Wire Size Calculator
Estimate the minimum cable size from current, length, supply voltage, and allowable voltage drop.
Introduction
The wire size calculator estimates a practical cable cross-sectional area from current, cable length, supply voltage, conductor material, and allowable voltage drop. This is one of the first sizing checks in electrical design because cables that are too small can cause excessive voltage drop, poor equipment performance, wasted energy, and heat buildup. Designers often use this type of calculation before performing the final code-based ampacity and protection checks.
This page focuses on voltage-drop-based sizing, which is especially important on long runs. The output is a preliminary design value and should be followed by checks for installation method, conductor temperature, grouping, ambient conditions, short-circuit performance, and local wiring regulations.
It works well as an early design tool when comparing cable options for feeders, branch circuits, and equipment connections that must balance performance and practical conductor size.
Formula
Single-phase: Vdrop = I x (2 x rho x L / A)
Three-phase: Vdrop = sqrt(3) x I x rho x L / A
Rearranging the voltage drop formula gives the minimum required conductor area A.
Variable Definitions
Vdrop = allowable voltage drop in volts
I = load current in amperes
rho = conductor resistivity in ohm mm2 per meter
L = one-way cable length in meters
A = conductor area in square millimeters
Units
Current is entered in amperes, cable length in meters, and supply voltage in volts. Allowable voltage drop is entered as a percentage of the supply voltage. The recommended wire size is shown in mm2, which is the common metric cable sizing unit used in many engineering and installation contexts.
Worked Example 1
For a 25 A single-phase load, 35 m one-way cable length, 230 V supply, and 3 percent allowable drop using
copper:
Allowable drop = 230 x 0.03 = 6.9 V
Required area = (2 x 25 x 0.017241 x 35) / 6.9
Required area is approximately 4.37 mm2
The next practical standard size is 6 mm2.
Worked Example 2
For a balanced three-phase 40 A load on a 400 V system with 50 m cable length, 5 percent allowable drop, and
aluminum conductors:
Allowable drop = 400 x 0.05 = 20 V
Required area = (sqrt(3) x 40 x 0.0282 x 50) / 20
The result is about 4.89 mm2
A practical next step is to consider a 6 mm2 cable and then verify ampacity and code limits.
Practical Notes
Voltage drop is only one part of wire sizing. A final conductor choice must also satisfy current-carrying capacity, insulation temperature rating, installation environment, grouping factors, protective device coordination, and any local electrical code requirements. Use this calculator for fast planning, not as a final code approval result.