Landscape Lighting Transformer Sizing
Transformer size, voltage drop and layout for low-voltage lighting — and why 12 volts loses 100 times more of its supply than 120 does.
Cable gauge against this run
A 12 V system loses 100× more of its supply than a 120 V one
Voltage drop follows the current, and current is watts ÷ volts — so the drop as a percentage goes as P/V², and (120/12)² is exactly 100. In numbers: 100 W over 100 ft of 12 AWG drops 2.65 V either way, which is 0.22% of a 120 V supply and 22% of a 12 V one. That single ratio is the whole reason landscape lighting is a wiring problem where household lighting isn't.
LED moved the constraint. The rules everyone quotes were written for halogen, which draws five times the current for the same light and dims visibly below 10.8 V. Five 20 W halogens on 100 ft of 12 AWG arrive at 9.35 V — dim, yellow and failing. Five 4 W LEDs of similar output arrive at 11.47 V against a floor nearer 9. On 12 AWG, 100 W of halogen runs about 45 feet before it fails; the LED equivalent runs hundreds. The limit that used to dominate every design now rarely binds.
Daisy-chain drop grows as N(N+1) — very nearly the square of the fixture count, because each fixture carries the current of everything beyond it. Eight fixtures put 3.81 V of drop on the last one where two put 0.32. The same eight on a hub see 0.95 V, and all see the same voltage rather than a gradient. Layout is a bigger lever than wire gauge, and it costs nothing but planning — though the hub's advantage only begins at three fixtures. At two, the two layouts are arithmetically identical.
- Size the transformer to 80% loaded. The margin leaves room to add a fixture without rewiring, keeps the unit cooler, and stops a magnetic transformer humming near its limit.
- Multi-tap transformers fix drop by starting higher, not by wiring better. The fixtures nearest the transformer then get the higher voltage too — which shortens halogen life and is simply wasted on LED. Use taps to correct a run you can't rewire.
- Buy cable one size heavier than the calculation says if you're burying it. It's the cheapest part of the job and the only part you can't change without digging the garden up again.
The 12 V side of this is genuinely low-risk, which is most of why low-voltage lighting is a homeowner job at all. The 120 V side is not: an exterior receptacle wants ground-fault protection and a weatherproof in-use cover, and adding one is electrician and permit territory. Burial depth is a code question and it varies — call for a locate before digging, whatever the depth turns out to be.
How to use
- Add up the fixture wattage and size the transformer to about 80 per cent loaded.
- Check the voltage reaching the far fixture, not just the total load.
- Try a hub layout before buying heavier cable — it is usually the bigger lever.
- Leave the 120 volt side of the transformer to an electrician.
Frequently asked questions
What size transformer do I need for landscape lighting?
Total the fixture wattage and divide by 0.8, then round up to a real transformer size — 240 watts of fixtures wants a 300 watt unit. The margin is not caution for its own sake: it leaves room to add a fixture without rewiring, keeps the transformer cooler and longer-lived, and stops a magnetic unit humming near its limit.
Why is voltage drop such a problem in 12 volt lighting?
Because the drop as a percentage goes as watts divided by volts SQUARED. Current is watts over volts, so a 12 volt system draws ten times the current of a 120 volt one for the same power — and that drop is measured against a supply ten times smaller. The two effects multiply, and (120/12) squared is exactly 100. A hundred watts over a hundred feet of 12 AWG loses 0.22 per cent at 120 volts and 22 per cent at 12.
Does LED change the voltage drop rules?
Substantially, and most published advice has not caught up. LED draws about a fifth of the current of halogen for the same light, and tolerates a much wider voltage range because the driver regulates. Five 20 watt halogens on 100 feet of 12 AWG arrive at 9.35 volts and look dim and yellow; five 4 watt LEDs of similar output arrive at 11.47 against a floor nearer 9. The constraint that used to dominate every design rarely binds now.
How far can I run landscape lighting cable?
On 12 AWG at 12 volts, keeping above 10.8 volts: about 91 feet for 50 watts of halogen, 45 for 100 watts, and 15 for 300. Those are short enough to bind at ordinary garden distances, which is why halogen systems were so fussy. The same fixture count in LED runs several hundred feet on the same cable.
What is the best way to wire landscape lights?
A hub — one feed to a junction point, then an individual leg to each fixture. Every fixture then sees the same voltage rather than a gradient, and the total drop is far lower. A daisy chain is the obvious approach and the worst one, because each fixture carries the current of everything beyond it as well as its own.
How much worse is a daisy chain than a hub?
The drop to the last fixture grows as N times N-plus-one — very nearly the square of the fixture count. Eight fixtures on a chain put 3.81 volts on the last one where two put 0.32; the same eight on a hub see 0.95, and every one of them sees the same. Doubling the fixtures on a chain roughly quadruples the drop at the far end.
Is a hub always better than a daisy chain?
From three fixtures upward, yes, and the gap widens with every fixture added. At exactly two they are arithmetically identical — the last segment of a chain carries one fixture worth of current over one spacing, which is precisely what a hub leg is. It is worth knowing which side of that line you are on before running a junction out to the middle of a bed for two path lights.
What gauge cable should I use for landscape lighting?
Whatever keeps the far fixture above its minimum — and then one size heavier if you are burying it. The cable is the cheapest part of the job and the only part you cannot change without digging the garden up again, and the temptation to add fixtures later is close to universal. The reverse mistake, heavy cable with a chain layout, is money spent on the wrong lever.
What do the 13, 14 and 15 volt taps on a transformer do?
They start the run higher so that a long cable still delivers enough at the end. They fix drop by compensating rather than by wiring better, and there is a cost: the fixtures nearest the transformer get the higher voltage too, which shortens halogen lamp life noticeably and is simply wasted on LED. Use them to correct a run you cannot rewire, not instead of laying it out properly.
Why are my far landscape lights dimmer than the near ones?
Cumulative voltage drop along a daisy chain — each fixture is fed through all the cable and all the current of the ones before it. On halogen the difference is visible as both brightness and colour, because a halogen filament shifts yellow as voltage falls. Switching to a hub layout removes the gradient entirely, and switching to LED makes it largely irrelevant.
Is low-voltage landscape lighting safe to install myself?
The 12 volt side genuinely is, and that is most of the reason these systems exist in the form they do. The 120 volt side is not: an exterior receptacle needs ground-fault protection and a weatherproof in-use cover, and adding one is electrician and permit territory in most places. Cable burial depth is a code question that varies, and calling for a utility locate before digging is worth doing regardless of depth.
Can I mix halogen and LED fixtures on one run?
Electrically yes, and it complicates the design for no benefit. The halogen fixtures set the voltage floor for the whole run at about 10.8 volts, so you keep the tighter constraint while gaining only part of the current saving. If the run is being reworked anyway, converting the whole thing is usually simpler than calculating around a mixture.
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