Exterior and Landscape Lighting Installation

Purpose

Exterior lighting fails in two places and both are decided at design, not at install. The first is voltage drop, which on a 12 V system is a large fraction of the supply rather than a rounding error, so a run that measures fine at the transformer starves the fixture at the end of it. The second is water at the connection, where the control is the listing and location marking on every enclosure, splice and fitting, and no amount of tape substitutes.

The failure this prevents is the system that looks perfect on the night it is commissioned and dims from the far end inward over the following season, which the customer reads as a lamp problem and which is actually a wire gauge and tap decision made months earlier.

Scope

Covers line-voltage exterior lighting and listed low-voltage landscape lighting on residential and small commercial premises: run mapping, the line-voltage against low-voltage fork, voltage-drop sizing, burial depth, wet-location connections, and the commissioning measurements.

Does not cover receptacle in-use cover verification, owned by that SOP; pool, spa and fountain lighting, which sits under its own article and its own SOPs; the feeder to a detached structure; or utility-owned street and area lighting.

Roles and handoffs

Procedure

1. Map every run with distances measured on the ground, and classify each fixture position as a damp or a wet location. A soffit fixture under a deep eave and a path light standing in a sprinkler zone are not the same environment, and NEC 410.10 and 406.9 both turn on that distinction. Acceptance: a run map with a measured distance from the source to each fixture or hub, and a damp or wet classification written at each position. Wrong looks like distances scaled off a landscape drawing that shows planting beds rather than cable routes. Stop rule: a fixture position with no classification does not get a fixture selected for it. Hazard clause: none here, measurement on the ground.

2. Take the line-voltage against low-voltage decision per run, and record the voltage ceiling the article sets. Listed low-voltage lighting systems fall under NEC Article 411, which sets the maximum secondary voltage and sets a lower ceiling in a wet location, so read both figures from the adopted edition rather than assuming one number covers both. Acceptance: each run marked line-voltage or low-voltage, with the article's ceiling for its location written beside it. Wrong looks like a single voltage assumed across a system whose fixtures sit in standing irrigation water. Stop rule: a run that cannot meet the wet-location ceiling changes system rather than changing location classification. Hazard clause: none here, a design decision from the step 1 map.

3. Compute voltage drop to the far end of every low-voltage run and pick the gauge and the transformer tap from the result. Drop equals twice the one-way length, times the current, times the conductor's resistance per foot, with the resistance read from NEC Chapter 9 Table 8 for the size and material you are actually using. Compare the delivered voltage against the lamp's published operating range, not against a percentage. Acceptance: the drop computed and written for each run, the delivered voltage stated, and both ends of the lamp's operating range written next to it. Wrong looks like a 3 percent rule of thumb borrowed from a 120 V branch circuit, where 3 percent is 3.6 V of headroom and on a 12 V system it is 0.36 V. Stop rule: a computed delivered voltage below the lamp's minimum is corrected by gauge or tap before the cable is bought, not by adding a second transformer later. Hazard clause: none here, arithmetic at a desk.

4. Read burial depth off the whole row of the table with the column named, because the landscape row is narrow. NEC Table 300.5 gives a shallow figure for circuits controlling irrigation and landscape lighting, and that row is conditioned on the circuit being limited to the stated low voltage and installed in type UF or another identified cable or raceway. A line-voltage exterior circuit does not qualify for it. Acceptance: method and column named on the job sheet with the required cover written as a number for each run. Wrong looks like a 120 V path-light circuit buried at the landscape depth. Stop rule: where the route cannot achieve the depth for the chosen method, the method changes, not the depth. Hazard clause: utilities are located before any trenching, and if a gas line is struck everyone leaves the area immediately and moves upwind, nobody starts an engine, uses a phone or operates a switch near the break, and 911 and the utility are called from a distance.

5. Make every connection with a device listed for the location it will actually live in. Direct-buried and submerged splices take a listed direct-burial or submersible connector, enclosures in wet locations are listed for wet locations and arranged to drain, and every luminaire carries the damp or wet marking its position requires. Acceptance: a line per connection type naming the listed device used and the location it is listed for. Wrong looks like a twist-on connector in a plastic bag at the bottom of a bed, which is the single most common exterior lighting defect and the one that produces intermittent failures a year later. Stop rule: any splice that cannot be made with a listed device at that location moves to an accessible wet-location enclosure above grade. Hazard clause: none here, selection and assembly at grade.

6. Install the transformer and the fixtures, mounting into masonry with dust control. Set the transformer at the height its instructions require above grade, keep its enclosure and any receptacle serving it in a weatherproof arrangement, and mount fixtures so a mower and a snow blade cannot reach them. Acceptance: transformer mounted to its stated clearance, fixtures set plumb at the mapped positions, and every masonry anchor drilled and sealed. Wrong looks like a transformer hung low enough to sit in a snowbank. Stop rule: a fixture position that cannot be protected from equipment is relocated on the map and the run recomputed. Hazard clause: drilling brick, block or stone releases respirable crystalline silica, so use a shrouded bit on HEPA dust collection per 29 CFR 1926.1153 for construction work; and eave or soffit fixtures worked from a ladder follow the ladder rules for the work you are doing, being 29 CFR 1926.1053 on construction work and 29 CFR 1910.23 in general industry, with the ladder set at its rated angle and the tech's belt buckle inside the rails.

7. Energize and measure what the computation predicted, at the far end of every run. Isolate, lock and tag under 29 CFR 1910.333(b)(2) and prove dead on a known live source before and after per NFPA 70E-2021, 120.5 before landing anything on the line-voltage side. Acceptance: every outdoor receptacle and every required device tested for ground-fault protection by tripping and resetting on its own button; voltage measured at the last fixture of each run under full load and falling inside the lamp's published operating range; and the photocell or timer proved by covering the sensor or advancing the clock and watching the system change state. Wrong looks like a run commissioned on a voltage reading taken at the transformer. Stop rule: a far-end reading below the lamp minimum stops the job at the tap or the gauge, and a reading above the lamp maximum stops it just as hard, because that end burns lamps. Hazard clause: the first energization happens on wet ground with the customer's family nearby, so the crew stands clear of standing water at the panel, the area around each fixture is clear of people, and nobody handles a fixture body while the run is live.

The record this produces

One exterior lighting sheet per job: the run map with measured distances and a damp or wet classification per position; the system decision per run with the article's voltage ceiling for that location; the drop computation per run showing the length, current, resistance figure and its source, the computed drop and the delivered voltage, set against both ends of the lamp's operating range; the burial method and column with required cover; the listed connector used at each connection type; the transformer mounting height; and the commissioning measurements at the last fixture of every run.

The office keeps the lamp operating range, because a lamp substitution two years later changes what the same measured voltage means. The far-end readings are the only evidence the system was ever right.

Worked pass: ten-fixture low-voltage path and uplight run, hub 120 ft from the transformer

Steps 1 and 2: ten fixtures teeing off a single hub, hub measured at 120 ft of cable from the transformer, every position classified wet because the beds are irrigated. All runs low-voltage under the adopted edition's wet-location ceiling.

Step 3: total connected load 50 W at 12 V, so the current is 50 divided by 12, which is 4.17 A. Cable is 12 AWG copper, whose direct-current resistance from NEC Chapter 9 Table 8 is about 1.93 ohms per 1,000 ft, that is 0.00193 ohms per foot. Drop is 2 times 120 ft times 4.17 A times 0.00193, which is 1.93 V. On the 12 V tap the delivered voltage would be 12.0 minus 1.93, or 10.07 V. The lamp specification sheet gives an operating range of 10.5 V to 15.0 V, so 10.1 V sits below the minimum by 0.4 V.

Step 3 fails on its own acceptance and takes its stop rule before cable is bought. Two corrections are compared: upsizing to 10 AWG, whose Table 8 resistance is about 1.21 ohms per 1,000 ft, would give 2 times 120 times 4.17 times 0.00121, or 1.21 V of drop and 10.79 V delivered, which clears the 10.5 V minimum by only 0.29 V; or moving to the transformer's 15 V tap, which on the original 12 AWG predicts 15.0 minus 1.93, or 13.07 V, comfortably inside the 10.5 to 15.0 V range. The 15 V tap is chosen and written on the sheet, with the 12 AWG retained.

Step 4: the landscape row of the table is used because this run meets its conditions on voltage and cable type, and the required cover is written as a number on the job sheet. Step 5: all ten hub connections made with listed direct-burial connectors, and the hub itself set in a wet-location enclosure arranged to drain.

Step 6: transformer mounted to the height its instructions state above grade; two masonry anchors for uplights drilled with a shrouded bit on HEPA collection.

Step 7: with the system on the 15 V tap and all ten fixtures running, the voltage measured at the last fixture reads 13.0 V, inside the 10.5 to 15.0 V range and within 0.07 V of the 13.07 V predicted. The outdoor receptacle serving the transformer trips and resets on its ground-fault button, and the photocell is proved by covering it and watching the system come on.

References

• NFPA 70, National Electrical Code, in the edition your authority having jurisdiction has adopted: 210.8 ground-fault protection, 406.9 receptacles in damp and wet locations, 410.10 luminaires in specific locations, Article 411 low-voltage lighting systems, Table 300.5 minimum cover, and Chapter 9 Table 8 for conductor resistance. Read the whole burial row and name its column; the landscape row carries conditions.
• Luminaire and lamp manufacturer specification sheets for the published operating voltage range the commissioning measurement is judged against.
• 29 CFR 1926.1153 respirable crystalline silica; 29 CFR 1926.1053 ladders on construction work and 29 CFR 1910.23 in general industry; 29 CFR 1910.333(b)(2) electrical work practices, with proving per NFPA 70E-2021, 120.5.
• See related: outdoor outlet and in-use cover verification; detached structure feeder installation; under-cabinet and accent lighting installation.