Conduit Sizing

Where the cabling of an installation is to be installed in conduit, the conduit system must be designed and installed not only to accommodate the required cables but also to enable them to be easily pulled in once the conduit has been assembled. A cable being installed in conduit could be subjected to excessive pulling force if it jams or if there is too much friction between the cable and the conduit. Such excessive force may damage the cable insulation, or stretch the conductor, thereby reducing its cross-sectional area and leading to ‘hot spots’, or cause one or more strands of the conductor to fracture, resulting in intermittent or permanent discontinuity. The Wiring Regulations require that a wiring system is selected and erected to avoid during installation, use or maintenance, damage to the sheath or insulation of cables and their terminations.

There are two methods in common use for determining the minimum size of conduit required to accommodate a particular group of cables and permit the cables to be pulled in without risk of damage:

• The space factor method

• The unit system method

The space factor method must be used where sheathed cables are drawn into conduit, or have conduit fed over them in order to provide support or mechanical protection or both.

The unit system method applies to all likely situations where unsheathed cables are drawn into conduit.

In addition, where a run of conduit incorporates a bend, the radius of that bend is such as to allow compliance with the minimum bending radii of the cables to be installed in the conduit. In the unit system method and in the space factor method, only mechanical considerations are taken into account. Account must also be taken of the fact that, as the number of circuits in a conduit increases, the current-carrying capacities of the cables must be reduced according to the appropriate grouping factors.

XS Engineering can provide computer generated calculations to specify the minimum conduit size for each group of cables installed in conduit.

TRUNKING SIZING

Where the wiring system selected is cables enclosed in trunking, the trunking system must be designed and installed to accommodate the required number, type and size of cables. There are two methods in common use for determining the minimum size of trunking required to accommodate a particular group of cables:

• The space factor method

• The unit system method

The unit system method is the more common method to use however if the unit system data for the required trunking and cable sizes is not available, it would be necessary to use the space factor method.

Where a run of trunking incorporates a bend, the shape of the bend should be such as to allow compliance with the minimum bending radii of the cables to be installed in the trunking.
In the unit system method, and in the space factor method, only mechanical considerations are taken into account. Account must also be taken of the fact that, the higher the number of circuits in a trunking the more the current-carrying capacities of the cables tend to be reduced in accordance with the appropriate grouping factors in BS 7671.

Determination of the size of trunking to accommodate required cables
In the unit system method, factors based on practical work and experimentation are allocated to standard sizes of trunking and to types and sizes of cable commonly used in trunking systems. The minimum size of trunking for a particular group of cables is determined by summing the factors for the individual cables in the group. The total is then compared with the factor for the various sizes of trunking. The minimum size of trunking for the group of cables is that having the smallest trunking factor equal to or greater than the sum of the factors for the individual cables in the group.

Consideration should be given to the provision of spare capacity for the future addition of cables. However, it should be noted that the addition of cables to an existing wiring system can affect the current-carrying capacity of the existing cables. An increase in the number of circuits grouped together tends to reduce the group correction factor and the current-carrying capacity of the cables. Consequently, if cables are added to a trunking system, the current-carrying capacity of existing cables in the trunking system may become inadequate, requiring their replacement.
Before adding cables to an existing trunking system, either the unit system method or an equivalent means, should be used to determine whether the installed trunking is adequately sized to accommodate the additional cables, and any larger cables needed to replace those that have insufficient current-carrying capacity due to the increase in the number of cables.

Cables installed in trunking (diagrammatic)

XS Engineering can provide computer generated calculations to specify the minimum trunking size for the groups of cables installed.