The famous question "how many amps can 1.5 mm² take?" has no blanket answer – it depends on installation method, temperature and neighbouring cables. The Cable Ampacity Calculator (with Grouping Factors) converts the table values of DIN VDE 0298-4 to your real installation situation.
The principle
Iz = Iz,table × f_temperature × f_grouping – the base value from the standard applies to 25 °C ambient and a single cable. Every deviation reduces the capacity: 40 °C ambient costs around 13–18% depending on insulation, four grouped circuits on a cable tray around 35%.
Step by step
- Select installation method: A1/A2 (in thermally insulated wall), B1/B2 (in conduit on the wall), C (directly on the wall) or E (cable tray with free air)
- Select cross-section: 1.5 to 35 mm²
- Enter ambient temperature: attics in summer and boiler rooms are well above 25 °C
- Specify grouping: number of jointly routed, simultaneously loaded circuits
- Check the result: real capacity Iz and the maximum permissible fuse
Worked example
1.5 mm² in installation method A2 (sheathed cable in an insulated wall): table value 15.5 A. At 40 °C ambient (f = 0.87) and three grouped circuits (f = 0.7), 9.4 A remain – the usual 16 A fuse is not permissible here; you need 2.5 mm² or a 10 A fuse. Exactly this combination is the most frequently overlooked in practice.
Practical tips
The installation method is the biggest lever: the same cable carries around 30% more on a cable tray (E) than in an insulated wall (A2). For long runs, also check the voltage drop – it often limits earlier than heating does. And document the factors: the acceptance test per DIN VDE 0100-600 asks exactly for them. The binding assessment remains with the qualified electrician.
