Explanation

Cables should not occupy more than 45% of the inside of a conduit or trunking. This is to prevent damage to cables being drawn in to too confined an enclosure and allow cables to dissipate the heat generated under load.

This is not as straightforward as first appears as:-

1. Trunking is described in terms of two edge lengths whilst cables are described in terms of the CSA of the conductor section of the cable.
2. Cables have an outer layer of insulation surrounding the inner conductor upon which the cable gets its name/size indicator. As such a 1.5mm² single cable has a CSA considerably greater than 1.5mm² A quick read of manufacturer specs reveals a 1.5mm² has an OD of 3.1mm and a CSA of nearer 7.5mm² whilst a 6.0mm² has an OD of 4.75mm and a CSA of nearer 17.7mm². Given that the insulation thickness of a conductor increases very slowly compared to increases in the cables conductor CSA even calculating cable factors is not straightforwards.

Lucky for us, the nice people who brought you the Regs have calculated all these factors for us so we can use a set of lookups that they have precalculated for us.

To calculate whether a set of conductors can safely be enclosed within a trunking the following steps need to be carried out:-

1. Find the qty of each size of cable and multiply by the appropriate factor from Table E5 on pg 150 of OSG.
2. Add the totals for each cable type and size together to obtain the total cable factor.
3. Find which sizes of trunking are suitable for these cables using Table E6 on pg 150 of OSG and looking for one that has a factor greater than or equal to the total obtained in step 2.

Note

1. Each cell is colour coded (red or green) to indicate whether or not the trunking is suitable for the current selection of cables.
2. Note after submitting the form it might appear as though nothing has happened. This is due to the form submitting to itself. Simply click the tabs to view the individual steps.

Tasks

The following steps should be carried out and your observations recorded in the answer sheet which is downloadable via the Download link below. The completed answers sheet should then be emailed as an attachment to Robert.Eyre@Hull-College.ac.uk

Find a suitable sized trunking capable of safely containing the following combinations of cables. For each scenario state the total factors for cables and the factor for that trunking.

1. Fifteen 10mm² BS 6004 stranded cables and fourteen 6mm² BS 7211 stranded cables.
2. Fifteen 10mm² BS 6004 stranded cables and eighteen 6mm²BS 6004 stranded cables and fourteen 6mm² BS 7211 stranded cables.
3. Thirty 2.5mm² BS 6004 stranded cables and eighteen 10mm² BS 7211 stranded cables.
4. Twenty four 25mm² BS 6004 stranded cables.
5. Twenty four 25mm² BS 7211 stranded cables.
6. Twenty four 25mm² BS 6004 stranded cables and twenty four 25mm² BS 7211 stranded cables.
7. Twenty four 25mm² BS 6004 stranded cables and thirty six 10mm² BS 7211 stranded cables and thirty six 25mm² BS 7211 stranded cables.

 

Find the minimum conduit size that can safely contain the following combinations of cables. For each scenario state the total factors for cables and the factor for that conduit.