Transformer Sizing Reference

Transformer sizing is one formula plus two rules. Get it right and the system has headroom for expansion; get it wrong and the far fixtures run dim and amber, or the transformer hums at its limit for years.

The formula

1. Sum the actual wattage of every fixture on the system (modern LED fixtures run 3 to 9 watts; check the spec, not the halogen-equivalent marketing number).
2. Multiply by 1.25 for headroom — this keeps the transformer at or under 80 percent load, where it runs cool and quiet, and leaves room for the add-on fixtures every client requests later.
3. Round up to the next standard transformer size: 150, 300, 600, 900 watts.

Example: 14 fixtures — eight 4-watt path lights, four 6-watt uplights, two 9-watt floods — totals 74 watts. Times 1.25 is 93 watts. A 150-watt stainless multi-tap covers it with true expansion room. Resist the urge to buy exactly to load; the price step between sizes is small and the callback to upgrade is not.

Voltage drop: the second half of sizing

LEDs want 9 to 15 volts at the fixture; long runs on thin wire drop below that. Plan wire like plumbing: 12/2 low-voltage cable is the standard trunk, 10/2 for long or loaded runs. As working limits on 12-volt taps, keep a 12/2 run under about 100 feet at 100 watts of load, and under 150 feet at 60 watts; 10/2 extends those runs roughly half again. Better practice than any single long run: hub or T layouts that keep each fixture within 25 feet of a central splice, balancing drop across the group.

• Use a multi-tap transformer (12, 13, 14, 15 V taps) and land long runs on higher taps to compensate measured drop — measure at the farthest fixture with everything on, and aim for 10.5 to 12 V at the LED driver.
• Split large properties into runs by area (front entry, drive, back patio) rather than one loaded daisy chain; daisy chains dim toward the tail.
• Keep total load on any single home-run cable under 150 watts even when the transformer has capacity — the cable, not the transformer, is usually the constraint.
• Mount transformers at least 12 inches above grade on a wall near a GFCI-protected outdoor circuit, with a photocell/timer or smart control per the spec.

Our lighting transformer calculator on the resources page runs the load math and flags voltage-drop risk from your run lengths — useful as a sanity check before the order goes in.

Size for the property, not the first phase. Most clients who install front-yard lighting add the back patio, a specimen tree, or a new fixture cluster within two years — a transformer with headroom and a labeled panel absorbs that add-on with a splice and a fixture, while an undersized unit forces a swap and a rewiring visit. The few dollars saved buying tight to the first-phase load rarely covers the cost of the second truck roll.