Group rated current replaces the simultaneity factor — sizing the main cable and distribution board

The cable cross-section & current-carrying capacity guide covers how to size the cross-section of a single circuit based on the current that circuit must carry. This article covers a question that comes before that: how much of the summed rated current of all connected final circuits a main cable or distribution board actually needs to carry simultaneously.

The old concept: simultaneity factor

Not every final circuit in an installation draws its full rated current at the same moment — a kitchen circuit, a socket-outlet circuit and a lighting circuit rarely peak simultaneously. The simultaneity factor (also called the diversity factor) was the traditional way to account for this when sizing a main cable or distribution board: a number between 0 and 1 by which the summed rated current of all circuits was multiplied to arrive at a more realistic design current. For building installations, a factor around 0.6 (60%) was traditionally common, and for industrial installations closer to 0.8 (80%).

The new concept: group rated current (Ing)

NEN-EN-IEC 61439-1 and -2 (3rd edition, parts 1 and 2 published in May 2021) replace the simultaneity-factor calculation step with a value that must be stated directly: the group rated current (Ing, "group rated current"). Instead of the installer having to estimate and apply a simultaneity factor themselves, the switchgear assembly manufacturer must now specify the group rated current per circuit directly as a current value, calculated as:

Ing = simultaneity factor × Inc

(where Inc is the rated current of the individual final circuit). The underlying principle — not every circuit draws its full current at the same time — stays the same; what changes is that the manufacturer states a concrete current value per circuit, instead of the installer applying a generic factor themselves.

Why the old 60% rule of thumb no longer holds

The reason for this change lies in the energy transition:

NEN 1010 §722 (electric vehicle charging installations) requires a simultaneity factor of 1 (100%) for most situations, unless a validated load-management system justifies a lower, demonstrable simultaneity — see the dynamic load balancing for charging plazas guide for what that load management looks like in practice.
PV installations likewise default to a simultaneity factor of 100% under the new NEN-EN-IEC 61439-2.
The combination of charge points, heat pumps and PV inverters in a single home or commercial building pushes the actual simultaneous load of a modern installation well above the level for which the traditional 60% rule of thumb (established in practice without these loads) was ever set.

Note: this is not a tightening of the rules "just to be safe" — it is a recognition that the old 60% assumption simply no longer matches the actual simultaneous current draw of a growing share of today's installations (with charge points, heat pumps or PV).

Practical relevance

When having a distribution board (consumer unit, main board) assembled or specified, it must be explicitly asked what group rated current (Ing) the manufacturer applies for the specific combination of circuits involved — not fall back from memory on the old 60% rule of thumb, especially once charge points, a heat pump or a PV inverter are part of the installation. For those loads, the full (100%) share generally applies, not a reduced share.

Common mistakes

Applying the traditional 60% rule of thumb to an installation with charge points, a heat pump or PV — under the current standard these loads generally require a simultaneity factor of 100%, not 60%.
Estimating a simultaneity factor yourself instead of requesting the stated group rated current (Ing) from the panel builder/manufacturer — the standard explicitly shifts this responsibility to the switchgear assembly manufacturer.
Treating "group rated current" and "simultaneity factor" as two separate, stackable corrections — Ing is the direct successor to (and replaces) the simultaneity-factor calculation step, not an additional correction on top of it.