Wire Gauge Calculator Formula
Understand the math behind the wire gauge calculator. Each variable explained with a worked example.
Formulas Used
Max Resistance Allowed
max_ohms_per_kft = max_resistanceMin Circular Mils
circular_mils = amps > 0 && max_drop_pct > 0 ? 2 * 10.8 * amps * distance / (voltage * max_drop_pct / 100) : 0Variables
| Variable | Description | Default |
|---|---|---|
amps | Circuit Amperage(A) | 20 |
distance | One-Way Distance(ft) | 50 |
voltage | System Voltage(V) | 120 |
max_drop_pct | Max Voltage Drop(%) | 3 |
max_resistance | Derived value= voltage * (max_drop_pct / 100) / amps / (2 * distance) * 1000 | calculated |
How It Works
Wire Gauge Selection
CM = 2 x K x I x D / Vdrop
Where K = 10.8 (copper resistivity constant), I is current in amps, D is one-way distance in feet, and Vdrop is the allowable voltage drop in volts. Compare the resulting circular mils to an AWG table to pick the minimum wire size.
Worked Example
20A circuit, 50 ft run, 120V, 3% max drop.
- 01Allowable drop = 120 x 0.03 = 3.6V
- 02CM = 2 x 10.8 x 20 x 50 / 3.6 = 6,000 CM
- 0312 AWG = 6,530 CM, which satisfies the requirement
- 04Use 12 AWG copper wire.
Frequently Asked Questions
What is the NEC voltage drop recommendation?
The NEC recommends no more than 3% voltage drop on branch circuits, and no more than 5% total (feeder plus branch). These are recommendations, not hard code requirements.
Why multiply distance by 2?
Current must travel to the load and return, so the total conductor length is twice the one-way distance.
Learn More
Guide
Electrical Load Calculation Guide for Residential Buildings
Learn how to calculate electrical loads for a home. Covers general lighting loads, appliance circuits, demand factors, panel sizing, and NEC code requirements.
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