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 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.