In our previous issue, we said that LV systems must be listed as complete systems. Any listing requirement depends upon the conditions of use. The NEC poses limitations on these systems in certain locations [411.5].
If you install an LV system in:
- Walls, floors, or ceilings, you must use a Chapter 3 wiring method or the wiring supplied by a listed Class 2 power source. If the latter, install per 725.130.
- Pools, spas or other such water vessels, you must keep the LV wiring at least 10 feet from the edge of the water except as permitted by Article 680.
Article 411 also puts limitations on secondary circuits [411.6]. But it does not define what these are, nor is there a definition in Article 100. So what are they? Remember, LV systems require a low voltage power supply. The circuits coming off the secondary side of this transformer are secondary circuits. The circuit that feeds the primary side is a branch circuit (max 20A).
The transformer itself must be an isolating transformer that separates the branch circuit from the secondary circuit [411.6(B)]. Now, one thing you don’t want to do is couple the two sides by bonding one of the secondary conductors to ground. So the NEC explicitly prohibits doing that [411.6(A)].
If the installation is indoors, you can have bare conductors [411.6(C)]. But they must be at least 7 feet above the finished floor unless specifically listed for a lower installation height.