Low Voltage Systems and Applications
This domain covers the systems themselves: what counts as Class 1, 2, or 3, how power-limited sources are bounded, and how real assemblies such as HVAC controls, 0-10 V dimming, access control, IP cameras, and paging are powered and interconnected.
Core rules
- Article 725 splits remote-control and signal circuits into Class 1 (not power-limited, treated much like power wiring) and Class 2/3 (power-limited). Source limits appear in Chapter 9 Table 11(A) for Class 2 and Table 11(B) for Class 3, both capped at 100 VA, with allowable current falling as voltage rises.
- Class 2/3 cable is marked CL2, CL3, CL2R/CL3R (riser), CL2P/CL3P (plenum), or CL2X (limited use). Substitution runs downward only: CL3P may serve where CL2, CL2R, or CL2P is required; the reverse is not permitted.
- A 0-10 V dimming pair is a Class 2 control circuit, but the class follows the driver's listing, so confirm the driver before sharing a raceway with line voltage.
- Power over Ethernet: 802.3af delivers 15.4 W at the source and 12.95 W at the device; 802.3at (Type 2) 30 W/25.5 W; 802.3bt Type 3 60 W/51 W and Type 4 90 W/71.3 W, all over 44-57 VDC.
- TIA-568 horizontal cabling: a 90 m permanent link plus 10 m of patch cords equals a 100 m channel.
Worked example
A 48-port PoE switch is rated 370 W total. Twenty cameras at 12.95 W draw 259 W and ten phones at 6 W draw 60 W, totaling 319 W, inside the 370 W budget. But if each port allocates only 15.4 W and a camera's inrush is 18 W for two seconds, negotiation succeeds while startup may still brown out. Check the aggregate budget and the per-port allocation separately.
Traps that cost marks
Treating watts and VA as interchangeable; assuming Class 2 cable may share a raceway with line voltage; and forgetting that a 0-10 V pair is polarity-sensitive, so reversed conductors can hold a driver at full output.