How to Size Wire for Landscape and Low-Voltage Lighting
Low-voltage landscape lighting runs at 12 V AC (or DC on some modern systems), which means even modest resistance in the cable can chew through a significant percentage of your available voltage before a single fixture lights up. Unlike a 120 V branch circuit where a 1 V drop is barely 1%, a 1 V drop on a 12 V system is already 8%. Getting the wire size right is less about ampacity and almost entirely about keeping that drop within acceptable bounds.
Why voltage drop dominates low-voltage lighting
A 12 V halogen or LED fixture is rated at a specific input voltage. Most landscape transformers output 12 V, and fixture manufacturers typically specify a tolerance of plus or minus 10%, meaning the fixture should still work down to about 10.8 V. Drop below that and LEDs dim noticeably, color temperature shifts, and halogens look orange and underperform.
The governing formula is the same one used in any DC circuit: Voltage drop = Current × Resistance, where resistance depends on wire length, conductor cross-section, and the resistivity of copper. For a complete circuit, current travels out on one conductor and returns on the other, so you use twice the run length in your calculation. A 50-foot home run becomes 100 feet of conductor.
For a deeper look at the math, voltage drop explained walks through the formula from first principles.
Summing fixture wattage to find your current draw
Before picking a gauge, you need to know how many amps the circuit carries. That starts with total wattage.
Add up every fixture on the run. A typical LED path light draws 3 to 5 W; a spotlight might be 7 to 10 W; a flood could hit 20 W. Suppose you have eight 5 W path lights: that is 40 W total.
Amps = Watts / Volts. At 12 V, 40 W = 3.33 A. That current flows through the cable from the transformer to the fixtures and back.
One caution: if you mix fixture types, use the rated wattage of each, not a blended average. A single 20 W flood on a run with six 5 W lights adds 20 W, not something in between.
Common cable gauges for landscape lighting
Most outdoor landscape cable is rated for direct burial and comes as a two-conductor assembly. The four gauges you will encounter most often are:
| Gauge | Resistance (Ω per 1000 ft, round trip) | Typical max watts at 100-ft run | Typical max watts at 150-ft run |
|---|---|---|---|
| 16 AWG | ~8.2 Ω | ~100 W | ~65 W |
| 14 AWG | ~5.2 Ω | ~160 W | ~105 W |
| 12 AWG | ~3.2 Ω | ~250 W | ~165 W |
| 10 AWG | ~2.0 Ω | ~400 W | ~265 W |
These figures assume a 1 V maximum drop (roughly 8% of 12 V). Real-world allowances vary; some installers target 0.5 V drop for the most voltage-sensitive fixtures.
16 AWG is the standard cable supplied with most transformer kits. It works for short runs with light loads. For anything beyond 50 to 75 feet with more than five or six fixtures, 14 AWG or larger is usually the practical choice.
For runs where both distance and load push the limits, sizing cable for a long run covers the same trade-offs in a grid-tied and off-grid context that translates directly here.
Hub wiring vs. daisy-chain: balancing the drop
How you connect fixtures matters as much as which gauge you choose.
Daisy-chain (series of taps along one cable): each fixture is tapped off the main run at a different distance from the transformer. The fixture farthest away sees the highest drop because current has traveled the full length of cable. Common symptom: lights near the transformer glow bright while lights at the end of the run look dim.
Hub wiring (home runs from a central junction): all fixtures connect back to a single point that sits roughly in the middle of the zone, and the transformer feeds that hub. Each fixture sees nearly the same cable length and therefore nearly the same voltage. More cable is required, but drop is balanced.
A hybrid approach works well on longer properties: run a heavy 12 AWG or 10 AWG trunk from the transformer to a mid-zone hub, then short 16 AWG spurs to each fixture. The trunk carries the full load but covers the most distance at low resistance; the spurs are short enough that their higher resistance barely matters.
Worked example: eight LED path lights on a 120-foot run
Fixtures: 8 × 5 W LED path lights, daisy-chained along 120 feet of cable.
Step 1: Total load. 8 × 5 W = 40 W. At 12 V: 40 / 12 = 3.33 A.
Step 2: Round-trip conductor length. The farthest fixture is 120 ft from the transformer, so the conductor loop for that fixture is 240 ft.
Step 3: Resistance. 16 AWG copper is about 4.1 Ω per 1000 ft (one conductor). For 240 ft: (240 / 1000) × 4.1 = 0.98 Ω.
Step 4: Voltage drop at the end fixture. V = I × R. The current at the far end is only the current consumed by fixtures beyond any given tap. If you assume roughly equal spacing, the last fixture draws 5 W / 12 V = 0.42 A, but the cable from transformer to that point carried the cumulative current from all upstream fixtures. For a distributed load, effective drop is approximately half of what you would get if all load were at the far end. Full-load drop at 240 ft: 3.33 A × 0.98 Ω = 3.26 V. With distributed tap correction, the effective drop is closer to 1.6 V, which is 13% of 12 V, above the comfortable 10% limit.
Recommendation: upgrade to 14 AWG. Resistance drops to about 2.6 Ω per 1000 ft. Same math yields an effective drop of roughly 1.0 V, or 8.3%. Within spec and with margin for connector resistance.
For comparison, the approach for solar battery cable of similar length is covered in voltage drop in DC solar and battery systems, which uses the same formula with identical logic.
Safety, codes, and transformer sizing
Low-voltage landscape wiring (Class 2, operating below 30 V) falls outside the scope of NEC Article 210 branch-circuit rules, but the transformer itself connects to 120 V line voltage. That connection must follow the current edition of the NEC and, depending on your jurisdiction, may require a permit and a licensed electrician. Never splice or extend the line-voltage side of a transformer yourself.
Transformer capacity is its own constraint separate from wire sizing. If your total fixture load is 80 W, you need a transformer rated for at least 100 W to keep it from running hot. Most manufacturers recommend sizing to 75 to 80% of the transformer's maximum output.
Frequently asked questions
Can I use standard 14 AWG house wire for landscape lighting?
Standard NM-B (Romex) is not rated for direct burial or wet locations. You need cable labeled for direct burial, typically marked "UL Type UF" or with a "W" suffix indicating outdoor/wet suitability. The conductor size follows the same AWG scale, but the jacket material and listing matter.
My lights are dimming at the far end. What is the fastest fix?
The quickest fix is adding a second cable from the transformer to a mid-point on the existing run, effectively creating a hub. This halves the effective run length and cuts voltage drop significantly without requiring you to dig up and replace the existing cable.
How do I calculate ampacity for low-voltage landscape cable?
For typical residential landscape lighting loads, ampacity is rarely the binding constraint. A 16 AWG direct-burial cable is rated for 13 A or more in most conditions, far above what landscape fixtures draw. The ampacity explained article covers how temperature, burial depth, and bundling affect that rating if you are running higher loads.
Does wire gauge matter more for LED fixtures than for halogens?
LEDs are more sensitive to voltage variation than halogens in one respect: they often use internal drivers that have a minimum input voltage below which they stop working entirely, rather than just dimming. Halogens simply dim. This makes the 10% drop limit feel more binary with LEDs, which is one reason to target 8% or less on LED runs.