power sources are covered, which cable marking is permitted in a plenum, and which rule covers separation from power conductors. Start with Articles 720 (general requirements), 721 (power sources), 722 (cables and substitutions) and 723 (raceways and cable trays), then follow the cross-references into Articles 725 and 760. Practice with the soft-bound 2026 NEC you plan to bring to the exam. PSI allows highlighting, underlining and notes made before the exam, but does not allow loose-leaf, spiral-bound or ring-bound copies. Only permanent index tabs made or supplied by the NEC publisher are allowed; homemade and third-party tabs are not. For each lookup, write down the article, the section, the conditions or exceptions and your final installation decision, then repeat it on a timer without your notes.

What you are looking for	Where to look in the 2026 NEC
General requirements for these systems	Article 720
Power sources	Article 721
Cables, cable markings and substitutions	Article 722
Raceways and cable trays	Article 723
Class 1 power-limited circuits	Article 724
Non-power-limited remote-control and signaling circuits (formerly Class 1 non-power-limited)	Article 206
Class 2 and Class 3 circuits (thermostats, doorbells, control wiring)	Article 725
Grounding and bonding for these systems	Article 750
Fire alarm systems	Article 760
Communications outside or entering the building	Chapter 8

Class 2 in plain words

A Class 2 circuit is powered by a source that limits the energy it can deliver, such as a listed Class 2 transformer or power supply. Because a compliant Class 2 source limits the available energy, the NEC permits different wiring methods than for ordinary power circuits. Class 2 wiring still has rules for cable selection, separation, physical protection and support. The power source is what makes a circuit Class 2, not the voltage alone. A 24 V circuit is not Class 2 just because it is 24 V; check that its source is one the NEC accepts for Class 2, such as a listed Class 2 transformer.

Cable ratings by space

Class 2, Class 3 and power-limited fire alarm cables carry markings that identify their listed cable types. The table shows the cable markings commonly associated with each space, not a complete list of permitted installations; other cable types or wiring methods may be allowed under the 2026 NEC. The table covers these cable types only; non-power-limited fire alarm circuits have different wiring rules. Substitutions depend on the cable type, the circuit class and the space. A marking permitted in one space is not automatically permitted in another, so use the substitution rules in Article 722 rather than assuming a different marking works.

Space	Class 2	Class 3	Power-limited fire alarm
Plenum (a space used for environmental air where plenum-rated cable is required)	CL2P	CL3P	FPLP
Riser (a vertical run between floors where riser-rated cable is required)	CL2R	CL3R	FPLR
General use	CL2	CL3	FPL

Check the substitution table in your 2026 NEC before you substitute one cable for another. A quick practice case: you have CL2R cable and the run goes through a plenum space used for environmental air. CL2R is a riser rating and does not qualify as CL2P, so select listed CL2P cable for the plenum run, or verify another cable type or wiring method the 2026 NEC specifically permits there. For every substitution, check Article 722, not just the letters on the jacket.

Keep it apart from power

Power-limited conductors must be separated from power and lighting conductors. In practice:

• Do not pull Class 2 cable in the same raceway as power conductors unless a specific exception applies.
• Do not treat a barrier or spacing as automatic permission to combine Class 2 and power wiring. First identify whether it is a raceway, box, enclosure or equipment compartment, because the exceptions differ. Start with the general limited-energy articles, 720 to 723, follow the 2026 cross-references into Article 725 for separation and wiring methods, and check whether the rule permits that exact arrangement, including any barrier, spacing or equipment conditions.
• When equipment has both line-voltage and Class 2 connections, like a furnace control board, find the terminals and wiring compartments from the equipment markings and installation instructions, and keep the two circuit types apart the way the NEC and the listing require.
• Practice: a Class 2 control run and a line-voltage circuit go to the same equipment. Before you decide whether they may share a raceway, write down what makes the control circuit Class 2 (Article 721), which separation rule applies, whether a specific exception fits, and the wiring method you would use if it does not.

Fire alarm circuits are either power-limited (PLFA) or non-power-limited (NPLFA), and the two are installed differently. FPL, FPLR and FPLP are cable markings for power-limited fire alarm circuits. Identify the circuit type first, then check its separation and cable rules in Article 760.

Support it on its own

Low-voltage cable generally cannot be strapped to the outside of a raceway as its means of support. A limited exception lets certain Class 2 cables be supported by the raceway that supplies power to the equipment they control, but only when the cable type, attachment method and other conditions are met. Check the 2026 support rules, including how the cable is secured to the raceway, before using this method. It does not allow low-voltage cable to be attached to any other raceway. Otherwise use suitable supports such as J-hooks, straps, hangers or cable trays, following the wiring rules and the manufacturer’s instructions. Route cable so it does not obstruct access to equipment above a ceiling. For abandoned cable, check the removal requirements and exceptions in the 2026 NEC that apply to the cable type and installation.

Worked example: is the thermostat transformer big enough?

A furnace has a 24 V Class 2 transformer rated 40 VA. It feeds three zone valves drawing 0.3 A each and a thermostat drawing 0.05 A, and nothing else on the furnace runs from that transformer.

1. Transformer rated output current: 40 VA ÷ 24 V = 1.67 A (about).
2. Load current: 3 × 0.3 + 0.05 = 0.95 A.
3. Assuming these are the complete simultaneous operating loads at 24 V, the load is 0.95 A, or 22.8 VA, which is below 1.67 A and the 40 VA rating. That leaves 17.2 VA of nominal capacity, so the 40 VA transformer has enough steady-state capacity under these assumptions. It does not prove the transformer handles every startup condition. On a real job, also check each device’s VA rating from the manufacturer, including any inrush or simultaneous operation, before you pick the transformer.

Check it in volt-amperes: 0.95 × 24 = 22.8 VA, which is less than 40 VA. Same answer.

Check yourself

1. Which cable marking may be installed in a plenum: CL2, CL2R or CL2P? CL2P.
2. Can you zip-tie Class 2 cable to the outside of any conduit as its support? Generally no. Use a permitted cable support method. A limited exception may apply to the raceway that supplies the equipment the Class 2 cable controls, but only when the installation meets its conditions.
3. A 24 V, 40 VA transformer: what is its rated current? About 1.67 A.

FAQ

Sources checked October 2026

• PSI Candidate Information Bulletin, Texas electrician examinations (updated September 15, 2026): https://test-takers.psiexams.com/api/content/bulletin/1682
• NFPA 70, National Electrical Code, 2026 Edition, Chapters 7 and 8 and Article 300
• IAEI Magazine, NEC 2026 significant code changes: https://iaeimagazine.org/standards/nec-2026-significant-code-changes/