DC Wire Size Calculator — Minimum Cable Cross-Section
Find the minimum copper or aluminium wire size for any DC circuit. Enter load current, wire length, supply voltage and the maximum acceptable voltage drop to get the cross-section in mm² and the nearest standard AWG gauge.
A
m
V
%
Conductor material
Theoretical minimum — use the next standard size above this value
- 1
Allowable voltage drop
12 × (3 ÷ 100) = 0.36Maximum voltage the wire resistance is allowed to waste. - 2
ρ × 2L × I (round-trip)
0.0178 × 2 × 5 × 10 = 1.78 - 3
Minimum cross-section
1.78 ÷ 0.36 = 4.944
How does this calculator work?
Minimum wire area (mm²) = (ρ × 2 × length × current) / voltage_drop, where ρ = 0.0178 for copper. A 10 A load, 5 m run, 12 V supply, 3% drop limit needs ΔV = 0.36 V, so A = (0.0178 × 10 × 10) / 0.36 = 0.49 mm² → use 0.5 mm² (AWG 20). Always round up to the next standard size.
Formula
How this is calculated
Every wire has resistance proportional to its length and inversely proportional to its cross-sectional area: R = ρ × L / A, where ρ is the material resistivity (copper ≈ 0.0178 Ω·mm²/m, aluminium ≈ 0.0282 Ω·mm²/m at 20 °C). In a DC circuit the current flows through both the positive and negative conductors, so the effective length is doubled: R_total = ρ × 2L / A. The voltage lost across the wire is V_drop = I × R_total. Rearranging for the minimum area: A = (ρ × 2L × I) / V_drop.
The calculator converts the percentage voltage drop limit into an absolute voltage (ΔV = V_supply × drop%), then solves for the minimum area. It then rounds up to the next standard IEC/EU size (0.5, 0.75, 1.0, 1.5, 2.5, 4.0, 6.0, 10.0 mm² … ) and maps to the equivalent AWG gauge. The results use recommended standard sizes; always use the next size up, never round down.
This calculation covers voltage drop only — it does not replace a full ampacity (current-carrying capacity) check, which depends on insulation rating, ambient temperature, bundling and installation method. For safety-critical installations always consult local wiring regulations (NEC in North America, IEC 60364 in Europe). The resistivity figures given are for soft-drawn copper and aluminium at 20 °C; resistance increases roughly 0.4% per °C for copper.
Frequently asked questions
Current must flow to the load AND back. A 5 m cable run uses 5 m of positive wire and 5 m of negative wire, giving 10 m of total conductor in the circuit. Both conductors contribute to the total resistance and voltage drop.
A common rule is 3% for general-purpose wiring and up to 5% for non-critical branch circuits. Low-voltage DC systems (12 V, 24 V) are more sensitive — even a 0.5 V drop is 4% of a 12 V supply. LED lighting typically requires ≤ 3%. Use 2–3% for longer runs or sensitive electronics.
No — it calculates minimum cross-section based on voltage drop only. Bundled cables, high ambient temperatures, or continuous high-current loads require a separate ampacity (thermal) check. Always verify against the appropriate wiring standard or use derating factors from the cable manufacturer.
Also known as
TG we-Calculate Editorial Team. (2026). DC Wire Size Calculator — Minimum Cable Cross-Section [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/dc-wire-size-calculator
TG we-Calculate Editorial Team. "DC Wire Size Calculator — Minimum Cable Cross-Section." TG we-Calculate. 2026. https://we-calculate.com/calculator/dc-wire-size-calculator.
TG we-Calculate Editorial Team, "DC Wire Size Calculator — Minimum Cable Cross-Section," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/dc-wire-size-calculator
@misc{wecalculate_dc_wire_size_calculator, title = {DC Wire Size Calculator — Minimum Cable Cross-Section}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/dc-wire-size-calculator}}, year = {2026}, note = {TG we-Calculate} }
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