The experts at NICEIC look at several methods that may be used to determine an appropriate diameter of conduit for the installation of conductors, taking account of the number of conductors to be installed and the most onerous section of the conduit system into which they will be drawn.
Before we start, it should be noted that cable grouping factors are not considered in this article.
Regulation 522.8.1 requires that wiring systems shall be selected and erected to avoid damage to the cables and their terminations during installation, use or maintenance. BS 7671 does not provide guidance on how compliance with this regulation may be achieved.
When considering a conduit system, there are generally two accepted methods for determining the diameter of the conduit to be used:
- The unit system method, and
- The space factor method.
There is a tendency amongst some electrical contractors to select the conduit size based on previous experience. However, this often results in the selection of conduits of insufficient diameter such that it becomes difficult to draw the cables in, increasing the likelihood of damage to the cables.
Method 1: The use of conduit and cable factors (unit system method)
This well-established method for sizing conduit is based on practical tests that take into account both the distance between drawing-in points and the number of bends in a particular run, both of which hinder the ability to draw the cables into the conduit.
This method involves the designer using standard tables to obtain factors for both conduit and conductors to ensure that, for the particular conduit installation, the conduit factor is larger than or equal to the sum of the factors for the cables to be installed, with the intention that single-core cables can be pulled in without the use of undue force.
For short, straight runs of conduit, the friction between cable and conduit is less than for longer runs or for runs containing bends. As a result, separate tables are used for conduits arranged in:
- straight runs not exceeding 3 m in length, or
- straight runs exceeding 3 m in length, or runs of any length incorporating bends or sets.
In the examples shown in this article, use is made of data given in the tables listed in Appendix A of IET Guidance Note 1 (GN1). Reference to this publication may aid the reader when working through the given examples.
Example 1
Four 4 mm² single-core PVC conductors are to be drawn into a 3 m length of conduit with no bends. Using the per unit method, determine the minimum size conduit that should be used.
Answer 1
From Table A1 of GN1: For 4 mm² conductors the factor is 58.
So, for 4 x 4 mm² conductors the total cable factor is 4 x 58 = 232
From Table A2 of GN1: For a straight 3 m length the smallest conduit with a factor equal or larger than 232 is 16 mm, which has a conduit factor of 290.
Example 2
Four 4 mm² single core PVC conductors are to be drawn into the conduit system shown in Fig 1.
Using the per unit method, determine the size of conduit that should be used.
Note: A double set can be treated as being equivalent to a single right angle bend (see A1 of GN1).
Answer 2
From Table A3 of GN1: For 4 mm² conductors the factor is 43.
So, for 4 x 4 mm² conductors the total cable factor is 4 x 43 = 172
From Table A4 of GN1: For Section 1, having one bend and one double set (effectively a conduit run having two right angle bends) in 7 m, the smallest conduit with a factor equal or larger than 172 is 25mm, having a conduit factor of 311.
For Section 2, having three bends in 5 m, the smallest conduit with a factor equal or larger than 172 is 25 mm, having a conduit factor of 260.
Consequently, for the proposed system, the smallest size conduit that should be used in either section is 25 mm.
It should be recognised that the position of the drawing-in points may impact the size of conduit used. Altering the location of the drawing-in point or adding an extra drawing-in point may provide the designer with the option of reducing the size of conduit used.
Comparing the answers given in examples 1 and 2 illustrates that, for a short straight run, a conduit with a smaller diameter may be used due to the friction between conduit and cable being less than that for the longer, more complex shape.
Method 2: The use of a space factor
A space factor may be used as an alternative method for determining the minimum size of conduit for those types not covered by the tables in Appendix A of GN1 and Appendix E of the OSG.
The current edition of GN1 recommends that the area occupied by the cables should not be more than 35% of the internal area of the conduit (see Fig 2).
In order to use this method, the following information, typically provided by the manufacturer is required:
- The overall diameter of the cable, including its insulation, and
- The internal diameter of commonly available sizes of conduit.
Where a conduit contains cables of the same size, the minimum internal diameter of the conduit is given by:
Where:
n = number of cables in conduit
d = diameter of the cables in the conduit (including insulation)
0.35 = conduit space factor
However, where the conduit contains cables of different sizes, the minimum internal diameter of the conduit is given by:
Where d1 and n1 are the diameter and number of the first set of cables and d2 and n2 are the diameter and number of the second set of cables.
Example 3
Determine the minimum size steel conduit that will accommodate four 4.0 mm2 single-core cables with an overall diameter of 4.8 mm and two 2.5 mm2 single-core cables with an overall diameter of 4.2 mm. It can be assumed that the heavy duty conduit used has a nominal wall thickness of 1.6 mm.
Answer 3
Using the formula to determine the minimum internal diameter of the conduit:
Taking the nominal wall thickness of the conduit as 1.6 mm would give a minimum internal diameter of 19.1 + (1.6 x 2) = 22.3 mm.
Therefore, the minimum standard size of conduit that complies with the 35 % space factor is 25 mm.
Summary
When conduit is used it is necessary to determine the maximum number of cables that can be drawn in to ensure there is sufficient free space remaining to prevent overheating.
There are two methods of achieving this; one uses the unit system method, which relies on using cable factor tables, the second involves verifying that the area occupied by the cables does not exceed 35 % of the internal area of the conduit.
To get more details about NICEIC registration, click here
