Brighter Nights · Field Tools No. 03
Voltage drop,
before you bury the wire.
Two families, two different machines. 12V landscape runs on 14/2 low-voltage wire from a 150W or 300W transformer, and every fixture on the run is a watt of load. 24V controller systems run on coaxial lead line from a 200W supply, and the load is spread along the track. Pick the family and size the run right.
Pick one.
12V landscape · the run
Watts on the wire.
Volts at the fixture.
Landscape drop is driven by wattage, not by fixture count. Every watt you hang on the run pulls current, and current on 14/2 is what costs you volts.
24V controller · the run
Lead line carries it.
Track spreads it.
A 24V system has two very different lengths. The coaxial lead line from the supply carries the whole load as one lump. The track itself sheds load steadily as it goes, so it costs you roughly half what the same footage would cost as a lump.
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The answer
What lands at the far end.
Voltage at the last fixture
Steady
12.0V —
If you need to claw volts back
Volts along the run
Drag across it.
Voltage plotted against distance from the transformer.
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The physics, out loud
No black box.
Here is every number.
What formula is this using?
Ohm’s law, not a lookup table. Voltage drop = 2 × length × current × resistance per foot, where current is watts divided by volts. The doubling is because current travels out to the fixture and back. For 14 AWG copper the resistance is 2.525 ohms per 1,000 feet.
Why not use the cable constants everyone publishes?
Because they are the same equation with the voltage baked in. The published constant is 1,000 × volts ÷ resistance — at 12V that gives 4,752 for 14 AWG, which is the 4,750 you see in the tables. Computing from resistance instead means the answer stays correct when you move to a 13V, 14V or 15V tap, which a fixed constant cannot do.
How much drop is acceptable?
The Illuminating Engineering Society guidance is to keep every fixture within 10% of the transformer output — on a 12V tap that means no fixture below 10.8V. Fifteen percent is common in practice and LED fixtures tolerate it better than halogen did, but 10% is the number to design to.
Why does wiring method change the answer so much?
On a daisy chain the first fixture sees the most voltage and every one after it sees less, because each span carries the current for everything downstream. The hub method runs equal-length legs from one central point, so every fixture sees the same voltage. It is the only method that delivers equal volts, and it is why the pros use it.
How big a transformer do I need?
The American Lighting Association guidance is to load a transformer to no more than 80% of its rating. A 150W transformer carries 120W of fixtures; a 300W carries 240W. Some manufacturers derate harder still — plenty of multi-tap units are plated at 70% max load, so read yours rather than trusting a rule. This calculator sizes to 85%, which is deliberately one notch tighter than the hardware allows and one notch looser than the ALA line; it is the number Brighter Nights builds to. Pack shipping follows the stricter 80% — every pack ships with a 150W until the run reaches 120W, then a 300W. A sconce 12-pack is 24 MR16 lamps at 5W, which is 120W on the nose. A PAR36 8-pack is 8 × 15W, also 120W. Both ship with the 300W.
How many watts is each fixture?
MR16 uplights are 5W. PAR36 floods are 15W. A sconce takes two MR16 lamps, so it is 10W. The G4 bi-pin pathway fixture is not published — it must be under 10W, because a 12-pack ships on a 150W transformer, but no tighter figure is available.
Can I just use heavier wire?
It is the most effective single fix — 12 AWG has about 63% the resistance of 14 AWG, and 10 AWG about 40%. Brighter Nights ships 14/2 and only 14/2, so heavier gauge means sourcing it separately. Splitting the load across two runs or moving the transformer closer is usually cheaper and always faster.
