How to Determine Cable Ampacity by Installation Method
A cable's current-carrying capacity depends far more on the installation method and grouping than on cross-section alone. This guide shows how to derive the permissible continuous ampacity to VDE 0298-4 using reference methods, correction factors and temperature.
What does ampacity depend on?
Ampacity is the continuous current a conductor may carry without exceeding the permissible operating temperature of its insulation. For PVC this limit is 70 °C, for cross-linked polyethylene (XLPE) it is 90 °C at the conductor. The decisive question is how well the heat generated in the conductor is dissipated to the surroundings.
This is exactly where the installation method matters: a cable run freely in air cools far better than one inside a conduit under plaster or buried in thermal insulation. VDE 0298‑4 (the application of IEC 60364‑5‑52) assigns each situation a reference method with base values.
- Conductor cross-section and material (copper or aluminium).
- Insulation material and its maximum operating temperature (PVC 70 °C, XLPE 90 °C).
- Installation method per the reference table (in air, in conduit, in wall, buried).
- Ambient temperature or, for buried cables, soil temperature.
- Grouping of several loaded cores or cables side by side.
Which reference installation methods exist?
VDE 0298‑4 and IEC 60364‑5‑52 condense practice into reference methods A to E. Each describes a typical mounting situation with a defined heat dissipation. From it comes the base ampacity, which is then corrected with factors.
How do grouping and temperature affect it?
The base value applies to a single circuit at 30 °C ambient (in air) or 20 °C in the ground. When reality differs, the value is multiplied by two correction factors: the temperature factor and the grouping factor. The permissible current equals the base value times both factors.
Grouping reduces capacity further because neighbouring loaded cores heat one another. For two or three bundled circuits the factor is often 0.70 to 0.80; with six or more it falls below 0.60. Both factors are multiplied together.
- 2 loaded cores bundled: factor approx. 0.80.
- 5 loaded cores bundled: factor approx. 0.60.
- Example: 27 A x 0.87 (40 °C) x 0.70 (grouping) = about 16 A permissible.
- Always check the protective device rating stays below this value.
Frequently asked questions
What exactly does VDE 0298-4 cover?
It defines the permissible current-carrying capacity of fixed-installed cables and wires as a function of installation method, ambient temperature and grouping. It is the German application standard to IEC 60364‑5‑52.
Why does grouping lower ampacity?
When several loaded cores lie close together they heat one another and dissipate heat less effectively. The grouping factor corrects the base value downward so the insulation temperature is kept within limits.
Which ambient temperature is the reference?
For installation in air the reference is 30 °C, for buried cables 20 °C soil temperature. At higher temperatures the base value is multiplied by the temperature factor; at lower ones a slight increase is permitted.
Copper or aluminium at the same cross-section?
Aluminium has a higher resistivity and carries about 20 to 25 percent less current than copper at the same cross-section. A larger aluminium cross-section is needed for the same load.
Need to size the right cable cross-section?
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