Field guide
Low voltage landscape lighting transformer size
How to estimate transformer wattage, headroom and cable-run risk for outdoor lighting.
Add load first
Multiply the number of fittings by their wattage, then add spare capacity. Running a transformer near its limit is rarely a good planning choice.
Cable length matters
Longer low-voltage cable runs can create voltage drop. Splitting the garden into zones often works better than one long run.
Leave room for changes
Outdoor lighting plans evolve. Spare transformer headroom prevents one extra fitting from forcing a full redesign.