Transformer & Voltage Drop Calculator
Size a low-voltage transformer and check voltage drop on a wire run before you buy or troubleshoot.
1 · Transformer size
2 · Voltage drop on one run
Uses round-trip conductor resistance (2 × length) and assumes the whole load sits at the end of the run (worst case). Always confirm the operating range on your fixtures' spec sheet and measure with a meter.
Sizing a transformer: the 80% rule
A low-voltage transformer should not run at its full rating. Loading it to about 80% leaves headroom for heat, start-up current and future fixtures, and helps it last.
Add up the wattage (or VA, if your LED fixtures list it) of every fixture, divide by 0.8, then choose the next standard size up. A 120 W load needs at least 150 W of capacity; a 250 W load needs at least about 313 W, so a 600 W unit gives room to grow, while a 300 W unit would be overloaded.
Why voltage drop matters
Wire has resistance. The longer the run and the more current it carries, the more voltage is lost before it reaches the fixtures. Halogen lamps showed this as dim, yellow light at the end of a run. Many LEDs tolerate a wider voltage range, but too little voltage can still cause flicker, dimming or shortened life.
| Wire | Approx. ohms per 1,000 ft (per conductor) | Typical use |
|---|---|---|
| 16/2 | 4.02 | Short leads to individual fixtures |
| 14/2 | 2.53 | Light loads, short runs |
| 12/2 | 1.59 | The workhorse for most home runs |
| 10/2 | 1.00 | Long or heavily loaded runs |
| 8/2 | 0.63 | Very long runs to distant zones |
The calculator uses the worst case: all the load at the far end of the run. Spreading fixtures along the run, using a hub layout or a T-method connection reduces real-world drop. Multi-tap transformers (12, 13, 14, 15 V) let an installer push a little extra voltage down longer runs.
Read the full guide: How to size a landscape lighting transformer. If your lights are dim at the end of a run, our repair service can diagnose it.
Frequently asked questions
What size transformer do I need for 20 LED lights?
At 5 W each, 20 fixtures draw about 100 W. Divided by 0.8 that is 125 W, so a 150 W transformer works, and a 200 to 300 W unit leaves room to add fixtures later.
What voltage should landscape lights receive?
Many systems aim for about 10.5 to 12 volts at the fixture, but always follow the manufacturer's stated operating range.
Is a bigger transformer wasteful?
A modestly oversized transformer is normal and allows expansion. Grossly oversized units can waste a little more energy at idle, so size for your load plus reasonable growth.
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