Voltage Drop, Single Phase
Also known as single phase VD · wire size voltage drop
Worked example: 20 A, 50 m, 4 mm² copper → 8.6 V drop — press Try an example to run it live, then adjust anything.
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Voltage Drop, Single Phase explained
Every conductor is a resistor, and Ohm's law does the rest: the current travels out and back, so the drop is 2 × ρL/A × I. Work in SI and no mystery constant is needed — copper is about 1.72 × 10⁻⁸ Ω·m (1.72 μΩ·cm) at 20 °C, aluminium about 2.82 × 10⁻⁸. Feeding a 20 A load 50 m away on 4 mm² copper drops 2 × 1.72e−8 × 50 × 20 / 4e−6 = 8.6 V, unacceptable on a 230 V circuit and a clear call for larger cable.
North American practice hides the same physics in the constant K in Vd = 2KIL/cmil, where K ≈ 12.9 Ω·cmil/ft for copper and 21.2 for aluminium — those numbers are just ρ expressed in circular-mil-feet. Two traps: use the one-way run length (the 2 is already there), and remember K rises with temperature, which is why 12.9 is a 75 °C figure while cold-copper calculations use about 10.4.
Voltage Drop, Single Phase formula
- = Voltage drop (V)
- = Conductor resistivity (Ω·m)
- = One-way run length (m)
- = Load current (A)
- = Conductor area (m²)
Missing one of these? Work it out first, then come back
- Voltage drop — Percent Voltage Drop, Voltage Drop, Three Phase
- Conductor resistivity — Voltage Drop, Three Phase, Skin Depth
- One-way run length — Voltage Drop, Three Phase, Studs on a Wall at a Given Spacing
- Load current — Available Short-Circuit Current from Percent Impedance, Transformer Full-Load Current
- Conductor area — Voltage Drop, Three Phase, Conduit Fill