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Amp to Wire Size Calculator

Selecting the right wire gauge is critical for safe and efficient electrical installations. This calculator determines the minimum cross-sectional area needed based on your system's amperage, voltage, distance from source to load, and allowable voltage drop. Engineers, electricians, and installers use this method to prevent overheating, voltage loss, and safety hazards in circuits ranging from low-voltage DC systems to high-amperage three-phase industrial connections.

Last updated:

Creators Davide Borchia, PhD
4,155 people find this calculator helpful

Wire Sizing Fundamentals

Choosing the correct wire size requires understanding how current, distance, and voltage drop interact. A wire that's too thin generates excessive heat and voltage loss; too thick wastes material and cost. The National Electrical Code (NEC) and similar standards limit voltage drop to 3% for branch circuits and 5% for combined feeder and branch circuits.

Several factors influence the wire size you need:

  • System amperage: The actual current draw under normal operating conditions.
  • Voltage configuration: Single-phase AC, three-phase AC, or direct current (DC) systems have different current distribution patterns.
  • Conductor material: Copper and aluminium have different electrical resistivity values; copper is more conductive and thus requires smaller gauges.
  • Distance: Longer cable runs increase voltage drop significantly. A 100-amp circuit over 50 feet needs a substantially larger wire than the same circuit over 10 feet.
  • Operating temperature: Higher temperatures increase conductor resistivity, reducing ampacity and requiring larger wire sizes.

Wire Cross-Sectional Area Calculation

The cross-sectional area (A) required for a cable is derived from Ohm's law and voltage drop constraints. This formula accounts for the resistance of the conductor material and the acceptable voltage loss across the run.

A = (ρ × I × L × φ) ÷ (V × ΔV)

  • A — Required wire cross-sectional area in square millimetres
  • ρ — Conductor resistivity in ohm·millimetres per metre, temperature-corrected
  • I — System amperage (current in amperes)
  • L — One-way distance from source to load in metres
  • φ — Phase factor (2 for single-phase AC, √3 ≈ 1.732 for three-phase AC, 1 for DC)
  • V — Source voltage in volts
  • ΔV — Allowable voltage drop in volts (e.g., 3% or 5% of nominal voltage)

Frequently Asked Questions

Why does distance matter when choosing wire gauge?

Voltage drop increases proportionally with cable length because resistance multiplies across the run. A 70-amp circuit over 200 feet requires a much larger conductor than the same circuit over 20 feet to stay within acceptable voltage loss limits. This is why branch circuits feeding distant equipment often need two or three wire sizes larger than circuits serving nearby loads.

How does temperature affect wire ampacity?

As conductor temperature rises, electrical resistivity increases, reducing the wire's ability to carry current safely. A 14 AWG copper wire rated for 15 amps at 60°C (standard conditions) may only handle 12 amps at 90°C in an enclosed conduit. Always account for the maximum expected operating temperature in your installation environment.

Can I use aluminium wire instead of copper to save cost?

Aluminium has roughly 1.6 times the resistivity of copper, meaning an aluminium conductor must be two sizes larger than copper to carry the same amperage. While initially cheaper, aluminium requires larger conduit, larger breakers, and often costs more overall. It's best suited for long-distance, high-voltage transmission where the cost advantage is significant.

What's the difference between wire gauge (AWG) and cross-sectional area?

American Wire Gauge (AWG) is a logarithmic standardized sizing system; each step down in gauge number increases cross-sectional area by roughly 26%. Cross-sectional area (in square millimetres or square inches) is the actual physical dimension used in resistance calculations. The calculator converts between both formats for your convenience.

Is it safe to use a smaller wire if voltage drop isn't critical?

No. Wire size is determined by two independent limits: ampacity (current-carrying capacity) and voltage drop. Even if voltage drop is acceptable, undersizing the wire can cause overheating, insulation breakdown, and fire risk. Always meet or exceed the ampacity requirement for your system's maximum expected current.

How do single-phase and three-phase systems differ in wire sizing?

Three-phase AC systems distribute current across three conductors, reducing the effective current per wire and allowing smaller gauges than single-phase for the same total power. The phase factor of √3 (≈1.732) in three-phase calculations reflects this advantage. For example, a three-phase 100-amp circuit typically requires smaller wires than a single-phase 100-amp circuit at the same voltage over the same distance.

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