The whole problem in one sentence
At 120 volts a one volt loss in the wire is under one percent and nobody notices. At 12 volts the same one volt is eight and a third percent, and it is the difference between a fixture at full output and a fixture that is visibly dimmer than the one next to it. Low voltage landscape lighting is not forgiving because the denominator is small, and every decision about wire and layout follows from that.
The arithmetic is the standard two-wire drop formula. Voltage drop equals 2 x K x I x L divided by CM, where K is about 12.9 for copper, I is the current in amps, L is the one-way length of the run in feet, and CM is the cross-sectional area of the conductor in circular mils. The 2 is there because current goes out along one conductor and back along the other, so the loop is twice the distance you measured. Circular mils for the common gauges are 2,583 for 16 AWG, 4,107 for 14, 6,530 for 12, 10,380 for 10 and 16,510 for 8.
Worked through
Eight fixtures at 4 watts each is 32 watts. At 12 volts that is 2.67 amps. Run 100 feet in 12 AWG with everything grouped at the far end: 2 x 12.9 x 2.67 x 100 / 6,530 gives 1.05 volts, which is 8.8 percent, and the fixtures see about 10.9 volts. Move the same load to 16 AWG and the drop is 2.67 volts, or over 22 percent, and the far fixtures are running at 9.3 volts. Move it to 10 AWG and the drop falls to 0.66 volts.
That is why landscape cable looks so heavy for the power it carries. Twelve gauge for 32 watts would be laughable at line voltage. Here it is a reasonable minimum, and long runs routinely use 10 or 8.
Layout beats gauge
Before upsizing an entire run, look at where the fixtures sit on it. The worst possible arrangement is a single daisy chain with the load concentrated at the far end, because every amp travels the whole length. Spreading fixtures evenly along the same run roughly halves the drop, since the current in the near half of the cable is carrying load that leaves before the end. Splitting into legs from a hub near the centre of the group, or running a tee that divides the load two ways, does better still and usually costs less than doubling the copper for the whole distance.
| Layout | What it does to the drop | Cost of doing it |
|---|---|---|
| Daisy chain, load at the end | Worst case, full length at full current | Least cable, worst result |
| Daisy chain, evenly spaced | Roughly half the worst case | Free — it is just where you place fixtures |
| Tee split | Halves the current on each leg | One connection, some extra cable |
| Hub with separate legs | Each leg carries only its own load | More cable, far more even output |
The other lever is the transformer tap. Many landscape transformers provide output taps above 12 volts specifically so a long run can start higher and land near 12 at the fixtures. That fixes the far end and it can overdrive the near end, so it works best when the load is grouped rather than strung out.
Sizing the transformer
Add up the actual watts of what is connected and leave real headroom above it. Running a transformer at its nameplate limit leaves nothing for the fixture you add next spring, and continuous operation at the limit is not where these units are happiest. Twenty to twenty-five percent spare is a reasonable starting point, and more if the plan is obviously going to grow. Units are commonly sold in steps around 60, 100, 150, 200, 300 and 600 VA, so in practice you are picking the next step up from the number rather than matching it.
The LED transition changed the sizing question more than anything else. A dozen 20 watt halogen fixtures was 240 watts and a serious transformer. A dozen LED fixtures replacing them might be 50 watts total. The wire is still heavy, because the percentage drop problem does not go away, but the transformer is much smaller and there is a great deal of headroom on an old one.
Two things this page will not do
Before any of this turns into a shovel in the ground: a free national utility-locate service operates in every US state, and calling it is how buried gas, electric, water and communication lines get marked. It costs nothing. Striking a gas main or an electric service is not an inconvenience or a repair bill, it is a life-safety event, and it happens most often on shallow residential digs exactly like this one. Look up the locate service for your state, request the marks, and wait until they are on the ground before you dig.
The other is the supply side. Everything above concerns the low voltage output after the transformer. The transformer itself has to be fed from a line voltage circuit, outdoors, with the right protection, enclosure and bonding, and that is licensed electrical work in essentially every jurisdiction — not a grey area, and not something to take from a web page. If the low voltage arithmetic is what you came for, the general voltage drop calculator handles the same formula across other voltages and conductor sizes, and the lighting layout calculator covers spacing and coverage rather than electrical loss.
Questions people ask
What size wire do I need for low voltage landscape lighting?
It depends on the load and the distance, and the honest answer is heavier than the wattage suggests. Work out the current — watts divided by 12 — then run the drop formula: 2 x 12.9 x amps x one-way feet, divided by the circular mils of the gauge. Thirty-two watts over a hundred feet in 12 AWG drops about a volt, which is 8.8 percent, and the same run in 16 AWG drops over two and a half volts. Twelve gauge is a reasonable floor for anything more than a short run, and 10 or 8 is normal on long ones.
What percentage voltage drop is acceptable on a 12 V run?
There is no single threshold, and what matters is the input range of the fixtures you actually bought. Halogen fixtures showed drop immediately as a dimmer, warmer light, so installers worked to keep everything within a few percent. LED fixtures generally accept a wider input band and hold output flat across it, then fall off or shift colour once they leave it. That makes them much more forgiving in the middle and much more abrupt at the edge. Read the acceptable input voltage on the fixture spec, and keep enough margin that adding a fixture later does not push the far end out of range.
What size transformer do I need?
Add the actual watts of every fixture on the unit and add headroom above it — twenty to twenty-five percent is a common working figure, more if you know the run will grow. Eight 4 watt fixtures is 32 watts, which with 25 percent headroom is 40 VA, so the next common size up is a 60 VA unit. What trips people is using the wattage of the fixture it replaced rather than the fixture that is there: LED retrofits often draw a fifth of what the halogen did, so an existing transformer usually has enormous spare capacity after a conversion.
Why are my far lights dimmer than the near ones?
Because the current feeding them has travelled further and lost more voltage on the way. On a single daisy chain the near fixtures tap in where the voltage is still high and the far ones get what is left, and the effect gets worse as the run gets longer or the wire gets lighter. Three things fix it, in rough order of cost-effectiveness: rearrange the layout into legs from a hub so no fixture is at the end of everything, move to heavier wire, or use a higher transformer tap so the run starts above 12 volts. Which one is right depends on how the fixtures are physically distributed.
Can I do the wiring myself?
The low voltage side after the transformer is generally treated as low risk work, and this page gives you the arithmetic for it. The supply side is not: the circuit feeding the transformer is line voltage, it is outdoors, and the requirements for its protection, enclosure, mounting and bonding are licensed electrical work in essentially every jurisdiction. No procedure for it appears here. The other half of the answer is the ground itself — buried gas, electric, water and communication lines are marked free of charge by a national locate service that operates in every US state, and calling it before any digging, including a shallow trench for cable, is not optional.