Fill factor of cable trays and ducts — why 40% isn't simply 40%

The cable sizing & current-carrying capacity guide covers the f2 correction factor for grouping/bundling of cables — the closer cables lie to each other, the poorer the mutual heat dissipation, the lower the permissible current-carrying capacity per cable. This article covers a related but separate requirement: the maximum fill factor of the tray, duct or conduit itself — a mechanical/thermal design limit on the containment system, independent of the electrical f2 calculation per cable.

What fill factor is

The fill factor (space factor) is the ratio between the total cross-sectional area of all the cables together and the available internal cross-section of the tray, duct or conduit in which they lie. An excessive fill factor not only restricts heat dissipation (which is already accounted for via f2 in the current-carrying capacity calculation), but also hampers future expansion, inspection and cable replacement — a tray that is already full at commissioning leaves no room for an additional circuit without installing a new tray.

The common limit values

Under IEC 60364-5-52, the space factor in conduits and ducting is limited to a maximum fill factor of 40% of the internal cross-section — regardless of the number of cables drawn in. For open cable trays and ladder runs (ladder racks, closed trunking), practice typically distinguishes between:

• Power cables: max. 40% fill factor.
• Control/data cables: max. 50% fill factor — these cables generally generate less heat themselves and are less critically affected by mutual bundling.

Note: these percentages are the common practical limits from the IEC 60364-5-52 context for containment systems; the exact limit value and the precise calculation method (including any national deviations) is set out in the full, current standard text and must be verified per project against the chosen cable type and containment system.

Why fill factor and f2 are two separate checks

An installation can have correctly applied the f2 correction (each cable individually sized correctly for the present bundling) and yet still have a cable tray that exceeds the maximum fill factor, simply because too many cables have been laid in too small a tray. Both checks are needed, independently of each other.

Practical measures

• Determine the fill factor at the design stage, not after installation — an undersized cable tray is costly to replace afterwards.
• For new-build or a major renovation, reserve extra capacity by default (for example, designing for 60–70% of the maximum fill factor) so that future expansion is possible without installing a new tray.
• Keep power cables and control/data cables in separate trays or runs wherever possible — this simplifies both the fill-factor and the EMC assessment (see the EMC separation of power/data cables guide).

Practical relevance

When designing or extending a cable containment system (tray, ladder run or conduit), both the f2 grouping correction on the individual cable cross-section and the maximum fill factor of the chosen containment system must be verified — an installation that has only been checked against f2 may still have a physically overfilled cable tray, with limited room for future expansion or replacement.

Common mistakes

1. Treating fill factor and the f2 grouping correction as the same check — both must be verified separately; one check does not cover the other.
2. Designing a cable tray for exactly the current need, without margin for future expansion — the next additional circuit then requires an entirely new tray instead of an added cable in the existing one.
3. Bundling power cables and control cables in the same tray without distinction — the fill-factor limit of 40% (power) versus 50% (control) precisely assumes a degree of separation between the two cable types.

Related

• Cable sizing & current-carrying capacity
• Cable containment systems — trays and ladders
• EMC separation of power/data cables