Wiring & calculations

Voltage drop and cable size chart

Copper cross-section required for 12V and 24V constant-voltage strips by load and cable run, with the formulas and when to raise the voltage instead of thickening the cable.

Last updated：2026-10-31

On this page

How to read it
12V and 24V charts
Formulas and assumptions
When to stop thickening the cable
How to measure on site

On constant-voltage strip jobs the usual problem is not a failed driver but voltage drop at the far end. This chart gives the copper cross-section for 12V and 24V systems at common loads and distances, working back from a 5% drop limit.

How to read it

The columns are the one-way cable run (driver to the first LED, in metres) and the rows are the total load. The cell is the copper cross-section in mm² that keeps the drop within 5%, rounded up to the next standard size.

Example: a 24V system, 100 W total, 10 m run → 1.5 mm². If only 0.75 mm² is on site, the far end will be visibly dim.

The headline result: at the same load and distance, 24V needs roughly a quarter of the copper that 12V needs. For long runs, raise the voltage before you reach for thicker cable.

12V and 24V charts

12 V system

Total load	5 m	10 m	15 m	20 m	30 m
30 W	0.75	1.5	2.5	4	6
60 W	1.5	4	6	6	10
100 W	2.5	6	10	10	>10
150 W	4	10	>10	>10	>10
200 W	6	10	>10	>10	>10
300 W	10	>10	>10	>10	>10

24 V system

Total load	5 m	10 m	15 m	20 m	30 m
30 W	0.5	0.5	0.75	0.75	1.5
60 W	0.5	0.75	1.5	1.5	2.5
100 W	0.75	1.5	2.5	2.5	4
150 W	1	2.5	4	4	6
200 W	1.5	2.5	4	6	10
300 W	2.5	4	6	10	>10

Formulas and assumptions

Working current I = P ÷ V
Allowed drop ΔV = V × 5% (the usual lighting limit; beyond it the dimming is visible)
Maximum loop resistance R_max = ΔV ÷ I
Copper cross-section S = ρ × 2L ÷ R_max, with ρ = 0.0175 Ω·mm²/m at room temperature and 2L for the return path

Every figure here comes from those formulas and matches the sizing calculator — the two cross-check each other.

When to stop thickening the cable

Once the answer passes 4 mm², more copper is usually the wrong spend. In order of effectiveness:

Raise the output voltage — 12V to 24V halves the current and doubles the allowed drop, cutting the required cross-section to about a quarter
Feed from more than one point — feeding the middle or both ends halves or quarters the effective L
Move the driver closer — subject to its own cooling and ingress-protection requirements

How to measure on site

Measure under full load — an open-circuit reading that looks fine is the classic misdiagnosis
Take two readings: at the driver terminals V₁ and at the far end of the strip V₂
Drop % = (V₁ − V₂) ÷ V₁ × 100%
Act above 5%; by 10% the colour temperature at the far end has shifted too and white looks visibly warmer

This chart covers DC-side resistive drop only — not the AC input, connector contact resistance or dimmer losses. A poorly crimped terminal on site routinely costs more than choosing one cable size too small.

Want it worked out for your job? Send us the wattage, voltage, run length and dimming method — we will size the cable and the driver together. Contact →