Wire Size Calculator

Calculate the minimum wire size needed for your electrical application

Calculate Required Wire Size

Current (A)

Cable Length (m)

System Voltage (V)

Max Voltage Drop (%)

Standard: 3% for most applications

Authoritative References

American Wire Gauge

Background on the conductor-size system commonly used in harness drawings.

Wire harness definition

Useful context when reviewing branch lengths, routing, and bundled conductors.

Frequently Asked Questions

What does this calculator estimate?

It estimates the minimum copper conductor area needed to stay within a target voltage-drop limit for a given load, length, and system voltage.

Does wire size alone guarantee safety?

No. Wire size is only one part of the decision. Final approval must also consider ampacity, insulation system, ambient temperature, bundling, and local code requirements.

Why is round-trip length included in the formula?

Current must travel out and back, so a complete circuit consists of both supply and return conductor resistance.

Can I use this for harness branches?

Yes, but each branch should be reviewed separately if current levels, duty cycle, or routing length change across the harness.

When should I choose a larger gauge than the output?

Move up when the application faces high ambient temperature, tighter bundles, start-up surge current, or long-term service risk that justifies extra margin.

Wire Sizing Guide

Why Wire Size Matters

Choosing the correct wire size is critical for safety, efficiency, and equipment performance. Undersized wire can cause excessive voltage drop, overheating, fire hazards, and equipment damage. Oversized wire wastes money and space but provides a safety margin.

Wire Sizing Formula

Minimum Area (mm²) = (2 × ρ × I × L) / ΔV

Where:

ρ = Resistivity of copper = 0.0172 Ω·mm²/m (at 20°C)
I = Current in Amperes (A)
L = One-way cable length in meters (m)
ΔV = Maximum allowed voltage drop in Volts (V)
2 = Factor for round trip (positive and negative conductors)

Key Sizing Factors

1. Voltage Drop

Wire must be large enough to keep voltage drop within acceptable limits (typically ≤3%).

2. Current Capacity (Ampacity)

Wire must handle the current without overheating. This calculator focuses on voltage drop; always verify ampacity separately.

3. Temperature Rating

Higher ambient temperatures reduce current capacity. Derating may be required.

4. Installation Method

Conduit, free air, or bundled cables have different ampacity ratings.

Common Applications & Wire Sizes

Application	Current	Typical AWG	mm²
LED lighting	1-5 A	18-16	0.82-1.31
12V automotive	10-20 A	14-12	2.08-3.31
Industrial control	5-15 A	16-14	1.31-2.08
Power tools	15-20 A	14-12	2.08-3.31

Important Safety Notes

⚠️ Warning:

This calculator provides wire size recommendations based on voltage drop only. Always verify that the wire meets ampacity (current capacity) requirements and complies with local electrical codes.

Always use wire rated for your voltage and application
Check local electrical codes and regulations
Consider derating factors (temperature, bundling, etc.)
Use appropriate insulation type for environment
When in doubt, consult a licensed electrician

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