12V Landscape Lighting and Article 411
A homeowner hires a contractor to install a 12V landscape lighting system around a new patio. The transformer is a listed 300W, 120V primary/12V secondary unit plugged into an exterior GFCI receptacle. The contractor runs 12 AWG UF cable 18 in. deep from the transformer to the first fixture, then daisy-chains 14 fixtures, each rated 9W, using 16 AWG cable for the last 60 ft. The total cable length from transformer to the last fixture is 180 ft. The transformer secondary is not grounded, but the contractor bonded the secondary to the building grounding electrode. The fixtures are listed for wet locations. The 16 AWG cable is direct-buried at 6 in. depth. The local inspector questions the installation under Article 411.
1. Calculate the total load and evaluate the 300W transformer. Is it adequate?
Read the worked answer
14 fixtures x 9 W = 126 W. The 300 W transformer is adequate in capacity and is loaded to about 42%. At 12 V, load current = 126 / 12 = 10.5 A. But the 16 AWG section and voltage drop may be the real problem. Check the transformer's listing for continuous load and the secondary circuit rating. A 300 W unit is not wrong by size alone.
2. Identify the Article 411 and related violations regarding the secondary grounding and bonding and burial depth.
Read the worked answer
Article 411 covers low-voltage lighting systems operating at 30 V or less. The system must be listed and installed per its listing. The secondary of a listed low-voltage system is generally isolated from the primary, so bonding the secondary to the building grounding electrode is not permitted unless the listing or instructions require it. The 12V secondary should not be grounded. For burial, the 12 AWG UF at 18 in. is generally acceptable, but the 16 AWG at 6 in. must be checked against Table 300.5 and local amendments; do not assume that a shallow direct-burial cable is permitted just because the circuit is low voltage.
3. What corrections would you recommend for voltage drop and code compliance?
Read the worked answer
Remove the secondary bonding to the grounding electrode and follow the manufacturer's isolation requirements. Replace the 16 AWG daisy-chain section with 12 AWG or reduce load or length. For example, 10.5 A over the 16 AWG section creates excessive drop; 12 AWG throughout gives much better voltage regulation. Use listed direct-burial connectors, wet-location fixtures, and GFCI protection, and inspect before backfill. Verify burial depth with the local code.