Wire Size Calculator
Determine the correct electrical wire gauge (AWG) for DC, AC single-phase, and three-phase circuits. Free online wire size calculator with voltage drop analysis.
About This Calculator
The Wire Size Calculator helps electricians, engineers, and DIY enthusiasts determine the correct electrical wire gauge for any electrical installation. Whether you are wiring a 12V DC solar panel, a 120V AC household circuit, or a three-phase industrial motor, this tool computes the minimum cross-sectional area required to keep voltage drop within safe limits.
The calculation follows Ohm's law combined with Pouillet's law: for DC and single-phase AC systems, the wire cross-sectional area is A = I × ρ × 2L / V, where I is the current in amps, ρ is the resistivity of the conductor at operating temperature, L is the one-way cable length in meters, and V is the allowable voltage drop. For three-phase systems, the formula is A = √3 × I × ρ × L / V to account for the use of three conductors. The calculator automatically corrects resistivity for operating temperature using the material-specific temperature coefficient.
Regional Notes
United States: NEC (National Electrical Code) recommends 3% voltage drop for branch circuits and 5% total. Standard voltages are 120V / 240V split-phase for residential and 208V / 480V three-phase for commercial. Common wire sizes: 14 AWG for 15A lighting, 12 AWG for 20A outlets, 10 AWG for 30A dryers/AC units.
United Kingdom: BS 7671 (IET Wiring Regulations) specifies voltage drop limits of 3% for lighting (6.9V for 230V) and 5% for other circuits (11.5V for 230V). Standard UK supply is 230V single-phase / 400V three-phase. Cable sizes are typically specified in mm².
India: IS 732 (Indian Standard for Electrical Wiring) follows similar guidelines with 5% max voltage drop. Standard supply is 230V single-phase / 415V three-phase at 50 Hz. Common house wiring uses 1.5 mm² for lighting and 2.5 mm² for power outlets.
Materials
Copper: Resistivity 1.68 × 10⁻⁸ Ω·m at 20°C, temperature coefficient 0.00404/°C. Preferred for most residential and commercial wiring due to high conductivity and durability.
Aluminum: Resistivity 2.65 × 10⁻⁸ Ω·m at 20°C, temperature coefficient 0.00391/°C. Used for large feeder cables and overhead lines where weight and cost savings justify the larger gauge required.
Frequently Asked Questions
How is wire size calculated?
Wire size is calculated using Ohm's law and Pouillet's law: A = I × ρ × 2L / V for DC/single-phase and A = √3 × I × ρ × L / V for three-phase, where I is current, ρ is resistivity, L is one-way distance, and V is allowable voltage drop.
What wire size do I need for 20 amps?
For 20 amps at 120V with 3% voltage drop over 30m using copper, you need approximately 12 AWG wire. Always derate for longer distances and higher temperatures.
Does temperature affect wire size?
Yes, higher temperatures increase electrical resistance, which reduces current-carrying capacity. The calculator corrects resistivity using the material's temperature coefficient to determine the appropriate wire gauge for operating conditions.
What is the difference between AWG and mm²?
AWG (American Wire Gauge) is a US standard where smaller numbers indicate thicker wires. mm² is the metric cross-sectional area. For example, 12 AWG ≈ 3.31 mm², 10 AWG ≈ 5.26 mm², and 8 AWG ≈ 8.37 mm².
Why does a three-phase system use less copper?
Three-phase systems use three conductors instead of two, reducing the required cross-sectional area per conductor by a factor of √3/2 compared to single-phase, making them more efficient for high-power distribution.
Should I use copper or aluminum wire?
Copper has lower resistivity (1.68×10⁻⁸ Ω·m) than aluminum (2.65×10⁻⁸ Ω·m), so copper can carry more current for the same wire size. Aluminum is lighter and cheaper but requires a larger gauge for the same current capacity.
What is the standard voltage drop limit?
The National Electrical Code (NEC) recommends a maximum 3% voltage drop for branch circuits and 5% total for feeder and branch circuits combined. The UK IET Wiring Regulations similarly recommend 3-5% depending on the circuit type.
Can I use this calculator for automotive wiring?
Yes, this calculator works for DC systems including automotive, marine, and solar installations. For 12V automotive systems, enter 12V source voltage with 3% voltage drop (0.36V) for critical electronics or 10% for lighting.