Deck lighting and voltage drop
Low-voltage lighting fails in one specific way: the first fixtures are bright and the last ones are dim or flickering, because 12 volts does not survive a long, thin cable. The calculation is resistance, and it takes ten seconds.
Your run
How this is calculated
- Current = total watts ÷ system voltage.
- Voltage drop = 2 × length × current × resistance per foot. The 2 is the return path: current travels out and back.
- Voltage at the last fixture = supply − drop. This assumes the full load is at the far end, which is the conservative case.
- Maximum run = the length at which drop reaches 5% of supply.
Worked example
Eight 4 W fixtures on 12 V, 80 ft of 16 AWG:
- Load = 32 W; current = 32 ÷ 12 = 2.67 A
- Drop = 2 × 80 × 2.67 × 0.00489 = 2.09 V, which is 17% of 12 V
- Last fixture sees 9.9 V — visibly dim, and some LED drivers will not start
- At 5% (0.6 V) the maximum run on 16 AWG is 23 ft
- 14 AWG would allow 36 ft, 12 AWG 58 ft, 10 AWG 92 ft
This is why landscape lighting is run as a home run with heavier cable, or split into several shorter runs from a multi-tap transformer, rather than daisy-chained on 18 AWG.
The two design fixes
Run separate legs. Four fixtures on 40 ft of 14 AWG drops about 1.4 V; eight fixtures on 80 ft of the same cable drops 2.9 V. Splitting the load is more effective than upgrading the cable, and usually cheaper.
Use the transformer's taps. Many low-voltage transformers have 12, 13, 14 and 15 V outputs. Running a long leg on the 14 V tap compensates for the drop at the far end, at the cost of overdriving the near fixtures slightly.
What the fixture count does not solve
- It does not do electrical code. Burial depth, connection methods, GFCI protection and transformer location are code questions. The arithmetic here is physics, not compliance.
- It assumes a concentrated load at the end. Fixtures distributed along the run drop less voltage than this calculation predicts, so the result is conservative — good for design, pessimistic for diagnosis.
- It ignores connector losses. Every connection adds resistance. On a long run with many splices, real drop exceeds the calculated figure.
- It does not size the transformer. The figure shown is total load; add 20–25% headroom for the driver and for future fixtures.
Questions people actually ask
Why are my deck lights dim at the end of the run?
Voltage drop. At 12 V the current is high for the wattage, and thin cable over a long run loses a significant fraction of the supply. The last fixtures see less than 12 V and dim or flicker.
What wire gauge for 12 V landscape lighting?
It depends on load and length, not on a fixed rule. 12 or 14 AWG covers most residential runs; 16 AWG is only adequate for short runs at low load. The calculator gives the maximum length for each.
How much voltage drop is acceptable?
Aim for 5% or less at the last fixture. Above 10%, LED drivers may fail to start and incandescent lamps visibly dim.
How many lights can one transformer run?
Transformer wattage divided by fixture wattage, minus 20–25% headroom. A 60 W transformer comfortably runs twelve 4 W fixtures, not fifteen.
Should I use 12 V or 24 V?
24 V halves the current for the same wattage and therefore quarters the voltage drop over the same cable. On long runs it is the better choice; on short ones it makes little difference.