Understanding voltage drop in landscape lighting

Why voltage changes along a cable

A cable has electrical resistance. When current flows through it, some voltage is lost along the circuit. In a low-voltage lighting system, that loss can affect fixture operation, particularly on long cable runs or heavily loaded branches.

The main factors

Longer cables generally have more resistance. A larger conductor generally reduces resistance compared with a smaller conductor of the same material and length. Higher current increases the voltage loss. Connections also need attention because poor joints can introduce additional resistance.

The effect depends on the arrangement of the load. A run with fixtures spread along its length is different from a run with the entire load located at the far end. Simple calculations can be useful for planning, but the actual layout must be understood.

Practical planning choices

• Record branch lengths and the fixtures connected to each branch.
• Consider distributing the load over suitable separate runs.
• Select cable size using the actual load and installation requirements.
• Check the acceptable input range specified for the fixtures.
• Use model-specific guidance when considering transformers with multiple output taps.

Verify the installed system

Performance should be checked under the intended operating load. Measurements at the relevant locations help determine whether the circuit is delivering suitable voltage. Raising an output tap without checking the nearest fixtures can create a different problem, so any adjustment should be made by a qualified person following the product documentation.

Prepare useful information for a quotation

A list of fixture quantities, rated wattages, cable lengths and intended output voltage is more useful than a transformer wattage alone. Include these details when discussing a system so that product selection can be reviewed in context.