Grouping factor Cg — why bundled cables may carry less than separated cables

The guide on cable cross-section & current-carrying capacity already mentions the correction factor f2 for grouping as part of the formula Iz = Itabel × f1 × f2 × f3. This article goes deeper into how that grouping factor (also referred to as Cg) actually works, and why the way cables are laid next to each other — bundled and touching, or spaced in a single layer — makes a material difference to the permitted current.

Why grouping reduces current-carrying capacity

A cable dissipates heat into its surroundings. If a cable is on its own, that heat can escape in every direction. If several current-carrying circuits lie right next to or against each other, each cable also heats the air or structure around its neighbour — the combined heat output is higher than for a single cable, so each cable in the group reaches a higher conductor temperature at the same current. The grouping factor Cg compensates for this by reducing the permitted current per cable as more circuits are grouped together.

Two different grouping arrangements

IEC 60364-5-52 annex B distinguishes, among others, two tables that lead to clearly different factors:

Grouping arrangement	Table	Characteristic	Effect on Cg
Bundled/touching (e.g. in conduit, trunking or a cable trough, cables against each other)	B.52.17	Cables touch, no air gap between them	Strongest reduction — for around 6 touching circuits Cg is roughly in the order of 0.55-0.6
Single layer, spaced (e.g. separated on a cable tray or ladder, each cable surrounded by air)	B.52.20	Cables do not touch, air gap between each cable for circulation	Noticeably less severe — for the same number of circuits Cg is markedly higher than in the touching arrangement

The practical consequence: the same six circuits allow a noticeably higher permitted current per cable when laid separately on a cable tray than when bundled or bunched together in one conduit — purely because of the difference in air circulation between them.

Practical relevance

When designing a cable route with multiple circuits (for example a distribution board with several outgoing circuits that share part of their route), it is not only the number of circuits but also the physical arrangement — bundled/touching or spaced in a single layer — that determines which grouping factor applies. In a tight cable tray where bundling the cables together seems the easiest fit, it can be worth deliberately spacing the cables apart (or choosing a wider tray) to qualify for a less severe grouping factor and so get away with a smaller cross-section.

Common mistakes

Applying the grouping factor as if all arrangements were equal — bundled/touching and single-layer-spaced give a materially different Cg value for the same number of circuits.
Not accounting for circuits added later in the grouping factor — when extra cables are added to the same trough or conduit after commissioning, the grouping factor changes for every cable in that group, including those already in place.
Ignoring spare circuits that are not (yet) connected when determining the number of grouped circuits — if an empty cable will still exchange heat with its neighbours once it is put into use later, this must be accounted for in the sizing beforehand.
Confusing the grouping factor with the reference method — the reference method (A1/B1/C/E) sets the base table value for the installation method, the grouping factor is a separate, multiplying correction on top of that.

Grouping factor Cg — why bundled cables may carry less than separated cables

Why grouping reduces current-carrying capacity

Two different grouping arrangements

Practical relevance

Common mistakes

Related

Further reading