Long runs need thicker wire to keep voltage drop low. This sizes the wire for voltage drop only – it must also be large enough for the current (ampacity) under your local electrical code.
How to use the Wire Size Calculator
- Current type
- Conductor
- Source voltage
- Allowed voltage drop (%)
- Current
- One-way length
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Choose the system and conductor, enter the voltage, allowed drop, current and one-way length.
Inputs explained
| Field | What to enter |
|---|---|
| Current type | Choose one: DC / single-phase AC, Three-phase AC |
| Conductor | Choose one: Copper, Aluminium |
| Source voltage | Units: V, kV, mV |
| Allowed voltage drop (%) | A number |
| Current | Units: A, mA, µA, kA |
| One-way length | Units: m, cm, mm, km, in, ft … |
What each result means
- Minimum cross-section area = [Current type]×[Conductor]×[One-way length]×[Current]÷([Source voltage]×[Allowed voltage drop (%)]÷100)
- Minimum diameter = sqrt(4×[Minimum cross-section area]÷pi)
- Smallest suitable AWG = floor(36 – 39×ln([Minimum diameter]÷0.000127)÷ln(92))
Example values
How the main result changes when Source voltage changes and everything else stays at the example values above:
| Source voltage (V) | Minimum cross-section area (mm²) |
|---|---|
| 115 | 3.9884 |
| 172.5 | 2.6589 |
| 230 | 1.9942 |
| 287.5 | 1.5954 |
| 345 | 1.3295 |
| 460 | 0.9971 |
Formula
A = 2·ρ·L·I / V_drop (DC, 1-phase) or √3·ρ·L·I / V_drop (3-phase), AWG = 36 − 39·log₉₂(d/0.127 mm)
Worked example
Inputs — Current type: 1; Conductor: Copper; Source voltage: 230 V; Allowed voltage drop (%): 3; Current: 20 A; One-way length: 40 m.
Results — Minimum cross-section area = 1.9942 mm²; Minimum diameter = 1.5935 mm; Smallest suitable AWG = 14; AWG note = Choose this gauge or thicker (lower number).
Key terms
- Ampacity
- Current a conductor can carry continuously.
Frequently asked questions
Is voltage drop the only factor?
No – the wire must also carry the current safely (ampacity).
What drop is acceptable?
Typically 3% for branch circuits, 5% total.
Why is aluminium thicker?
It has about 64% higher resistivity than copper.
Do I need an electrician?
Yes – wiring must meet local codes.