Landscape Lighting Transformer Calculator

Transformer Size is based purely on your total wattage load (Number of Fixtures times Watts per Fixture) plus a 20% headroom margin -- the transformer just needs to supply enough power, regardless of what gauge wire carries it. Cable Gauge instead affects Voltage Drop and Max Run Distance, since thicker wire (a lower gauge number) has less electrical resistance.

The calculator sizes up to the smallest standard transformer that keeps your load at or below 80% of its rated capacity, a 20% headroom margin that's standard practice in low-voltage lighting design. Running a transformer continuously at its full rated capacity shortens its working life and leaves no room to add fixtures later without upgrading.

It's the longest cable run, at your current fixture load and chosen wire gauge, before voltage drop crosses the commonly used 10% guideline -- beyond that point, fixtures near the end of the run will visibly dim compared to those closer to the transformer, since less voltage reaches them after resistance losses along the wire.

More fixtures mean more total current flowing through the cable, and voltage drop increases with current for a given wire resistance and length. So the same cable run that was fine for a smaller fixture count can cross the 10% voltage-drop threshold sooner once enough additional fixtures raise the total current draw.

It uses simplified per-1000-foot resistance values for common landscape lighting wire gauges and doesn't account for connector or splice losses, wire temperature effects on resistance, or voltage drop from a multi-tap transformer feeding several separate cable runs -- all of which can add real-world voltage drop beyond this single-run estimate.