Landscape lighting wire size and voltage drop: plan each run

Start with a run-level load schedule

Separate the project into actual home runs. For each run, record one-way cable length, every fixture model, the documented VA load, where each load sits along the route, the conductor being considered, and the transformer tap.

A single whole-project wattage and a longest-distance number lose the information needed to reason about delivered voltage.

Worked schedule example: ten fixtures documented at 4 VA each create a 40 VA run-level load. Record that 40 VA alongside the actual one-way path and each fixture position; do not turn the total into a gauge recommendation until the equipment-specific voltage-drop method, conductor, topology, and tap are known.

Use the manufacturer method for the represented system

Kichler publishes a voltage-drop equation using twice the cable length, total VA on the run, and a conductor constant: voltage drop = (2 × one-way cable length × total run VA) ÷ cable constant. It then calculates end-fixture voltage as transformer tap voltage minus voltage drop. That method is useful when its assumptions match the Kichler system being designed.

Transparent substitution example: for a 60-foot run carrying 40 VA, the numerator is 2 × 60 × 40 = 4,800. Divide 4,800 by the exact cable constant in the current Kichler chart, then subtract that voltage-drop result from the selected tap. This draft deliberately does not invent the cable constant or turn the example into a gauge recommendation.

Kichler's current guidance says the calculated end-fixture voltage should be greater than 10 V and recommends confirming calculations with a voltmeter. Do not silently move that threshold, formula, or cable constant to another manufacturer, conductor material, or system voltage.

Load placement and branching matter

A conservative far-end calculation can be a useful screen, but fixtures distributed along a run do not all see the same conductor length under load. Hub, T, loop, and daisy-chain layouts change the analysis and fault-finding process.

Connections also add real-world failure points. Specify connectors approved for the cable, environment, and equipment rather than treating every low-voltage splice as equivalent.

Calculate, then measure

Document the chosen equation, assumptions, taps, and target voltage range before installation. After staging the system with its intended load, measure at the locations and under the conditions the manufacturer directs.

An out-of-range result is a design signal. Review conductor size, run length, topology, load placement, connections, and tap selection instead of treating a higher tap as an automatic cure.