Every wire has resistance, and every meter of wire drops some voltage. For short household runs, this is negligible. But for solar installations, long cable runs, or high-current applications, voltage drop can be the difference between a working system and a failed one.
How Voltage Drop Works
Voltage drop = current × wire resistance. Wire resistance increases with length and decreases with diameter. The formula: V_drop = I × R, where R = resistivity × length ÷ area.
Copper resistivity: 1.68 × 10⁻⁸ Ω·m. For 100 meters of 2.5 mm² copper wire: R = 1.68×10⁻⁸ × 100 ÷ (2.5×10⁻⁶) = 0.672 Ω. At 10A: voltage drop = 10 × 0.672 = 6.72V — significant on a 12V system but negligible on 230V.
Voltage Drop by Application
Solar installations (12–48V): Critical because even small drops represent large percentages. A 1V drop on a 12V system is 8.3% loss. Use thick cables (6–10 mm²) and keep runs short.
Household wiring (120/230V): NEC recommends max 3% drop on branch circuits and 5% total (feeder + branch). For 230V at 20A over 30m: use 4 mm² wire for 2.3% drop.
EV charging (240V, 32–48A): Over long distances (50m+), upsizing wire is essential. A 50m run at 40A needs 10 mm² to stay under 3% drop.
Minimizing Voltage Drop
- Increase wire size: Doubling cross-section halves the resistance
- Increase voltage: Same power at double voltage = half current = quarter the power loss
- Shorten the run: Route cables as directly as possible
- Use copper over aluminum: Copper has 60% lower resistivity
This is why power transmission lines use extremely high voltages (up to 765 kV) — it dramatically reduces current and therefore reduces I²R losses.
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