The path lights along our front walk came in a box from a big-box store, and the instructions were four pictures long. Push the stakes in, clip the fixtures onto the cable, plug the transformer into the outdoor outlet, done. I buried the cable about as deep as the trowel happened to go, which is to say barely, and I did it without a second thought because the whole thing ran on twelve volts. Two summers later I put an edger through the cable, and while I was splicing it back together in the dark I finally read the transformer label properly. It had two very different sides, and I had spent two years thinking about only one of them.
Two circuits pretending to be one
A landscape lighting setup looks like a single system because you buy it as a single kit. Electrically it's two systems bolted together at a box on the wall. On one side of that box is an ordinary 120-volt branch circuit, the same voltage as everything else in the house, arriving through a cord and plug or through fixed wiring. On the other side is the low-voltage secondary — the buried cable, the fixtures, the part everybody means when they say "it's just twelve volts."
The two halves have almost nothing in common. The low-voltage side is deliberately built so that touching it is not a shock hazard, which is the entire reason these systems exist for gardens where people walk barefoot and children dig. The 120-volt side is exactly as dangerous outdoors as it is indoors, and it lives in a spot that gets rained on, snowed on, and mowed past. Most of what goes wrong with landscape lighting happens on the half that nobody thinks about, or at the seam where the two halves meet.
What the Code calls this, and what it requires
The National Electrical Code covers these installations in Article 411, titled Low-Voltage Lighting. Its scope sets the ceiling: lighting systems operating at no more than 30 volts ac or 60 volts dc, and where wet contact is likely to occur, no more than 15 volts ac or 30 volts dc. That second, lower limit is the one landscape lighting is really designed around, which is why so many products land at twelve volts rather than pressing up against thirty.
Article 411 also cares how the pieces were assembled. Section 411.4 gives two acceptable paths. Either every component — luminaires, power supply, and fittings — is listed for use as part of the same identified lighting system, or the installation is put together from individually listed parts with a suitably rated cord, cable, or other fixed wiring method carrying the secondary. What is not on the list is the arrangement most homeowners drift into over a few seasons: one brand's transformer, another brand's cable, a set of fixtures from a third, and a couple of connectors improvised from whatever was in the garage.
The product standard behind the listed version is UL 1838, Low Voltage Landscape Lighting Systems, which covers an isolating power unit, low-voltage cable or flexible cord, and the luminaires themselves, with each secondary circuit limited to 15 volts ac, 25 amperes, and 300 volt-amperes. Those numbers are the reason a transformer has a wattage rating printed on it, and the reason that rating is a ceiling rather than a suggestion.
Six inches, and the footnote that moves it
Buried cable has a published depth, and it is shallower than people expect. Table 300.5 in the Code sets minimum cover for underground wiring, and its fifth column is written specifically for circuits controlling irrigation and landscape lighting that are limited to not more than 30 volts. In essentially every location that column asks for six inches of cover — under a lawn, under a driveway at a house, under an exterior slab. A footnote allows less where the installation instructions of a listed low-voltage lighting system say so, which is how kits that call for a shallow trench stay legitimate.
Six inches is not much, and it is still roughly three times what I had done. The depth is not there to protect you from the cable; it's there to protect the cable from you, and from the aerator, the edger, the fence-post digger, and the shrub that gets moved in year four. A nicked low-voltage cable rarely announces itself. It corrodes at the wound, gets warm under load, and shows up months later as a run of fixtures that has gone dim.
The ground fault device protects the half you worried about least
Outdoor receptacles at a dwelling are required to be ground-fault protected, and that protection is genuinely doing something: it watches the 120-volt circuit feeding the transformer, including the cord and the receptacle itself, which is the part of the installation sitting outside in the weather. What it does not do is reach past the transformer. An isolating power unit is designed to keep the two circuits electrically separate, so a fault out among the fixtures is not something the ground-fault device on the supply side can see or respond to. That's a feature, and it also means nothing on the low-voltage side is being monitored for you. The Outdoor Electrical Safety Check published by the Electrical Safety Foundation International, in cooperation with the Consumer Product Safety Commission, is a good plain-language walk through the outdoor half of this, and it makes a point worth stealing: when you plan an outdoor circuit, add up the ratings of everything that will share it — garden lights, an electric grill, a hot tub — rather than adding them one at a time until something trips.
Where twelve volts stops being a free pass
Low voltage removes the shock problem. It does not remove the heat problem, and heat is what actually damages these systems. A transformer loaded beyond its rating, a long run of undersized cable, or a splice made with a twist and some tape all produce the same thing: resistance where there shouldn't be any, and a warm spot in a bed of dry mulch. Watch for a transformer that is hot rather than warm, connectors that have gone green or chalky, cable insulation that has cracked from ultraviolet exposure, fixtures at the far end of a run that are visibly dimmer than the ones near the transformer, and any splice buried without a connector rated for direct burial. Voltage drop over a long cable is normal and is designed around; a sudden change in how one section behaves is not.
Water features are their own conversation
Article 411 has long kept low-voltage lighting systems back from the nearest edge of a pool, spa, or fountain by ten feet unless Article 680 specifically permits otherwise. Recent Code editions do carve out an exception for particular listed luminaire and transformer combinations, which is why you will see landscape fixtures legitimately installed close to modern pools. Whether a given fixture qualifies is a question about listings and markings rather than appearance, and getting it wrong puts equipment near water on the strength of a guess. If a pool, a spa, or a water feature is anywhere in the plan, that part of the layout belongs to someone who reads those listings for a living.
A ten-minute walk-around
None of this requires opening anything. Start at the transformer: read its volt-ampere rating and note it, check that it's mounted the way its instructions call for rather than resting on the ground, and confirm the receptacle it plugs into has a weatherproof cover that closes over the plug. Then walk the run. Look for cable surfacing through mulch, connectors sitting in standing water, fixtures leaning where a stake has rotted, and lenses fogged from the inside, which means the seal has failed. Count your fixtures and compare the total against the transformer's rating. Finally, mark where the cable crosses anything you dig, mow, or edge on a sketch you keep with the house records, which is what will matter the next time somebody plants a shrub.