Landscape Lighting Calculator

How it’s calculated. Each run’s current is I = total watts ÷ 12 V. Voltage drop uses the two-conductor formula VD = 2 × R × I × L × (N+1)/(2N), where R is the copper resistance per foot (20°C), L is the distance to the furthest light, and the (N+1)/(2N) factor accounts for the load being spread across N fixtures on a one-direction daisy chain (one fixture = worst case; many fixtures ≈ half the drop). The far fixture voltage is the chosen tap minus the drop.

Tap & lamp window. The tool picks the lowest transformer tap (12–22 V) that still delivers the minimum voltage to the furthest light — 10.8 V for halogen (tight 10.8–12 V window) or 9 V for LED (wide 9–15 V). Raising the tap raises voltage on the whole run, so it warns when the near fixtures would over-volt — a real problem for halogen, which is why long halogen runs need heavier wire or looping.

Looping vs. one direction. These calculations assume the cable runs in one direction (not looped). When a run’s drop is too high, the tool tells you to step up the wire gauge, loop the run (feed both ends, which roughly halves the drop), or split it into shorter runs. Transformer size is the connected watts divided by 0.8 (never load a low-voltage transformer past ~80%), rounded up to a standard size (75–1200 W; larger jobs use multiple transformers).

This is an educational planning aid — always verify the finished system with a voltmeter at the fixtures. Designing landscape or architectural lighting for a commercial property? Our Tucson commercial electricians handle site and landscape lighting as part of the build.