Calculate cable size, voltage loss and end voltage for LED strip, garden lights and low voltage lighting runs. Results update instantly as you type.
Total connected wattage on this run.
Optional — if entered, this current is used instead of calculating from watts.
One-way run: the ×2 return path is applied automatically.
Compare cable sizes for your run
Voltage drop for every common cable size using your inputs above. The smallest cable that passes is highlighted.
| Cable size | Resistance (Ω/m) | Voltage drop | Drop % | End voltage | Result |
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How the calculation works
No black box — these are the exact formulas behind the numbers above, shown live with your inputs.
The formulas
Assumptions
- Stranded copper conductors, DC resistance per IEC 60228 at 20°C (e.g. 2.5mm² = 0.00741 Ω/m).
- Two-conductor DC circuit — drop occurs on both the outgoing and return conductor.
- Pass ≤ 3% drop · Warning 3–5% · Fail > 5%, in line with common low voltage lighting practice.
- Connector, joint and terminal resistance is not included — keep joins to a minimum on long runs.
- Resistance rises with temperature (~0.4%/°C) — allow margin for cable in hot roof spaces or conduit in full sun.
Voltage drop, explained simply
What is voltage drop?
Every cable resists the flow of electricity. As current travels along a cable, some voltage is lost overcoming that resistance — so the voltage arriving at your light is always lower than the voltage leaving the driver. That loss is voltage drop. It grows with longer cable, higher current and thinner conductors.
Why it matters for LED strip
LED strip is voltage-sensitive. A 24V strip receiving 22V won't just be slightly dim — brightness falls noticeably, whites drift warmer, and on RGB strip the colour mix shifts. Because the strip itself also has resistance, the LEDs furthest from the feed are hit hardest, producing a visible brightness gradient along the run.
Why 24V beats 12V over distance
Power = volts × amps. Deliver the same wattage at 24V instead of 12V and the current halves. Half the current means half the voltage drop in volts — and because the system voltage doubled too, the drop as a percentage is quartered. The result: a 24V run can go roughly four times further on the same cable. For any run over a few metres, choose 24V.
When to use thicker cable
Step up a cable size whenever the calculator shows more than 3% drop, when a run is long but you're locked into 12V, or when you may add more lights to the circuit later. Doubling the conductor area roughly halves the resistance. Heavier cable costs a little more per metre, but it's far cheaper than re-pulling cable after you notice dim lights.
Split long runs or feed both ends
Past a certain length, thicker cable stops being the smart fix. Splitting one long run into two shorter runs fed from a central driver halves the distance each amp travels. Feeding an LED strip from both ends (or injecting power mid-run) does the same job — voltage drop falls dramatically and brightness evens out across the whole strip.
A worked example
You're feeding 5 metres of 9.6W/m LED strip (48W total) on a 24V system, with the driver 10 metres away on 2.5mm² copper cable:
Drop = 2 × 10m × 2.0A × 0.00741 Ω/m = 0.30V
Drop % = 0.30V ÷ 24V × 100 = 1.2% → PASS
End volts = 24V − 0.30V = 23.70V
The same load and cable on a 12V system draws 4A and drops 0.59V — a 4.9% loss, right at the edge of failure. Same lights, same cable, four times the percentage drop. That single comparison is why 24V is the default choice for longer LED runs.
Everything for a clean low voltage install
Stocked in Australia and matched to the maths above — 24V strip for longer runs, properly sized drivers and genuine copper cable.
