Wire Size Calculator determines the minimum conductor size needed based on system voltage, current, circuit length, phase, and allowable voltage drop. It also allows optional ampacity checks using current density, helping you size conductors during early electrical design.
About voltage drop. Voltage drop is the loss of electrical potential along the length of a conductor due to its resistance. Excessive voltage drop reduces equipment performance, increases energy losses, and can overheat conductors. Electrical codes such as NEC recommend limiting voltage drop to around 3% for branch circuits and 5% overall (feeder + branch). This calculator estimates the minimum cross-sectional area required to stay within the specified voltage drop.
About conductor materials. Copper and aluminum are the two most common conductor materials. Copper has lower resistivity (about 0.017 Ω·mm²/m) than aluminum (about 0.028 Ω·mm²/m), which means aluminum conductors must be larger to carry the same current without excessive voltage drop. The calculator uses these typical resistivity values in its computations.
About current density (ampacity check). Ampacity is the maximum current a conductor can carry continuously without exceeding its temperature rating. This calculator optionally checks ampacity by comparing the current against an allowable current density (A/mm²). This gives a quick estimate for planning purposes, though final ampacity must be verified using NEC or IEC tables based on installation conditions.
About standard wire sizes. Conductor sizes are standardized in both metric (mm²) and imperial (AWG/kcmil) systems. The calculator computes the minimum required area, then suggests the next larger standard size in mm² along with its approximate AWG and kcmil equivalents. This helps you select a real-world size rather than a theoretical value.
Calculation. The calculator determines the minimum cross-sectional area based on the voltage drop formula:
Voltage drop: ΔV = K × ρ × I × L / A
Required area from drop: Adrop = K × ρ × I × L / ΔVallow
Ampacity area (optional): Aamp = I / Jallow
Final required area: A = max(Adrop, Aamp)
Where: K = 2 for single-phase or √3 for three-phase circuits; ρ is conductor resistivity (Ω·mm²/m); I is current (A); L is one-way length (m); A is conductor cross-sectional area (mm²).
- Example 1: 230 V, 20 A, 30 m, 1φ, copper, 3% drop → ≈ 4.7 mm² → next standard = 6 mm²
- Example 2: 400 V, 40 A, 50 m, 3φ, copper, 5 V drop → ≈ 10.6 mm² → next standard = 16 mm²
- Example 3: Same as Example 1 but aluminum → ≈ 7.8 mm² → next standard = 10 mm²
Corresponding tools. To verify voltage drop after selecting a wire size, use the Voltage Drop Calculator. To size protective devices, use the Breaker Size Calculator or the Motor Circuit Calculator.
