Wire Sizer & Ampacity Calculator

Sizing a wire requires checking two limits: Thermal Ampacity (so the insulation doesn’t melt) and Voltage Drop (so the equipment gets enough power).

This calculator uses the full NEC 310.16 standard. It applies temperature and bundling derating factors to find the safe current limit, while simultaneously calculating the voltage drop over distance.

Cable Specification

Wire Size

Insulation

Ambient Temp

°C

Conductors in Cable

Run Length & Load

Drop Check

Load Current

System Voltage

One-Way Length

Safety Analysis

Result

Thermal Ampacity Limit

— A

(Base × Temp × Bundle)

Voltage Drop Check

— %

Voltage at Load: — V

Formulas Explained

Safe Ampacity

I_safe = I_base × F_temp × F_bundle

Base rating multiplied by derating factors.

Voltage Drop

V_drop = I_load × R_wire

Current × Total Resistance of the wire run.

Total Resistance

R_wire = (ρ × L × 2) / Area

Includes length there and back (round trip).

% Voltage Drop

% Drop = (V_drop / V_source) × 100

Recommended limit is 3% for feeders.

How to Use

Follow these steps to size your wire correctly.

Select Wire Specs

Choose the Material, Size (AWG), and Insulation type. Set the Ambient Temp if the wire runs through a hot area.

Define Load & Length

Enter the Load Current (Amps) and the Wire Length. This activates the Voltage Drop Check.

Check the Status

Look for the Green PASS box.
• If Thermal fails: Increase wire size (AWG).
• If Voltage Drop fails: Increase wire size to reduce resistance.