The most frequently asked electrical question of all has no single answer: how much current a cross-section can carry depends on how the cable is installed. The same 1.5 mm² carries 13 A in a thermally insulated wall and 18.5 A on a cable tray. Here are the reference values per DIN VDE 0298-4 (PVC-insulated cables, two loaded conductors or three-phase, 25 °C ambient*).
Ampacity Iz by installation method
| Cross-section | A1 (insulated wall, single cores) | A2 (insulated wall, sheathed) | B1 (conduit on wall) | B2 (conduit, sheathed) | C (on wall) | E (cable tray) |
|---|---|---|---|---|---|---|
| 1.5 mm² | 13.5 A | 13 A | 15.5 A | 15 A | 17.5 A | 18.5 A |
| 2.5 mm² | 18 A | 17.5 A | 21 A | 20 A | 24 A | 25 A |
| 4 mm² | 24 A | 23 A | 28 A | 27 A | 32 A | 34 A |
| 6 mm² | 31 A | 29 A | 36 A | 34 A | 41 A | 43 A |
| 10 mm² | 42 A | 39 A | 50 A | 46 A | 57 A | 60 A |
| 16 mm² | 56 A | 52 A | 66 A | 62 A | 76 A | 80 A |
*Guide values; identical to the base values of our Cable Ampacity Calculator (with Grouping Factors). The standard and the assessment by a qualified electrician govern.
Don't forget the correction factors
The table values apply to a single cable at moderate ambient temperature. In practice, two derating factors multiply in: temperature (at 40 °C only factor 0.87, at 50 °C factor 0.71) and grouping (two bundled circuits: 0.8; four: 0.65; nine: 0.5). Example: 1.5 mm² in A2 at 40 °C with three grouped circuits → 13 × 0.87 × 0.7 ≈ 7.9 A – the usual 16 A breaker is then two sizes too big.
Assigning the fuse
Basic rule: the rated current of the protective device must not exceed the (corrected!) ampacity. This yields the familiar standard assignments in residential construction – B16 on 1.5 mm² only with favourable installation, otherwise B13 or 2.5 mm²; B20 on 2.5 mm²; B32 on 6 mm² (cooker). Wallboxes, instantaneous water heaters and long feeders always deserve an individual calculation.
The second criterion: voltage drop
On long runs it is often not heating but voltage drop that limits (recommended ≤ 3% from the meter): a 30 m feeder to the garage at 16 A already needs 4 mm² for that, although 1.5 mm² would suffice thermally. The Cable Cross-Section Calculator handles both checks together; the installation-method factors in detail are computed by the Cable Ampacity Calculator (with Grouping Factors).
