Learn & Understand

Low Voltage in the Garden: Ohm, Watts, and Voltage Drop

A landscape lighting system runs on the same electrical laws that govern any circuit, scaled down to a safe low voltage and stretched across a yard. Sizing a transformer and wiring a run of fixtures is really an exercise in Ohm's law and the stubborn problem of voltage drop — the reason the last light on a long cable can glow dimmer than the first.

Why Low Voltage

Most garden lighting runs at around twelve volts rather than household mains voltage, stepped down by a transformer. The reason is safety: low-voltage cable can be buried shallow and handled with far less risk of dangerous shock, which is what makes it a homeowner-friendly system in wet, dug soil. The transformer is the heart of the system, converting mains power to the gentle voltage the fixtures expect.

Watts, Volts, and the Transformer

Electrical power follows a simple relationship — watts equal volts times amps — and a transformer is rated in the total watts it can supply. Add up the wattage of every fixture and you have the load; the transformer must exceed it. Prudent practice sizes the transformer above the measured load, leaving headroom so it never runs at its absolute ceiling and so fixtures can be added later without a new transformer.

The Voltage Drop Problem

Wire has resistance, and current flowing through resistance loses voltage along the way. On a long cable run, fixtures far from the transformer receive less voltage than those near it — and even a small drop can visibly dim a lamp. This voltage drop is the defining challenge of laying out a low-voltage system: designers use thicker cable, split runs into legs, or tap fixtures at points that equalize the distance to keep the far lights as bright as the near ones.

Why Headroom Matters

Running any transformer at its exact rated capacity is a mistake on two counts. There is no room to add a fixture later without replacing the unit, and continuous operation at the ceiling shortens its life. Sizing with a margin above the calculated load — then rounding up to the next standard transformer size — buys both longevity and the freedom to expand, which is why the recommended size always sits above the raw total.

Sizing the Transformer

To total your fixture wattage and find a transformer size with proper headroom, use the Outdoor Landscape Lighting Calculator. Lay out the fixtures along paths and beds planned with the Paver Calculator and Tree Spacing Calculator, and light a fence line measured with the Fence Length Calculator.