Landscape lighting for a 140 ft run at 12 ft spacing with 20 W fixtures

Fixtures and transformer

13fixtures

260 W total over 140 ft — a 600 W transformer and heavier than 8 AWG cable

Fixtures12 ft apart along 140 ft	13fixtures
Total load13 × 20 W — 21.7 A	260W
Transformer325 W needed at the 80% rule — never run one at full rating	600W
Headroom leftroom for about 11 more fixtures	57%
Cable sizesplit the run into two circuits	over 8 AWG
— Voltage drop by cable —
16 AWGthe last fixtures will look dim	126.1%-3.1 V at the end
14 AWGthe last fixtures will look dim	79.4%2.5 V at the end
12 AWGthe last fixtures will look dim	50.1%6 V at the end
10 AWGthe last fixtures will look dim	31.3%8.2 V at the end
8 AWGthe last fixtures will look dim	19.7%9.6 V at the end

Run length: 140 ft
Fixture spacing: 12 ft
Fixture wattage: 20 W

Three numbers, and they have to be worked out together

13 fixtures at 12 ft along 140 ft, drawing 260 W — 21.7 A at 12 volts.

A 600 W transformer. The load needs 325 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.

larger than 8 AWG cable, which holds % drop and leaves 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 13 fixtures spread evenly, the full 21.7 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 140 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 11 more 20 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. Even 8 AWG cannot hold this run inside 10% — split it into two circuits fed from opposite ends of the transformer.
Current is halved in this calculation, and that is correct. With 13 fixtures spread evenly, the full 21.7 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 140 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 — 260 W of load wants a 600 W unit, not a 260 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 11 more 20 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.

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