My neighbor across the street, who was in the process of selling his house, came to me in a panic about his low-voltage decorative yard lighting not working properly. A few days before signing the final agreement for his home sale, he discovered that half the decorative lights on one circuit were dark. He was sure the potential new owners would decide against buying his home if everything wasn’t working properly.

He had installed the lighting system himself about two years before and explained that he had done everything in accordance with the manufacturer’s instructions, including using waterproof, silicone-filled wire nuts designed for direct burial. He had already excavated around a number of the inoperative lights, so I walked over to see if I could help him troubleshoot.

What’s the problem?

We determined that the problem was at the point of connection of the wire-nuts at the first two or three lights in the circuit. The drip irrigation for the bushes had been installed directly over the location where the low-voltage lighting was connected, and for two years the water for the bushes had caused deterioration of the wire-nut connection. Problem solved, sort of. 

I told my neighbor that, even though these wire-nuts were designed for direct burial and wet locations, direct submersion in water over a period of years caused this problem. Whenever I use these UL-listed waterproof wire-nuts, I said I always wrap the direct burial conductors and the wire-nuts with electrical tape or use shrink wrap to ensure longevity of the connection.

I also warned him that he should use proper outdoor low-voltage cable rated for direct burial, tested as waterproof and crush-resistant. The issue of
crush-resistance will help keep the cable safe when installed in gravel, sharp rocks and other decorative covering that could penetrate the insulation and result in deterioration. Most listed low-voltage direct burial cable is designed to be fairly robust, but must always be installed in accordance with the manufacturer’s instructions.

The NEC has answers

Section 300.5(A) and Table 300.5(A) in Column 5 address circuits for control of irrigation and landscape lighting limited to be not more than 30V AC or DC. The cables must be Type UF (underground feeder) cable or other identified cable or raceway, and must be buried at least 6 inches deep. 

Two notes at the bottom of Table 300.5(A) apply directly to circuits 30V and lower. Note 1 is “A lesser depth shall be permitted where specified in the installation instructions of a listed low-voltage lighting system.” Note 2 is “A depth of 150 mm (6 in.) shall be permitted for pool, spa, and fountain lighting, installed in a nonmetallic raceway, limited to not more than 30 volts where part of a listed low-voltage lighting system.”

Column 5 in Table 300.5(A) also provides different burial depths (other than 6 inches) for installations, such as 24 inches in depth where located under streets, highways, roads, alleys, driveways and parking lots, and 12 inches deep where located under one- and two-family dwelling driveways and outdoor parking areas used only for dwelling-related purposes. The reason for the increased depth is to take into consideration the flexing that occurs where vehicles run directly over the earth where the cables are installed.

Table 300.5(A) also has an additional Note 4 that states, “where one of the wiring method types listed in Columns 1 through 3 is used to install one of the circuit types in Columns 4 and 5, the shallowest depth of burial is permitted.” 

Column 4 applies to residential branch circuits rated 120V or less with GFCI protection and maximum overcurrent protection of 20A, so it doesn’t apply to low-voltage lighting. Column 1 applies to direct burial cables and conductors; Column 2 applies to rigid metal conduit and intermediate metal conduit; and Column 3 applies to electrical metallic tubing, nonmetallic raceways listed for direct burial without concrete encasement, or other approved raceways.

Article 411 covers low-voltage lighting systems and their associated components. The low-voltage lighting luminaires, power supply and fittings must be listed for use as part of the same identified lighting system or must be listed parts. 

The operating voltage of low-voltage lighting systems and associated components must not exceed 30V AC or 60V DC. If wet contact is likely to occur, the operating voltage of low-voltage lighting systems and associated components must not exceed 15V AC or 30V DC.

Fortunately, my neighbor successfully closed the sale of his home within days of fixing his low-voltage yard lighting. He still owes me a steak dinner for troubleshooting his system!

stock.adobe.com / Brian