DC Wire Size Calculator

Written by Great Medico

Updated September 25, 2026 •2 min read

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Electrical shortcut formulas are useful only when phase, units, and circuit assumptions are kept straight. DC Wire Size Calculator targets recommended DC conductor area and shows the relationship behind the number rather than treating it as a black-box design approval.

What this calculator does

The DC Wire Size Calculator centers on recommended DC conductor area using the fields that are actually present here: Source voltage, Allowable voltage drop, Conductor material, Current (I), One-way distance (D). Rather than treating every box as an independent input, use the equation below to identify the driving quantities and read the remaining fields as derived or supporting values when appropriate.

How to use it

Enter source voltage, allowable drop, conductor material, load current, one-way run length, and conductor temperature. If drop is entered as a percentage, the page converts it to volts before sizing area.

How the calculation works

The voltage-drop sizing path uses A = 2ρLI/ΔV for a two-conductor DC run, where L is one-way length. Copper or aluminum resistivity is adjusted from its 20 °C reference using ρ(T) = ρ₂₀[1 + α(T−20 °C)]. If the entered drop is a percentage, the page first converts it to volts from source voltage.

Example

For 120 V, 15 A, 30 m one-way copper, 3% allowable drop, and a 75 °C conductor temperature, ΔV = 3.6 V. The temperature-adjusted resistivity is about 2.04×10⁻⁸ Ω·m, giving A ≈ 5.11 mm² and an equivalent round-wire diameter near 2.55 mm.

How to interpret the result

Treat the calculated area as the cross-sectional area needed to satisfy the entered voltage-drop target in this simplified DC model. A longer run, higher current, hotter conductor, or tighter voltage-drop limit generally increases the required area; code-required ampacity can still demand a larger conductor.

Limitations and notes

This is voltage-drop sizing, not a complete electrical-code conductor selection. Ampacity, insulation rating, ambient temperature, terminal limits, bundling, overcurrent protection, installation method, fault current, and local code can require a larger conductor. Treat the result as an engineering check and have real installations verified by a qualified electrician or engineer.