Damper Actuators: Is It Class 2 or Class 1?
A building-automation retrofit in a six-story office building adds nine 24 VAC damper actuators rated 12 VA each, controlled from a panel in the mechanical room. The mechanical contractor supplies a 24 VAC, 150 VA transformer that is not marked as a Class 2 power source. Your crew has already pulled 18/2 CL2P plenum cable to each actuator through the return-air ceiling, and the runs from the mechanical room pass through a 4 in. wireway that also carries the 120 V control conductors for the air handlers. The project superintendent wants the actuators energized this week to prove the sequence of operation, and the electrical inspector has asked for a written basis for the cable type before he will look at the ceiling. Your stock trailer has CL2R, CL3P, and CL2P on hand.
1. Determine the classification of the 24 V actuator circuit and state whether the installed CL2P cable is acceptable.
Read the worked answer
Classify the source first
- A Class 2 circuit must be supplied from a listed Class 2 power source. The common Class 2 ceiling for a 24 V source is 100 VA (Table 725.121). A 150 VA transformer not marked as a Class 2 source is not one.
- Operating at 24 V with 150 VA available puts this in the Class 1 power-limited range (30 V, 1,000 VA - 725.42), so the circuit is Class 1, not Class 2.
Do the load math too
9 actuators x 12 VA = 108 VA, which is 4.5 A at 24 V. Even a listed 100 VA Class 2 transformer would be overloaded, so swapping to a single 100 VA unit does not solve it.
Compliance
- CL2P cable is listed for Class 2 circuits. On a Class 1 circuit it is the wrong wiring method: Class 1 conductors must be sized and installed under 725.49 (14 AWG minimum with limited exceptions) and separated per 725.48.
- Viable fixes: (a) supply the actuators from two listed Class 2 transformers at 60 VA each, keeping each circuit under 100 VA and keeping the CL2P; or (b) treat it as Class 1 and rewire with Class 1 conductors and methods. Write the basis down for the inspector either way.
2. The crew needs cable today. Of the CL2R, CL3P, and CL2P on the trailer, which may be installed in the return-air ceiling, and why?
Read the worked answer
Plenum rules
A ceiling used as a return-air plenum is governed by 300.22(C). Wiring there must be in metal raceway, metal-clad cable, or a cable listed as plenum rated for the application. Riser and general-purpose ratings do not qualify.
Ranking the stock
- CL2R: riser rated only. Not permitted in the plenum, even inside a J-hook or on a bridle ring.
- CL2P: correct class and correct plenum rating - the baseline choice.
- CL3P: Class 3 plenum cable. The substitution table in 725.154 permits a higher-class cable with equal or better fire rating to substitute for a lower one, so CL3P may be used in place of CL2P. The reverse is never allowed, and CL2R may never substitute for CL2P.
Practical judgment
Using CL3P on this job is code-legal but wasteful if CL2P is available and the circuit stays Class 2; reserve CL3P for where a Class 3 cable is genuinely required. Whatever you install, keep the plenum rating visible on the jacket at accessible points so the inspector can read it without pulling the ceiling apart.
3. The 24 V runs share a wireway with 120 V air-handler control conductors. How does the classification you determined change what is permitted there?
Read the worked answer
If it stays Class 2
725.136(A) prohibits Class 2 conductors in the same raceway, wireway, or box as power or Class 1 conductors unless a permanent barrier or a listed exception applies. The shared wireway would be a violation, and the fix is a separate wireway or a listed divider.
If it is Class 1, as calculated here
Separation is governed by 725.48 rather than 725.136. Mixing with power conductors is permitted only under the specific conditions of that section, and the 18 AWG CL2P cable still fails because it is not a Class 1 wiring method. So the shared wireway does not become legal just because the circuit is Class 1.
Practical guidance
- Give the control conductors their own wireway or a barrier, regardless of class.
- Where the runs parallel VFD output conductors, use shielded cable with the shield grounded at one end and cross power conductors at right angles; induced noise on 24 V actuator signals is a commissioning failure waiting to happen.
- Record the final class, the cable type, and the separation method on the as-built for the inspector's file.