# Wire Size Calculator

URL: https://speedycalculator.com/calculators/physics/wire-size/
Last updated: 2026-10-30

> The wire size calculator finds the minimum conductor cross-section and the nearest suitable AWG size for a circuit, from the voltage, allowed voltage drop, current, one-way length and conductor material.

## Inputs

- Current type
- Conductor
- Source voltage (V, kV, mV)
- Allowed voltage drop (%)
- Current (A, mA, µA, kA)
- One-way length (m, cm, mm, km, in, ft, yd, mi)

## Results

- Minimum cross-section area
- Minimum diameter
- Smallest suitable AWG
- AWG note

## 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)
```

## Explanation

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

FieldWhat to enterCurrent typeChoose one: DC / single-phase AC, Three-phase ACConductorChoose one: Copper, AluminiumSource voltageUnits: V, kV, mVAllowed voltage drop (%)A numberCurrentUnits: A, mA, µA, kAOne-way lengthUnits: 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)1153.9884172.52.65892301.9942287.51.59543451.32954600.9971

## 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.

## FAQ

**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.


## Sources

- [The Engineering ToolBox](https://www.engineeringtoolbox.com/)
- [NFPA 70 – National Electrical Code](https://www.nfpa.org/codes-and-standards/nfpa-70-standard-development/70)