Landscape lighting for a 150 ft run at 6 ft spacing with 4 W fixtures

Fixtures and transformer

26fixtures

104 W total over 150 ft — a 150 W transformer and 8 AWG cable

Fixtures6 ft apart along 150 ft	26fixtures
Total load26 × 4 W — 8.7 A	104W
Transformer130 W needed at the 80% rule — never run one at full rating	150W
Headroom leftroom for about 4 more fixtures	31%
Cable size8.4% drop — 11 V at the far end	8 AWG
— Voltage drop by cable —
16 AWGthe last fixtures will look dim	54.1%5.5 V at the end
14 AWGthe last fixtures will look dim	34%7.9 V at the end
12 AWGthe last fixtures will look dim	21.4%9.4 V at the end
10 AWGthe last fixtures will look dim	13.4%10.4 V at the end
8 AWGwithin 10%	8.4%11 V at the end

Run length: 150 ft
Fixture spacing: 6 ft
Fixture wattage: 4 W

Three numbers, and they have to be worked out together

26 fixtures at 6 ft along 150 ft, drawing 104 W — 8.7 A at 12 volts.

A 150 W transformer. The load needs 130 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.

8 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 26 fixtures spread evenly, the full 8.7 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 150 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 4 more 4 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. 8 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 26 fixtures spread evenly, the full 8.7 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 150 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 — 104 W of load wants a 150 W unit, not a 104 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 4 more 4 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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