Cable size calculator by current-carrying capacity
Pick the minimum cable size from the load current, conductor material, insulation, installation method, temperature and number of grouped circuits, with the correction factors applied, and optionally check voltage drop too.
Indicative Values come from IEC 60364-5-52 tables (Annex B). Confirm the result against your national wiring rules (e.g. BS 7671, SR HD 60364-5-52) before installation. The calculator checks cable heating and, optionally, voltage drop only.
How the size is chosen
I_req = max(I_b, I_n) design or protective-device current k = k_temp · k_group I_tab ≥ I_req / k smallest table size that satisfies it I_z = I_tab · k capacity under your conditions
- I_b = load current; I_n = rated current of the fuse or breaker
- k_temp = factor for air temperature (reference 30 °C) or ground temperature (reference 20 °C)
- k_group = factor for adjacent circuits
Worked example
A 32 A single-phase circuit in PVC copper cable, method B1, at 40 °C, grouped with 2 other circuits:
k_temp = 0.87 k_group = 0.70 k = 0.609 I_tab ≥ 32 / 0.609 = 52.5 A 6 mm² → 41 A (too small) 10 mm² → 57 A ✔ I_z = 57 · 0.609 = 34.7 A ≥ 32 A
The result is 10 mm². On a long route the size may increase because of voltage drop.
Installation methods available
- A1 – insulated conductors in conduit in a thermally insulated wall
- A2 – multicore cable in conduit in a thermally insulated wall
- B1 – conductors in conduit on or in a masonry wall
- B2 – multicore cable in conduit on or in a wall
- C – cable clipped directly on a wall
- D1 – multicore cable in a buried duct
- D2 – cable buried directly in the ground
- E – multicore cable in free air / on perforated tray
Current-carrying capacity: copper, PVC, three loaded conductors (A)
Air 30 °C, ground 20 °C, a single circuit. Source: IEC 60364-5-52, tables B.52.4 and B.52.10.
| mm² | A1 | B1 | C | E |
|---|---|---|---|---|
| 1.5 | 13.5 | 15.5 | 17.5 | 18.5 |
| 2.5 | 18 | 21 | 24 | 25 |
| 4 | 24 | 28 | 32 | 34 |
| 6 | 31 | 36 | 41 | 43 |
| 10 | 42 | 50 | 57 | 60 |
| 16 | 56 | 68 | 76 | 80 |
| 25 | 73 | 89 | 96 | 101 |
| 35 | 89 | 110 | 119 | 126 |
| 50 | 108 | 134 | 144 | 153 |
| 70 | 136 | 171 | 184 | 196 |
| 95 | 164 | 207 | 223 | 238 |
Air temperature factors
| °C | PVC | XLPE / EPR |
|---|---|---|
| 10 | 1.22 | 1.15 |
| 15 | 1.17 | 1.12 |
| 20 | 1.12 | 1.08 |
| 25 | 1.06 | 1.04 |
| 30 | 1 | 1 |
| 35 | 0.94 | 0.96 |
| 40 | 0.87 | 0.91 |
| 45 | 0.79 | 0.87 |
| 50 | 0.71 | 0.82 |
| 55 | 0.61 | 0.76 |
| 60 | 0.5 | 0.71 |
| 65 | – | 0.65 |
| 70 | – | 0.58 |
| 75 | – | 0.5 |
| 80 | – | 0.41 |
Grouping factors
Adjacent circuits or cables, single layer. For numbers in between, the factor for the next tabulated number is used.
| Circuits | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 12 | 16 | 20 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| A1, A2, B1, B2 (bunched) | 1 | 0.8 | 0.7 | 0.65 | 0.6 | 0.57 | 0.54 | 0.52 | 0.5 | 0.45 | 0.41 | 0.38 |
| C (on wall) | 1 | 0.85 | 0.79 | 0.75 | 0.73 | 0.72 | 0.72 | 0.71 | 0.7 | 0.7 | 0.7 | 0.7 |
Frequently asked questions
What is the current-carrying capacity of a cable?
It is the maximum current a cable can carry continuously without the insulation exceeding its rated temperature (70 °C for PVC, 90 °C for XLPE). It depends on cross-section, conductor material, insulation, installation method, ambient temperature and how many cables are grouped.
Why does the installation method matter?
Heat escapes differently: a cable in free air cools far better than one in conduit inside an insulated wall. The same 2.5 mm² PVC copper cable carries 27 A clipped to a wall (method C) but only 19.5 A in conduit in a thermally insulated wall (A1), with two loaded conductors.
What are “loaded conductors”?
They are the conductors carrying current in normal operation: 2 for single-phase (line and neutral), 3 for balanced three-phase. The protective conductor is not counted, and the neutral counts only if it carries current (for example with harmonics).
How are the correction factors applied?
Divide the required current by the product of the temperature and grouping factors and look the result up in the table: Itab ≥ Ib / (kT · kG). After correction the chosen cable must carry at least the design current and, if given, the rated current of the protective device.
Is the size found here the final size?
Not necessarily. Besides heating you must also check voltage drop (tick the option in the calculator), short-circuit withstand and coordination with the protective device. The result is indicative and must be confirmed against your national wiring rules.