7.8Designing for cables in conduit#

Appendix C6 of AS/NZS 3000 provides a guide to the maximum number of cables that can be installed in conduits. This can be determined using the formulae: 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑐𝑎𝑏𝑙𝑒𝑠= 𝑆𝑝𝑎𝑐𝑒 𝑓𝑎𝑐𝑡𝑜𝑟× 𝐼𝑛𝑡𝑒𝑟𝑛𝑎𝑙 𝑐𝑟𝑜𝑠𝑠 𝑠𝑒𝑐𝑡𝑖𝑜𝑛𝑎𝑙 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑒𝑛𝑐𝑙𝑜𝑠𝑢𝑟𝑒 𝐸𝑥𝑡𝑒𝑟𝑛𝑎𝑙 𝑐𝑟𝑜𝑠𝑠 𝑠𝑒𝑐𝑡𝑖𝑜𝑛𝑎𝑙 𝑎𝑟𝑒𝑎𝑠 𝑜𝑓 𝑐𝑎𝑏𝑙𝑒𝑠

Where there are cables of different sizes to be installed in an enclosure, it is often easier to ascertain that the space factor has not been exceeded. Rearranging the formula:

𝑆𝑝𝑎𝑐𝑒 𝑓𝑎𝑐𝑡𝑜𝑟 × 𝐶𝑟𝑜𝑠𝑠 𝑠𝑒𝑐𝑡𝑖𝑜𝑛 𝑜𝑓 𝑒𝑛𝑐𝑙𝑜𝑠𝑢𝑟𝑒 ≥Σ 𝐶𝑟𝑜𝑠𝑠 𝑠𝑒𝑐𝑡𝑖𝑜𝑛𝑎𝑙 𝑎𝑟𝑒𝑎𝑠 𝑜𝑓 𝑎𝑙𝑙 𝑐𝑎𝑏𝑙𝑒𝑠

Space factors are:

The following table provides the typical characteristics for rigid poly vinyl chloride (PVC) conduit to AS2053.2:

Note that conduit is measured outside diameter up to 65 mm diameter and inside diameter for conduits in this table.

Similar typical characteristics can be obtained for galvanised steel pipe to AS 1163 C350 light as follows (from Antec Steel Pipe Catalogue):

The following data are cable sizes for typical Transport and Main Roads cables (from Nexans catalogue):

Note that the dimensions are diameter for circular cables and width x height for flat cables. For example, will eight traffic feeder loop cables, two 36c multicore traffic signal cables and two 4c 16 mm² road lighting cables fit into a single 100 mm diameter PVC conduit?

Using the data from these tables, the following formula must be satisfied:

𝑆𝑝𝑎𝑐𝑒 𝑓𝑎𝑐𝑡𝑜𝑟 × 𝐶𝑟𝑜𝑠𝑠 𝑠𝑒𝑐𝑡𝑖𝑜𝑛 𝑜𝑓 𝑒𝑛𝑐𝑙𝑜𝑠𝑢𝑟𝑒 ≥Σ 𝐶𝑟𝑜𝑠𝑠 𝑠𝑒𝑐𝑡𝑖𝑜𝑛𝑎𝑙 𝑎𝑟𝑒𝑎𝑠 𝑜𝑓 𝑎𝑙𝑙 𝑐𝑎𝑏𝑙𝑒𝑠

Since space factor x cross-section of enclosure is greater than the cross-sectional area of all the cables, the cables do not exceed the maximum fill factor.

Drawings from the original pages

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