Wire Size Calculator | Minimum Cross-Section for Current
Calculate the minimum wire cross-sectional area (mm² or AWG) based on current, wire length, and allowable voltage drop.
How to Use
- Enter the maximum current the wire must carry in amperes.
- Enter the one-way wire length in meters (for a round trip, the formula uses 2× this length internally).
- Enter the maximum allowable voltage drop in volts.
- Select the conductor material: copper (lower resistance) or aluminum (lighter, higher resistance).
- Click 'Calculate' to get the minimum cross-section in mm² and AWG.
- Always select the next larger standard wire size from the calculated minimum.
About Wire Sizing
Wire Size Formula
The minimum wire cross-sectional area is calculated as: A = (ρ × 2 × L × I) / ΔV, where ρ is the resistivity of the conductor (copper ≈ 1.72×10⁻⁸ Ω·m, aluminum ≈ 2.82×10⁻⁸ Ω·m), L is the one-way wire length (m), I is the current (A), and ΔV is the allowable voltage drop (V). The factor 2 accounts for both the outgoing and return conductors.
Standard Wire Sizes
Wire cross-sections follow international standards. IEC standard sizes (mm²): 0.5, 0.75, 1.0, 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120 mm². AWG (American Wire Gauge) uses the inverse scale: smaller AWG = larger wire. Common AWG sizes: 24 (0.2mm²), 22 (0.33mm²), 20 (0.52mm²), 18 (0.82mm²), 16 (1.3mm²), 14 (2.1mm²), 12 (3.3mm²), 10 (5.3mm²).
Voltage Drop Standards
Most electrical codes recommend a maximum voltage drop of 3–5% for branch circuits. For 12V DC systems, 3% = 0.36V drop; for 24V systems, 3% = 0.72V. Low-voltage systems (12V) are more sensitive to voltage drop than 120/240V AC systems. For critical loads like motors and solenoids, keep voltage drop below 2–3%. For lighting, up to 5% is generally acceptable.
Ampacity vs. Voltage Drop
Wire sizing has two constraints: ampacity (thermal current limit) and voltage drop (resistance limit). The ampacity limit prevents the wire from overheating; the voltage drop limit ensures the load receives adequate voltage. In short runs at high current, ampacity usually dominates. In long runs at moderate current, voltage drop often requires a larger wire than ampacity alone would suggest. Always check both constraints.
Key Features
- Calculates minimum mm² and equivalent AWG from current and drop constraints
- Supports copper and aluminum conductors with correct resistivity
- Shows recommended standard wire size and actual voltage drop
- Voltage drop percentage for quick code compliance check
Common Applications
- Residential and commercial electrical wiring design
- Automotive and marine 12V/24V DC wiring
- Solar panel and battery cable sizing
- Industrial motor and control panel wiring
- LED strip and low-voltage lighting circuits