What Size Transformer Do I Need for Landscape Lighting?

How to use the transformer size calculator

This tool helps you choose a low-voltage landscape lighting transformer from the total wattage of your fixtures. Enter the total watts of every fixture that will be on the transformer. Choose a diversity factor if you want to reduce the assumed load, and choose a headroom (safety margin). You can also set a maximum wattage per run; 100 to 150 watts per run is typical. Select Calculate to see a recommended transformer size and how many runs to use for the load.

The calculator considers stocked transformer sizes of 150W, 300W, 600W, and 900W. If the required capacity is above the largest single unit, it will suggest more than one transformer so the load can be split.

Transformer sizing formula and rules of thumb

A widely used rule of thumb is straightforward: add the nameplate watts of all fixtures, multiply by 1.25 for headroom, then pick the next standard transformer size. The 1.25 factor is a 25 percent margin so the transformer is not loaded to its exact limit. That extra capacity leaves room for a few added fixtures later and helps the unit run cooler.

Example: a path and a set of up lights that total 180 watts. 180 × 1.25 equals 225 watts, so the next stocked size is 300W. Another example: 420 watts × 1.25 equals 525 watts, so the next size is 600W.

The calculator can go a step further. Diversity lowers the assumed load when not every fixture draws its full nameplate watts at the same time. Headroom then increases the required capacity by the percentage you select. The recommended default in the form is 20 percent headroom. If you prefer the simple 1.25 rule, set diversity to 0 percent and headroom to 25 percent, or apply 1.25 to your watt total and confirm that the suggested size matches.

Voltage drop over long runs

Sizing the transformer is only half of the electrical check. Voltage drop over long runs can leave the farthest fixtures dimmer than those near the transformer, even when total wattage is within the transformer rating. Longer cable, smaller wire, and higher wattage all increase drop. After you have a transformer size, use the landscape lighting wire gauge calculator to check each run. Keeping each run in a typical 100 to 150 watt range, as the max-watts-per-run field suggests, also helps limit drop and keeps branches easier to service.

If you are still laying out the path, the path and step light spacing calculator will give you a fixture count, which you can convert to watts before you return here.

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Browse landscape transformers and power supplies, landscape lighting, pathway and area lights, and low-voltage lighting wire and cable. You can also return to all landscape lighting calculators.

Transformer sizing FAQ

What size low-voltage transformer do I need?

Add the watts of every fixture, multiply by 1.25, and choose the next standard size: 150W, 300W, 600W, or 900W. Use the calculator if you also want run-count guidance.

Why is 25 percent headroom recommended?

It keeps the transformer from running at its limit, leaves room for added fixtures, and is the common 1.25 multiplier used for low-voltage landscape lighting.

How many watts should each run carry?

A typical range is 100 to 150 watts per run. Lower limits help on longer runs where voltage drop is a concern.

Does a larger transformer fix dim fixtures at the end of a long run?

Not by itself. If the cable is too small or the run is too long, voltage will still drop. Check wire gauge after you size the transformer.