Cable cross-section & current-carrying capacity
§523 — Cable cross-section & current-carrying capacity
The current-carrying capacity Iz of a cable is the maximum current the
cable may carry continuously without exceeding the permitted conductor
temperature. The design principle is easy to remember; the complexity lies
in the correction factors:
Ib ≤ In ≤ Iz — the load current (Ib) may never exceed the rated current of the protective device (In), which in turn may never exceed the current-carrying capacity of the cable (Iz).
The basic formula
Iz = Itable × f1 × f2 × f3
- Itable — the table value from the NEN 1010 annex (dependent on conductor material, insulation type and installation method), valid at reference conditions: 30 °C ambient temperature, a single cable, no additional thermal insulation.
- f1 — correction factor for ambient temperature (deviating from 30 °C).
- f2 — correction factor for bunching/grouping with other cables.
- f3 — correction factor for thermal insulation around the cable (e.g. cavity wall insulation, floor insulation).
When several deviating conditions occur simultaneously (a warm technical room combined with bunched cables), the factors are multiplied together — the reduction in Iz stacks up quickly.
Reference installation methods
The table value Itable depends heavily on how the cable is installed:
| Method | Description | Effect |
|---|---|---|
| A1 | Cable in an insulated wall/conduit, built in | Worst heat dissipation → lowest Iz for the same cross-section. |
| B1 | Cable in conduit/trunking against a wall | Better heat dissipation than A1. |
| C | Cable directly on/against the wall (no conduit) | Even better heat dissipation. |
| E | Cable on cable tray/ladder in free air | Best heat dissipation → highest Iz for the same cross-section. |
Practical consequence: the same 2.5 mm² cable has a higher permitted current on an open cable tray (E) than that same cable built into an insulated wall (A1) — when in doubt about the installation method, always use the more conservative (lower) table value.
Correction factors in practice
- f1 (temperature): in a technical room or attic that is structurally warmer than 30 °C (e.g. near a boiler or under a sloped roof in summer), f1 drops below 1.0 — the cable must then be sized more heavily than the table value suggests.
- f2 (bunching): multiple current-carrying cables laid next to each other in the same conduit or trunking limit each other's heat dissipation. The more cables bunched together, the lower f2 — for large bundles (>9 circuits) f2 can drop below 0.5.
- f3 (thermal insulation): a cable fully wrapped in insulation material (e.g. cavity wall insulation added afterwards) can no longer dissipate its heat — this is a common, often overlooked reduction factor during renovations.
Common mistakes
- Using only the table value without applying correction factors — leads to overloaded cables that become hotter than the insulation permits, resulting in accelerated ageing or fire risk.
- Basing the cable cross-section on the MCB rating instead of the other way around — the correct order is: determine Ib → select Iz (cable) ≥ Ib → select In (circuit breaker) between Ib and Iz, not the reverse.
- Forgetting bunching during renovation — a cable that originally ran freely and is later added into an existing, already filled conduit gets a lower f2 than in the original calculation.
- Not checking voltage drop separately — a cable may be adequate in terms of Iz but still produce excessive voltage drop over long lengths (see voltage-drop calculation in the exam-questions guide).