Cable Size Calculator

Formula used

Ib ≤ In ≤ Iz · ΔU% ≤ limit

Minimum copper cable size by protective device rating · three-phase (3 loaded conductors)

Smallest IEC 60364-5-52 cross-section whose reference current at 30 °C ambient and a single circuit is at least the device rating (Ib ≤ In ≤ Iz). Higher ambient temperature, grouped circuits and long runs need a larger size.

Minimum copper cable size by protective device rating · single-phase (2 loaded conductors)

Smallest IEC 60364-5-52 cross-section whose reference current at 30 °C ambient and a single circuit is at least the device rating (Ib ≤ In ≤ Iz). Higher ambient temperature, grouped circuits and long runs need a larger size. Turkish wiring regulations also require at least 2.5 mm² for socket circuits and 1.5 mm² for lighting circuits in buildings.

Longest three-phase run for a 3% voltage drop at full load (copper, 400 V)

Load current equals the PVC method C reference current of the cable; cos φ 0.9, conductor at 70 °C, reactance 0.08 Ω/km. At half the current the permitted length doubles.

Example: 30 kW three-phase load, 400 V, copper PVC cable on a surface

1. Design current: Ib = 30,000 / (√3 × 400 × 0.9 × 0.95) = 50.64 A; next standard device rating In = 63 A.
2. Ampacity: the smallest size with Iz ≥ 63 A is 16 mm² (Iz = 76 A).
3. At 80 m the voltage drop is 2.13% (limit 3%), so 16 mm² is enough.
4. At 200 m the 3% limit needs 35 mm² (2.51%); voltage drop, not ampacity, sets the size.

What cable size is needed for a 16 A breaker?

For PVC-insulated copper cable at 30 °C and a single circuit, the smallest IEC 60364-5-52 size that carries at least 16 A is 2.5 mm² in conduit or trunking (B2) and 1.5 mm² on a surface (C) for a three-phase circuit, and 1.5 mm² and 1.5 mm² respectively for a single-phase circuit. Long runs, higher ambient temperature or several circuits laid together can require a larger size; in buildings, socket circuits use at least 2.5 mm².

What cable size is needed for a 25 A breaker?

For PVC-insulated copper cable at 30 °C and a single circuit, the smallest IEC 60364-5-52 size that carries at least 25 A is 4 mm² in conduit or trunking (B2) and 4 mm² on a surface (C) for a three-phase circuit, and 4 mm² and 2.5 mm² respectively for a single-phase circuit. Long runs, higher ambient temperature or several circuits laid together can require a larger size; in buildings, socket circuits use at least 2.5 mm².

What cable size is needed for a 32 A breaker?

For PVC-insulated copper cable at 30 °C and a single circuit, the smallest IEC 60364-5-52 size that carries at least 32 A is 6 mm² in conduit or trunking (B2) and 4 mm² on a surface (C) for a three-phase circuit, and 6 mm² and 4 mm² respectively for a single-phase circuit. Long runs, higher ambient temperature or several circuits laid together can require a larger size; in buildings, socket circuits use at least 2.5 mm².

What cable size is needed for a 40 A breaker?

For PVC-insulated copper cable at 30 °C and a single circuit, the smallest IEC 60364-5-52 size that carries at least 40 A is 10 mm² in conduit or trunking (B2) and 6 mm² on a surface (C) for a three-phase circuit, and 10 mm² and 6 mm² respectively for a single-phase circuit. Long runs, higher ambient temperature or several circuits laid together can require a larger size; in buildings, socket circuits use at least 2.5 mm².

What cable size is needed for a 63 A breaker?

For PVC-insulated copper cable at 30 °C and a single circuit, the smallest IEC 60364-5-52 size that carries at least 63 A is 25 mm² in conduit or trunking (B2) and 16 mm² on a surface (C) for a three-phase circuit, and 16 mm² and 10 mm² respectively for a single-phase circuit. Long runs, higher ambient temperature or several circuits laid together can require a larger size; in buildings, socket circuits use at least 2.5 mm².

How is cable size calculated?

First find the load design current and choose the next standard breaker. Then find the smallest size that carries the breaker current after temperature and grouping correction, and the smallest size that keeps voltage drop within the limit; use the larger of the two.

Which criterion sets the size?

On short runs ampacity usually sets the size; on long runs voltage drop does. In buildings, socket circuits use at least 2.5 mm² and lighting circuits at least 1.5 mm²; short-circuit withstand and motor starting current are checked separately.