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The long-term current-carrying capacity Iz is the maximum current a conductor can carry continuously without exceeding the permissible operating temperature of its insulation (70 °C for PVC, 90 °C for XLPE). The calculator determines Iz for copper and aluminium conductors based on the current-carrying capacity tables of PN-HD 60364-5-52 and corrects it for real conditions.
A tool for electrical installation designers, licensed electricians, and inspectors. Current-carrying capacity depends on the conductor cross-section, material, insulation type, number of loaded conductors and — crucially — on the installation method (reference method A1–F). The base value Iz₀ from the tables is corrected with the factors k₁ (ambient temperature) and k₂ (circuit grouping).
Base values of long-term current-carrying capacity Iz₀ [A] for copper conductors with PVC insulation, 2 loaded conductors, under reference conditions (air temperature 30 °C / ground 20 °C, single circuit). Columns indicate the installation method per PN-HD 60364-5-52.
| Cross-section [mm²] | A1 | A2 | B1 | B2 | C | D1 | D2 | E | F |
|---|---|---|---|---|---|---|---|---|---|
| 1,5 | 14,5 | 14 | 17,5 | 16,5 | 19,5 | 22 | 22 | 22 | 25 |
| 2,5 | 19,5 | 18,5 | 24 | 23 | 27 | 29 | 29 | 30 | 33 |
| 4 | 26 | 25 | 32 | 30 | 36 | 38 | 38 | 40 | 45 |
| 6 | 34 | 32 | 41 | 38 | 46 | 47 | 47 | 51 | 57 |
| 10 | 46 | 43 | 57 | 52 | 63 | 63 | 63 | 70 | 76 |
| 16 | 61 | 57 | 76 | 69 | 85 | 81 | 81 | 94 | 101 |
| 25 | 80 | 75 | 101 | 90 | 112 | 104 | 104 | 119 | 135 |
| 35 | 99 | 92 | 125 | 111 | 138 | 125 | 125 | 148 | 169 |
| 50 | 119 | 110 | 151 | 133 | 168 | 148 | 148 | 180 | 207 |
| 70 | 151 | 139 | 192 | 168 | 213 | 183 | 183 | 232 | 268 |
| 95 | 182 | 167 | 232 | 201 | 258 | 216 | 216 | 282 | 328 |
| 120 | 210 | 192 | 269 | 232 | 299 | 246 | 246 | 328 | 382 |
| 150 | 240 | 219 | 300 | 258 | 344 | 278 | 278 | 379 | 441 |
| 185 | 273 | 248 | 341 | 294 | 392 | 312 | 312 | 434 | 506 |
| 240 | 321 | 291 | 400 | 344 | 461 | 361 | 361 | 514 | 599 |
The factor k₁ corrects the current-carrying capacity when the ambient temperature differs from the reference value (30 °C for air, 20 °C for ground). A higher temperature means a smaller k₁ and a lower current-carrying capacity. XLPE insulation (rated 90 °C) loses current-carrying capacity more slowly than PVC (70 °C), because it has a larger conductor temperature margin.
The factor k₂ accounts for the mutual heating of conductors installed in a group — the more circuits next to each other, the smaller k₂ (2 circuits → 0.80; 3 → 0.70; 6 → 0.57). The corrected current-carrying capacity is Iz = Iz₀ × k₁ × k₂. It is this Iz, not the raw table value, that must satisfy the protective device selection condition Ib ≤ In ≤ Iz.
Current-carrying capacity is one of the two criteria for selecting a cross-section — the other is voltage drop, which on long routes often forces a larger cross-section than current-carrying capacity alone. Single-core conductors in conduit in a thermally insulating wall (A1) give the lowest current-carrying capacity (poor heat dissipation), while free air (E/F) or trunking (C) give the highest. Aluminium conductors have a current-carrying capacity about 20–25 % lower than copper of the same cross-section and are not used below 16 mm². Remember that the overcurrent protection must protect the conductor: In ≤ Iz. The calculator treats the in-ground installation methods (D1/D2) in a simplified way — full sizing of buried cables additionally requires accounting for the thermal resistivity of the ground, the burial depth and the spacing between circuits.
The long-term current-carrying capacity (Iz) is the maximum current a conductor can carry continuously without exceeding the permissible operating temperature of its insulation — 70 °C for PVC and 90 °C for XLPE. Exceeding Iz causes the insulation to overheat, ages it prematurely, and creates a risk of short circuit or fire.
For a copper conductor with PVC insulation installed in conduit on a wall (method B1, 2 loaded conductors, 30 °C): 1.5 mm² → 17.5 A, 2.5 mm² → 24 A, 4 mm² → 32 A, 6 mm² → 41 A. The values depend on the installation method — the full table is above. Under real conditions they must be corrected with the factors k₁ and k₂.
On the conductor cross-section, the material (copper has a higher current-carrying capacity than aluminium), the insulation type (XLPE higher than PVC), the number of loaded conductors and the installation method (method A1–F). It is additionally reduced by high ambient temperature (factor k₁) and the grouping of multiple circuits in a bundle (factor k₂).
Current-carrying capacity (Iz) is a conductor parameter — how much current it can safely carry. Cross-section sizing is the reverse process: for a given design current Ib you look for the smallest cross-section whose Iz (after correction) satisfies Ib ≤ Iz while the voltage drop stays within the limit. This calculator gives Iz; full sizing is done by the cable cross-section sizing calculator.
A higher ambient temperature reduces the conductor's ability to dissipate heat, so it lowers the current-carrying capacity (factor k₁ < 1). For PVC in air at 40 °C, k₁ = 0.87, and at 50 °C, k₁ = 0.71. XLPE insulation withstands high temperatures better than PVC. The tables refer to 30 °C in air and 20 °C in the ground.
It is the current-carrying capacity under continuous operation — when current flows long enough for the conductor to reach a steady-state temperature. It is distinguished from the short-circuit (short-term) current-carrying capacity, which concerns fractions of a second during a fault and is described by the thermal condition I²t ≤ k²S². In installation design, it is the long-term Iz that governs the choice of cross-section and protection.
Current-carrying capacity is one of the pillars of electrical installation design. See also:
