4.5.2 - Ducting and trunking

Metal and plastic trunkings are very widely used in electrical installations. They must be manufactured to comply with the relevant British Standards, and must be installed so as to ensure that they will not be damaged by water or by corrosion (see {4.2.5}).

Table 4.13 Support spacings for trunking
Typical trunking size	Metal		Insulating
(mm)	Horizontal	Vertical	Horizontal	Vertical
Up to 25 x 25	0.75	1.0	0.5	0.5
Up to 50 x 25	1.25	1.5	0.5	0.5
Up to 50 x 50	1.75	2.0	1.25	1.25
Up to 100 x 50	3.0	3.0	1.75	2.0

If it is considered necessary to provide an additional protective conductor in parallel with steel trunking, it must be run inside the trunking or the presence of steel between the live and protective cables will often result in the reactance of the protective cable being so high that it will have little effect on fault loop impedance. Trunking must be supported as indicated in {Table 4.13}. The table does not apply to special lighting trunking which is provided with strengthened couplers. Where crossing a building expansion joint a suitable flexible joint should be included.

Where trunking or conduit passes through walls or floors the hole cut must be made good after the first fix on the construction site to give the partition the same degree of fire protection it had before the hole was cut. Since it is possible for fire to spread through the interior of the trunking or conduit, fire barriers must be inserted as shown in {Fig 4.18}. An exception is conduit or trunking with a cross-sectional area of less than 710 mm², so that conduits up to 32 mm in diameter and trunking up to 25 mm x 25 mm need not be provided with fire barriers. During installation, temporary fire barriers must be provided so that the integrity of the fire prevention system is always maintained.

Fig 4.18 Provision of fire barriers in ducts and trunking

Since trunking will not be solidly packed with cables (see {4.5.3}) there will be room for air movement. A very long vertical trunking run may thus become extremely hot at the top as air heated by the cables rises; this must be prevented by barriers as shown in {Fig 4.19}. In many cases the trunking will pass through floors as it rises, and the fire stop barriers needed will also act as barriers to rising hot air.

Lighting trunking is being used to a greater extent than previously In many cases, it includes copper conducting bars so that luminaires can be plugged in at any point, especially useful for display lighting.

The considerably improved life, efficiency and colour rendering properties of extra-low voltage tungsten halogen lamps has led to their increasing use, often fed by lighting trunking. It is important here to remember that whilst the voltage of a 12 V lamp is only one twentieth of normal mains potential, the current for the same power inputs will be twenty times greater. Thus, a trunking feeding six 50 W 12 V lamps will need to he rated at 25 A.

Fig 4.19 Heat barriers provided in vertical cable ways