ElectroHub

Wire Voltage Drop Calculator

Cable is not a perfect conductor. Over a long run at high current, the volts lost in the wire itself can matter more than anything in the circuit — and the loss appears twice, since current goes out and comes back.

Wire Gauge & Voltage Drop (copper)

Resistance, voltage drop and power lost along a copper cable run. The length is one-way — the current flows out and back, so the resistive path is twice that. Add the source voltage for a percentage drop.

Conductor diameter
Loop resistance
Voltage drop
Drop as percent of supply
Power lost in cable
  1. 1.Given: 14 AWG copper, one-way length = 10 m, current = 10 A
  2. 2.FormulaConductor diameter d = 0.127 mm × 92^((36−n)/39)Substituten = 14Resultd = 1.6277 mm
  3. 3.FormulaResistance R = ρ·(2L) ÷ A (ρ_Cu = 1.724×10⁻⁸ Ω·m, out-and-back)SubstituteR = ρ × 20 m ÷ AResultR = 0.1657 Ω
  4. 4.FormulaVoltage drop V = I × RSubstituteV = 10 × 0.1657ResultV = 1.657 V
  5. 5.FormulaPower lost in cable = I²·RSubstituteP = 10² × 0.1657ResultP = 16.5697 W
Common trap: A lower AWG number means a thicker wire — 10 AWG is far heavier than 20 AWG. Each 3 gauges roughly doubles (or halves) the cross-section and resistance. Keep drop under ~3% for power wiring, or the load misbehaves and the cable heats up.

The formula

R_cable = ρ × (2 × L) / A ; V_drop = I × R_cable

L
one-way run length (metres) — doubled for the return path
A
conductor cross-section (mm², converted to m²)
ρ
resistivity: copper 1.68 × 10⁻⁸, aluminium 2.82 × 10⁻⁸ Ω·m
I
load current (amperes)

Worked example

A 20 m run of 2.5 mm² copper carrying 16 A on a 230 V circuit.

  1. Total conductor length: 2 × 20 = 40 m
  2. R = 1.68×10⁻⁸ × 40 / 2.5×10⁻⁶ = 0.269 Ω
  3. V_drop = 16 × 0.269 = 4.30 V
  4. As a percentage: 4.30 / 230 = 1.87 %

4.3 V lost, or 1.87 % — inside the 3 % usually allowed for a final circuit.

Where you'll use it

Wiring installations, long DC runs to solar panels or LED strips, and diagnosing why a motor at the end of a long feeder never quite reaches full torque. Low-voltage DC is worst affected: 4.3 V is trivial on 230 V and catastrophic on 12 V.

The laws behind it

Parts this applies to

Resistor (fixed)Thermistor (NTC / PTC)Photoresistor (LDR)Resettable Fuse (PTC / Polyfuse)Temperature Sensor (thermocouple/RTD/IC)Humidity Sensor (DHT etc)Gas / Air-Quality Sensor (MQ)Load Cell / Strain Gauge

Related calculators

Resistivity Calculator (R = ρL/A)R = ρ × L / AOhm's Law CalculatorV = I × REnergy and Electricity Bill CalculatorUnits (kWh) = (Watts / 1000) × hoursJoule Heating CalculatorH = I² × R × t