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. Conduits are described in terms of their diameter whilst cables are described in terms of the CSA of the conductor section of the cable. This means that five 4mm² would not fill a 20mm conduit.
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 conduit 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 E3 on pg 147 of OSG.
2. Add the totals for each cable type and size together to obtain the total cable factor.
3. Find the appropriate conduit for these cables by finding one in Table E4 on pg 148 of OSG that has a factor greater than or equal to the total obtained in step 2.

You will see that the tables are split horizontally first into the no of bends (0 to 4) and within each of these groups into the 4 smaller conduit sizes (16mm to 32mm.

Within each column for a given number of bends and conduit size the factor decreases as the length of run increases and similarly for a given length of the same conduit the factor decreases as the number of bends increases.

Note

1. Each cell is colour coded (red or green) to indicate whether or not the conduit 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 the smallest size conduit capable of safely containing the following combinations of cables. For each scenario state the total factors for cables and the factor for that conduit.

1. Six 1.0mm² stranded cables and three 6mm² stranded cables in a 5m short straight run.
2. Nine 2.5mm² stranded cables in a 6m short straight run.
3. Nine 2.5mm² stranded cables in a 7m short straight run.
4. Nine 4mm² stranded cables in a 7m short straight run.
5. Nine 4mm² stranded cables in a 7m run containing 1 bend.
6. Five 6mm² stranded cables in a 1.5m run containing 1 bend.
7. Three 10mm² stranded cables in a 7m run containing 1 bend.
8. Six 2.5mm² stranded cables in a 6m run containing 2 bends.
9. Six 2.5mm² stranded cables in a 6m run containing 3 bends.
10. Three 2.5mm² stranded cables and three 6mm² stranded cables in a 6m run containing 1 bend.
11. Three 2.5mm² stranded cables and three 6mm² stranded cables in a 5m run containing 2 bends.
12. Three 2.5mm² stranded cables and three 6mm² stranded cables in a 4.5m run containing 3 bends.
13. Six 2.5mm² stranded cables and six 6mm² stranded cables in a 5m run containing 3 bends.

 

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

1. Six 2.5mm² stranded cables in a 20mm conduit run containing 2 bends.
2. Six 2.5mm² stranded cables in a 20mm conduit run containing 3 bends.
3. Six 2.5mm² stranded cables in a 20mm conduit run containing 4 bends.
4. Five 4mm² stranded cables and three 6mm² stranded cables in a 25mm conduit run containing no bends.
5. Five 4mm² stranded cables and three 6mm² stranded cables in a 25mm conduit run containing 1 bend.
6. Five 4mm² stranded cables and three 6mm² stranded cables in a 25mm conduit run containing 2 bends.
7. Five 4mm² stranded cables and three 6mm² stranded cables in a 25mm conduit run containing 3 bends.
8. Five 4mm² stranded cables and three 6mm² stranded cables in a 32mm conduit run containing 4 bends.

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.

1. Nine 2.5mm² stranded cables in a 5m run containing no bends.
2. Nine 2.5mm² stranded cables in a 5m run containing 1 bend.
3. Nine 2.5mm² stranded cables in a 5m run containing 2 bends.
4. Nine 2.5mm² stranded cables in a 5m run containing 3 bends.
5. Six 6mm² stranded cables and six 4mm² stranded cables in a 4.5m run containing 0 bends.
6. Six 6mm² stranded cables and six 4mm² stranded cables in a 4.5m run containing 1 bend.
7. Six 6mm² stranded cables and six 4mm² stranded cables in a 4.5m run containing 2 bends.
8. Six 6mm² stranded cables and six 4mm² stranded cables in a 4.5m run containing 3 bends.