Landscape lighting for a 120 ft run at 15 ft spacing with 9 W fixtures

Fixtures and transformer

9fixtures

81 W total over 120 ft — a 120 W transformer and 10 AWG cable

Fixtures15 ft apart along 120 ft	9fixtures
Total load9 × 9 W — 6.8 A	81W
Transformer101 W needed at the 80% rule — never run one at full rating	120W
Headroom leftroom for about 2 more fixtures	32%
Cable size8.4% drop — 11 V at the far end	10 AWG
— Voltage drop by cable —
16 AWGthe last fixtures will look dim	33.7%8 V at the end
14 AWGthe last fixtures will look dim	21.2%9.5 V at the end
12 AWGthe last fixtures will look dim	13.4%10.4 V at the end
10 AWGwithin 10%	8.4%11 V at the end
8 AWGwithin 10%	5.3%11.4 V at the end

Run length: 120 ft
Fixture spacing: 15 ft
Fixture wattage: 9 W

Three numbers, and they have to be worked out together

9 fixtures at 15 ft along 120 ft, drawing 81 W — 6.8 A at 12 volts.

A 120 W transformer. The load needs 101 W once you apply the 80% rule, and that rule is not conservatism: a transformer run at its nameplate rating runs hot, and heat is what kills them.

10 AWG cable, which holds 8.4% drop and leaves 11 V at the far end.

The one way these systems fail

The fixtures at the end are dimmer than the ones at the start. It is always voltage drop, and it is always discovered after the cable is buried.

12 V LED fixtures want 10.5–15 V. Below that they visibly dim and some flicker; above it they fail early. Ten percent is the practical ceiling on drop, and it is what the cable above is sized to.

Half the current, and that is correct

With 9 fixtures spread evenly, the full 6.8 A only flows in the first section of cable — by the far end almost nothing is left. This calculation uses the midpoint current, which is the honest way to do it. Treating the whole load as flowing the whole 120 ft doubles the apparent drop and sells you twice the copper.

Two free fixes

Feed the run from the middle rather than one end. That halves the effective distance and roughly quarters the drop, and it costs nothing but a different trench route.

And leave headroom on purpose. Landscape lighting always grows — one more fixture at the new tree, two more along the path. There is room for about 2 more 9 W fixtures on this transformer before it needs replacing.

Notes

Low-voltage landscape lighting fails in exactly one way: the fixtures at the end are dimmer than the ones at the start. 10 AWG keeps the far end at 11 V, inside the 10% the LEDs need.
Current is halved in this calculation, and that is correct. With 9 fixtures spread evenly, the full 6.8 A only flows in the first section of cable; by the far end almost nothing is left. Calculating as if all of it ran the whole 120 ft doubles the apparent drop and sells you twice the copper.
The 80% rule on the transformer is not conservatism. A transformer run at its nameplate rating runs hot, and heat is what kills them — 81 W of load wants a 120 W unit, not a 81 W one.
Leave headroom deliberately. Landscape lighting always grows: one more fixture at the new tree, two more along the path. There is room for about 2 more 9 W fixtures on this transformer before it needs replacing.
Feeding the run from the middle rather than one end halves the effective distance and roughly quarters the drop. It is the cheapest fix available and it costs nothing but a different trench route.
12 V LED fixtures want 10.5–15 V. Below that they dim visibly and some flicker; above it they fail early. Multi-tap transformers with 13, 14 and 15 V terminals exist precisely so you can compensate a long run — but sizing the cable properly is the better answer.

Nearby sizes

2700K vs 3000K: A Difference You Only See Side by SideShown separately, almost nobody can tell 2700K from 3000K. Shown together, everybody can. That single fact decides how you should buy them.
3000K or 4000K in a Kitchen? Look at the Worktop, Not the ChartMost advice picks a number and defends it. The honest answer is that 3000K and 4000K are both correct, in different kitchens, and the deciding factor is what the light lands on.
Vanity Lighting: Why the Fitting Position Beats the LumensA vanity is a face-lighting problem, not a room-lighting problem. Almost every bad bathroom light is a fitting in the wrong place rather than a fitting that is too dim.
Bedroom Lumens: The Room Where Less Is the SpecificationThe bedroom is the only room where the target is a range you should deliberately sit at the bottom of, and where the dimmest setting matters more than the brightest.