electrical cable size calculator

How this electrical cable size calculator works

Choosing the right cable size is a safety and performance decision, not just a cost decision. If a cable is too small, it can overheat, create excessive voltage drop, reduce equipment life, and in severe cases become a fire hazard. This calculator gives you a fast first-pass estimate of the required cable cross-sectional area in mm².

The tool combines two checks:

• Voltage drop check: Keeps terminal voltage within your acceptable range.
• Ampacity rule-of-thumb check: Ensures a practical minimum section for current carrying.

Inputs explained

1) System type

Select single-phase AC, three-phase AC, or DC. The formula uses different path factors: single-phase and DC use round-trip length (2L), while three-phase uses a √3 factor.

2) Conductor material

Copper and aluminum have different resistivity. Aluminum needs a larger cross-section for the same current and voltage drop target.

3) Load current

Enter the design current in amperes. If your load is intermittent, use the current value required by your local electrical code and duty cycle conditions.

4) One-way cable length

Use physical one-way run distance from source to load. The formula internally handles return path or phase factor.

5) Voltage and allowable drop

Typical design values are 2% to 5%, depending on equipment sensitivity and local standards. Lower allowed drop usually means larger cable.

Formula used in this calculator

Resistivity constants used:

• Copper: 0.0175 Ω·mm²/m
• Aluminum: 0.0282 Ω·mm²/m

Voltage-drop area estimate:

• Single-phase or DC: A = (2 × I × ρ × L) / V_drop
• Three-phase: A = (√3 × I × ρ × L) / V_drop

Ampacity rule-of-thumb estimate:

• Copper current density: 6 A/mm²
• Aluminum current density: 4 A/mm²

Final required area is the larger of the two values, then rounded up to the next common standard cable size.

Common standard cable sizes

Low/Medium Power	Higher Power / Feeders	Large Distribution
1.5, 2.5, 4, 6 mm²	10, 16, 25, 35, 50, 70 mm²	95, 120, 150, 185, 240, 300+ mm²

Practical factors not fully modeled here

This calculator is intentionally simple and useful for planning. Final engineering should include:

• Ambient temperature correction
• Installation method (in conduit, buried, tray, free air)
• Grouping/derating for multiple loaded cables
• Insulation type and conductor temperature rating
• Motor starting current and harmonic content
• Fault level and short-circuit thermal withstand
• Local code requirements (NEC, IEC, BS, AS/NZS, etc.)

Quick example

Suppose you have a 230 V single-phase load drawing 32 A over a 45 m run, with a maximum 3% voltage drop, copper conductor.

• Allowed drop = 230 × 3% = 6.9 V
• Voltage-drop area ≈ (2 × 32 × 0.0175 × 45) / 6.9 ≈ 7.3 mm²
• Ampacity estimate = 32 / 6 ≈ 5.3 mm²
• Take larger value: 7.3 mm² → round up to standard 10 mm²

Result: 10 mm² copper is the recommended minimum by this method.

Final note

This tool provides an estimate for design screening and educational use. Always verify with local electrical standards and a licensed electrician or engineer before installation.