Cable Size Calculator

Q: How do I choose the right cable size?

A cable must satisfy two constraints: (1) ampacity — its current rating must exceed the load current; (2) voltage drop — the drop along the cable must stay within 3–5% of supply voltage. The larger of the two results gives the minimum cable size.

Q: What is the maximum allowed voltage drop?

IEC and most national standards allow 3% voltage drop for lighting circuits and 5% for power circuits. For long cable runs or sensitive equipment, use 3% or less.

Q: What cable sizes are standard in IEC mm²?

Standard IEC cable sizes are: 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240 mm². Each step roughly doubles the cross-section and ampacity.

Find the minimum IEC cable size (mm²) for your load. The calculator checks both current-carrying capacity and voltage drop and recommends the cable that satisfies both.

This calculator provides estimates only. Always consult a qualified electrician and check local wiring regulations before installation.

Calculate Cable Size

Load Current (Amps)

Cable Length (metres, one-way)

System Voltage (V)

Max Voltage Drop (%)

Circuit Type

Recommended Cable

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Cable Size (mm²)

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Ampacity Rating (A)

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Actual Voltage Drop (%)

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Sized By

How It Works

The calculator applies two checks to each cable size from smallest to largest:

Ampacity check: cable rating ≥ load current
Voltage drop (single phase): VD% = (2 × L × I × R) ÷ (V × 1000) × 100
Voltage drop (three phase): VD% = (√3 × L × I × R) ÷ (V × 1000) × 100

Where L = cable length (m), I = current (A), R = conductor resistance (mΩ/m). The first cable that passes both checks is recommended.

IEC Cable Reference Table

Size (mm²)	Ampacity (A)	Resistance (mΩ/m)
1.5	16	12.1
2.5	25	7.41
4	32	4.61
6	41	3.08
10	57	1.83
16	76	1.15
25	101	0.727
35	125	0.524
50	151	0.387
70	192	0.268
95	232	0.193
120	269	0.153

Ampacity values based on IEC 60364-5-52 for copper conductors in free air at 30°C ambient. Apply derating factors for grouped cables or high ambient temperatures.

Practical Notes

These values are for copper conductors. Aluminium cables are about 60% of copper ampacity — go one or two sizes up for aluminium.
Grouped cables (bundled in conduit or trunking) must be derated — typically 0.57–0.8× depending on grouping.
For motor loads, use 125% of full-load amps to account for starting current and continuous loading rules.
Always verify against your local wiring regulations (BS7671, NEC, AS/NZS 3000, etc.) as derating conditions vary.
Use 5% voltage drop limit for power circuits, 3% for lighting.

Frequently Asked Questions

How do I choose the right cable size?

Check two things: (1) Current-carrying capacity — the cable's rated ampacity must exceed your load current. (2) Voltage drop — the drop across the cable at full load must stay within 3–5% of nominal voltage. Whichever constraint requires a larger cable governs the selection.

What is the maximum allowed voltage drop?

IEC and most standards allow 3% for lighting and 5% for power circuits from the supply point to the final outlet. For long runs or sensitive electronics, target 2–3%. Excessive voltage drop causes equipment to run hot and can shorten motor life.

What cable sizes are standard in IEC mm²?

Standard copper conductor sizes: 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240 mm². Each size up roughly doubles the cross-section. For residential wiring, 2.5mm² (lighting/sockets) and 6–10mm² (cookers, showers) are most common.

Does cable length affect the required size?

Yes, significantly. Longer runs increase total resistance, which increases voltage drop. A 2.5mm² cable adequate for a 20A load at 10 metres may need upgrading to 4 or 6mm² at 50 metres to keep voltage drop within 3%. This is why voltage drop is the governing factor on long runs.