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 crosssectional 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 singlecore 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 mm². Choose a suitable size.
Table 4.14  Cable factors (terms)
for conduit and trunking

Type of conductor 
Conductor c.s.a. (mm²) 
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. (mm²) 
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 mm², 30 for 2.5 mm² and
58 for 6 mm²

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 mm² and
8 x 4 mm² 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 mm² and four 2.5 mm² 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 mm²,
sixteen 6 mm², twelve 4 mm², and ten 2.5 mm² 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 10mm² at 36.3 
= 18 x 36.3 
=
653.4 
16 x 6mm² at 22.9 
= 16 x 22.9 
= 366.4 
12 x 4mm² at 15.2 
= 12 x 15.2 
= 182.4 
10 x 2.5mm² 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.