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- Choose the conductor material and wire size system.
- Choose the wire size and circuit type.
- Enter the supply voltage, load current and one-way cable length (from source to load).
Formel
Voltage drop is current times the resistance of the conductors. Resistance per meter is resistivity ÷ cross-sectional area, using values at a 75 °C conductor temperature. Single-phase circuits have two current-carrying conductors (out and back); three-phase uses √3.
R′ = ρ(75 °C) / A
Single-phase / DC: Vd = 2 × L × I × R′
Three-phase: Vd = √3 × L × I × R′
AWG diameter: d = 0.127 mm × 92^((36 − n)/39)
Example
- 12 AWG copper (3.31 mm²), 100 ft one way, 16 A at 120 V.
- R′ ≈ 2.097e-8 / 3.31e-6 ≈ 0.00633 Ω/m; L = 30.48 m.
- Vd = 2 × 30.48 × 16 × 0.00633 ≈ 6.2 V = 5.2%, so upsizing to 10 AWG is advisable.
Häufige Fragen
What voltage drop is acceptable?
The US NEC recommends (in informational notes) no more than 3% on a branch circuit and 5% total for feeder plus branch. Many other codes use similar limits.
Why is my result different from an NEC table?
This uses DC resistance at 75 °C and ignores reactance and conduit effects, which matter more for large conductors in AC circuits.
Does this size my wire for safety?
No. Wire must first be sized for ampacity (current-carrying capacity) and protection under your code; voltage drop is an additional check.