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⚠ Method & when not to trust it. Copper resistivity 1.72×10⁻⁸ Ω·m at 20°C, round-trip length modeled. It ignores: connector resistance (often bigger than the wire!), temperature (hot wire = more drop), and AC skin effect (irrelevant at DC). For precision work, measure at the load under full current.
Quick gauge guide (12V, 3% drop, round trip)
| Current | 2 m | 5 m | 10 m |
|---|---|---|---|
| 2A | 20 AWG | 18 AWG | 16 AWG |
| 5A | 18 AWG | 16 AWG | 14 AWG |
| 10A | 16 AWG | 14 AWG | 12 AWG |
| 20A | 14 AWG | 12 AWG | 10 AWG |
Worked example
A 5m LED-strip run at 12V pulling 5A over 18 AWG (0.75 mm²): round-trip 10m × 0.023 Ω/m = 0.23Ω → 5A × 0.23Ω = 1.16V drop = 9.6%. The verdict: undersized — step up to 14 AWG (2.5 mm²) for 2.9% or feed the strip from both ends.
FAQ
What is the 3% voltage drop rule?
The standard low-voltage wiring convention: 3% on the branch, 5% total. More than that and gear misbehaves — motors heat, LEDs dim, sensors drift.
One-way or round-trip length?
Round trip — current goes out and comes back. Enter the physical run length; this tool doubles it internally.
Why do connectors matter?
A cheap barrel connector can add 10-30 mΩ — at 10A that's 0.3V, more than meters of good wire. Crimp or solder high-current joints.
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