Boards & wires calculator
Wire gauge & voltage drop
A long thin wire is a resistor you did not draw: its volts are lost as heat.
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The example, worked out
Given
- Size given as
- AWG
- AWG
- 18
- Distance to the load
- 5 m
- Return wire
- Same cable back (×2)
- Current
- 3 A
- Material
- Copper
- Wire temperature
- 30 °C
- Supply
- 12 V
Working
- the resistivity at 30 °C (+0.39 % per °C)
- the length is there and back
- the drop, and the heat it becomes
Results
The formulas
- R = ρ × l / Aresistivity, length over section
- ΔV = R × I, P = R × I²the drop and the heat
- d(AWG) = 0.127 mm × 92^((36 − AWG) / 39)the gauge table
The usual traps
- Counting one wire when the current goes there and back.
- A cold-wire resistance: copper gains 0.4 % per degree.
Questions
How thick must my wire be, and how much voltage does it drop?
R = ρ × l / A: resistivity, length over section. ΔV = R × I, P = R × I²: the drop and the heat. d(AWG) = 0.127 mm × 92^((36 − AWG) / 39): the gauge table.
What does it give for typical values?
653 mV dropped over 5 m of AWG 18 at 3 A. Voltage drop: 653 mV; Resistance of the run: 218 mΩ.
What are the usual mistakes?
Counting one wire when the current goes there and back. A cold-wire resistance: copper gains 0.4 % per degree.
In the app
Your circuit, drawn live
Change a value: the schematic redraws with your numbers on the parts.
The part you can buy
Results rounded to standard values (E12, E24, E96) and power ratings.
The working, shown
Every step written out. Try it first by hand, then check.
Into your notebook
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Open the calculatorOpens in the app on 1 December, with Circuit Notebook 1.0
Also in Boards & wires
- PCB trace width The width a current needs (IPC-2221), and what the trace drops.
- Microstrip impedance Z0 of a trace over a ground plane, or the width for 50 Ω.
- Differential pair Zdiff of two coupled traces, or the width for 90 or 100 Ω.