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Transformer & Wattage Sizing Calculator
Enter the fixtures on the job and your longest wire run. Get the transformer size, the wire gauge, and the voltage drop at the last fixture — the three numbers that decide whether the install holds up.
- Fixtures20
- Total load98 W
- With 80% rule123 W
- Amps on longest run3.3 A
- Voltage drop0.78 V
- Last fixture sees11.2 V
Assumes 12V copper two-conductor wire and a 1.2V drop ceiling. Hub wiring changes the math — measure the longest single path.
Voltage drop by wire gauge on this run
| Gauge | Drop at 120 ft | Fixture voltage | Verdict |
|---|---|---|---|
| 16/2 | 3.15 V | 8.9 V | Too much drop |
| 14/2 | 1.98 V | 10.0 V | Too much drop |
| 12/2 | 1.24 V | 10.8 V | Too much drop |
| 10/2 | 0.78 V | 11.2 V | Good |
| 8/2 | 0.49 V | 11.5 V | Good |
How the sizing works
Two separate problems decide a low-voltage system. The first is capacity: add up every fixture's wattage and divide by 0.8, because running a transformer at more than 80% of its rating shortens its life and leaves nothing for the fixtures the client will ask for next spring. The second is voltage drop: resistance in the wire eats voltage over distance, and an LED that sees 10V instead of 12V shifts color and dims noticeably compared to the fixture next to it.
The drop formula is (2 × run length × amps × ohms per 1000 ft) ÷ 1000. Doubling appears because current travels out and back. Amps come from watts ÷ 12. This calculator sizes the wire so the furthest fixture still sees at least 10.8V, which is where most LED integrated fixtures stay color-stable. If your transformer has 13V and 14V multi-tap terminals, you have extra headroom — tap up rather than upsizing the wire on a run that's only slightly long.
Now show them what it'll look like.
Numbers win arguments. A photoreal nighttime render of their home wins the job. Three free renders, no credit card.
Common questions
What size transformer do I need for landscape lighting?
Add up the wattage of every fixture, then divide by 0.8 — you never want to load a low-voltage transformer past 80% of its rating. A 186W fixture load needs 233W of capacity, so you'd pick a 300W transformer. Sizing up also leaves room for the fixtures the client adds next season.
How do I calculate voltage drop on a low-voltage run?
Voltage drop = (2 × run length in feet × amps × ohms per 1000 ft) ÷ 1000. Amps is the wattage on that run divided by 12. Keep the drop under about 1.2V so the last fixture still sees 10.8V or better — below that, LED color shifts and halogen goes dim and orange.
What wire gauge should I use for landscape lighting?
12/2 handles most residential runs. Go to 10/2 on long runs or heavy loads, and 8/2 for a main trunk feeding a distant hub. 16/2 is only for a couple of fixtures within 30 feet of the transformer.
Should I run one big transformer or several small ones?
Several smaller transformers placed near fixture clusters almost always beat one large one on a big property. Shorter runs mean less voltage drop, cheaper wire, and a failed transformer only takes out one zone.
Do LED fixtures change transformer sizing?
Yes, dramatically. A 20-fixture LED job might pull 120W where the same job in halogen pulled 700W. Size for the LED load plus headroom, but check that your transformer has a low enough minimum load — some older magnetic units won't regulate properly under a small LED draw.