S 1.21 Sufficient dimensioning of cable trays and channels

Initiation responsibility: Planner, Head of IT, Building Services

Implementation responsibility: Building Services

Sufficiently large cable trays and channels (e.g. underfloor channels, window sill cable ducts, trays, outside cable conduits) must be planned. On one hand, there must be enough space available to accommodate an eventual expansion of the network. On the other hand, minimum distances between the cables may need to be maintained to prevent crosstalk (coupling of signals between cables). In particular, when power and IT cabling are routed together in a common channel or tray, it must be ensured that the routes are separated by a centre rail. Interference with the IT cables can usually be avoided simply by routing the power and IT cables separately.

If it is not possible to install trays and channels with enough reserve space, then it should at least be ensured that there is enough space in the trays and channels to accommodate expansions. If the sizes specified for the openings through walls and ceilings are sufficiently large, then noisy, dirty, and expensive work will be unnecessary later. When fire seals are used that can be installed after the cables are installed, then the openings can be equipped so that protection against smoke and fire is always ensured while enabling the trouble-free installation of additional cables at any time.

It must be noted that in order to obtain effective sealing of wall openings in walls with a fire resistance class, the openings may only be filled up to 60%. If necessary, openings should be made for later expansions, and these openings should be sealed for the time being using soft firestops or fire protection cushions.

It is important that the sizes of the cable trays and channels are always specified in conjunction with the type of cable selected (see S 1.20 Selection of cable types suited in terms of their physical/mechanical properties and S 5.3 Selection of cable types appropriate in terms of communications technology). For example, space can be saved by using a few multiwire cables instead of many small cables. Crosstalk can be prevented through the use of shielded cables or fibre optic cables. This then ensures problem-free operation, even in cable trays and channels with little space.

Review questions:

• Are the cable trays and channels adequately dimensioned with a view to possible expansions or minimum distances to be maintained?
• Are IT cabling and electrical cabling at least separated by a centre rail, however, preferably, routed in separate trays or channels?
• Is the maximum number of cables observed for wall openings in walls with a specific required fire resistance class?