to the 16th Edition IEE Regulations
   
   
   
 
 

chapter 5
Earthing

chapter 6
Circuits

Cables, conduits and trunking
  4.1 - Cable insulation materials 4.4 - Cable supports, joints and terminations
  4.2 - Cables 4.5 - Cable enclosures
  4.3 - Cable choice 4.6 - Conductor and cable identification


4.5.3 - Cable capacity of conduits and trunking

Not oflly must it be possible to draw cables into completed conduit and trunking systems, but neither the cables nor their enclosures must be damaged in the process. If too many cables are packed into the space available, there will be a greater increase in temperature during operation than if they were given more space. It is important to appreciate that grouping factors (see {4.3.5}) still apply to cables enclosed in conduit or trunking.

To calculate the number of cables which may be drawn into a conduit or trunkmg, we make use of four tables ({Tables 4.14 to 4.17}). For situations not covered by these tables, the requirement is that a space factor of 45% must not be exceeded. This means that not more than 45% of the space within the conduit or trunkmg must be occupied by cables, and involves calculating the cross-sectional area of each cable, including its insulation, for which the outside diameter must be known. The cable factors for cables with thermosetting insulation are higher than those for pvc insulation when the cables are installed in trunking, but the two are the same when drawn into conduit (see {Table 4.14})

Fig 4.20 Low voltage luminaire on lighting trunking

The figures in {Table 4.14} may be high when applied to some types of plastic trunking due to the large size of the internal lid fixing clips.

To use the ({Tables 4.14 to 4.17}). the cable factors for all the conductors must be added. The conduit or trunking selected must have a factor (otherwise called 'term') at least as large as this number,

Example 4.9
The following single-core p.v.c. insulated cables are to be run in a conduit 6 m long with a double set: 8 x 1,4 x 2.5 and 2 x 6 mm˛. Choose a suitable size.

Table 4.14 - Cable factors (terms) for conduit and trunking
Type of conductor Conductor c.s.a. (mm˛) Factor for conduit Factor for trunking pvc insulation Factor for trunking thermosetting insulation
Solid
1.0
16
3.6
3.8
Solid

1.5

22
8.0
8.6
Stranded
1.5
22
8.6
9.1
Solid
2.5
30
11.9
11.9
Stranded
2.5
30
12.6
13.9
Stranded
4.0
43
16.6
18.1
Stranded
6.0
58
21.2
22.9
Stranded
10.0
105
35.3
36.3
Stranded
16.0
145
47.8
50.3
Stranded
25.0
217
73.9
75.4



Table 4.15 - Cable factors (terms) for straight runs up to 3m.
Type of conductor Conductor c.s.a. (mm˛) Cable factor
Solid
1.0
22
Solid
1.5
27
Solid
2.5
39
Stranded
1.5
31
Stranded
2.5
43
Stranded
4.0
58
Stranded
6.0
88
Stranded
10.0
146



Table 4.16 - Conduit factors (terms)
Length of run between boxes (m)
 
1
2
3
4
5
6
8
10
Conduit, straight
-
16mm
290
290
290
171
171
167
158
150
20mm
460
460
460
286
278
270
256
244
25mm
800
800
800
514
500
487
463
442
32mm
1400
1400
1400
900
878
857
818
783
Conduit, one bend
-
16mm
188
177
167
158
150
143
130
120
20mm
303
286
270
256
244
233
213
196
25mm
543
514
487
463
442
422
388
258
32mm
947
900
857
818
783
750
692
643
Conduit, two bends
-
16mm
177
158
143
130
120
111
97
86
20mm
286
256
233
213
196
182
159
141
25mm
514
463
422
388
358
333
292
260
32mm
900
818
750
692
643
600
529
474
For 38mm conduit use the 32mm factor x 1.4.
For 50mm conduit use the 32mm factor x 2.6.
For 63mm conduit use the 32mm factor x 4.2.



Table 4.17 Trunking factors (terms)
Dimensions of trunking (mm x mm)
Factor
37.5 x 50
767
50 x 50
1037
25 x 75
738
37.5 x 75
1146
50 x 75
1555
75 x 75
2371
25 x 100
993
37.5 x 100
1542
50 x 100
2091
75 x 100
3189
100 x 100
4252

Consulting {Table 4.14} gives the following cable factors:

16 for 1 mm˛, 30 for 2.5 mm˛ and 58 for 6 mm˛
Total cable factor is then       (8 x 16) + (4 x 30) + (2 x 58)
= 128 + 120 + 116  = 364

The terrn "bend" means a right angle bend or a double set.

            {Table 4.16} gives a conduit factor for 20 mm conduit 6 m long with a double set as 233, which is less than 364 and thus too small. The next size has a conduit factor of 422 which will be acceptable since it is larger than 364.

            The correct conduit size is 25 mm diameter.

Example 4.10
The first conduit from a distribution board will be straight and 10 m long. It is to enclose 4 x 10 mm˛ and 8 x 4 mm˛ cables. Calculate a suitable size.

From {Table 4.14}, cable factors are 105 and 43 respectively. Total cable factor:

             =(4 x 105) + (8 x 43) = 420 + 344 = 764

From ({Table 4.15}, a 10 m long straight 25 mm conduit has a factor of 442. This is too small, so the next size, with a factor of 783 must be used.

            The correct conduit size is 32 mm diameter.

Example 4.11
A 1.5 m straight length of conduit from a consumer's unit encloses ten 1.5 mm˛ and four 2.5 mm˛ solid conductor p.v.c. insulated cables. Calculate a suitable con duit size.

From ({Table 4.15} (which is for short straight runs of conduit) total cable factor will be:

            = (10 x 27) + (4 x 39) = 426

Table 4.16 shows that 20 mm diameter conduit with a factor of 460 will be necessary.

Example 4.12
A length of trunking is to carry eighteen 10 mm˛, sixteen 6 mm˛, twelve 4 mm˛, and ten 2.5 mm˛ stranded single p.v.c. insulated cables. Calculate a suitable trunking size.

The total cable factor for trunking is calculated with data from {Table 4.14}.

18 x 10mm˛ at 36.3 = 18 x 36.3 =  653.4
16 x 6mm˛ at 22.9 = 16 x 22.9 =  366.4
12 x 4mm˛ at 15.2 = 12 x 15.2 =  182.4
10 x 2.5mm˛ at 11.4 = l0 x ll.4 =  114.0
-
Total cable factor = 1316.2

From the trunking factor {Table 4.17}, two standard trunking sizes have factors slightly greater than the cable factor, and either could be used .
They are 50 mm x 75 mm at 1555, and 37.5 mm x 100 mm at 1542.

 

 

Return to top of page

Extracted from The Electricians Guide Fifth Edition
by John Whitfield
Published by EPA Press Click Here to order your Copy

Click here for list of abbreviations