Installation
a) All the cables described in this publication can be used out doors, but some reservations are necessary concerning cables without a metal sheath for direct burial e.g.
- Unarmoured cables are not recommended for laying direct in ground.
- Cables for laying directly in the ground particularly in sustained wet conditions should have extruded bedding.
b) Sheath damage:
Core should be taken to ensure that the over sheath is not damaged during installation. This is especially important where aluminium armour is used since ingress of moisture could lead to corrosion and loss of earth continuity.
c) Minimum Bending Radius:
Cables should not be bent during installation to a radius smaller than that recommended below wherever possible.
Type of Cables |
Overall Diameter (mm) |
Minimum Internal Radius of Bend |
Circular copper conductor unarmoured |
10 – 25 mm |
4 D |
Above 25 mm |
6 D |
Circular copper conductor armoured |
any |
6 D |
d) Connections:
For P.V.C. compression or soldered may be used.
e) Cable Support Spacing:
The following table gives the recommended spacing of supports for cable covered by this publication. The dimensional range has been extended so as to cover a greater number of cable sizes.
* Stranded Copper or Aluminium Conductor Cables:
Overall Cable Diameter (mm) |
Support Spacing (mm) |
Horizontal |
Vertical |
Up to 15 |
350 |
450 |
15 – 20 |
400 |
550 |
20 - 40 |
450 |
600 |
40 - 60 |
700 |
900 |
60 and above |
1100 |
1300 |
Cable Rating Factors
a) Cables Laid Directly in Ground
The current ratings for cables installed directly in the ground are based on values of soil temperature and soil thermal resistivity.
1- Rating Factors for Ground Temp
Insulation |
Ground Temperature |
15 °C |
20 °C |
25 °C |
30 °C |
35 °C |
40 °C |
45 °C |
PVC |
1.25 |
1.19 |
1.13 |
1.06 |
1.00 |
0.93 |
0.84 |
2- Rating Factors for Variation in Thermal Resistivity of Soil
Size of Cable ( mm 2 ) |
Soil Thermal Resistivity in (°C m/W) |
0.8 |
0.9 |
1.0 |
1.2 |
1.5 |
2.0 |
2.5 |
3.0 |
Single core cables |
|
|
|
|
|
|
|
|
Up to 150 |
1.16 |
1.12 |
1.07 |
1.0 |
0.91 |
0.81 |
0.73 |
0.66 |
Multicore cables |
|
|
|
|
|
|
|
|
Up to 16 |
1.12 |
1.08 |
1.05 |
1.0 |
0.93 |
0.84 |
0.77 |
0.72 |
From 25 to 150 |
1.14 |
1.10 |
1.06 |
1.0 |
0.92 |
0.82 |
0.75 |
0.69 |
3- Rating Factors for Depth of Laying
(to center of cable or trefoil group of cables)
Depth of Laying (M) |
600/1000 Volt Rated Cables Conductor Size |
Up to 50 mm 2 |
70 mm 2 to 300 mm 2 |
Above 300 mm 2 |
0.50 |
1.00 |
1.00 |
1.00 |
0.60 |
0.99 |
0.98 |
0.97 |
0.80 |
0.97 |
0.96 |
0.94 |
1.00 |
0.95 |
0.93 |
0.92 |
1.25 |
0.94 |
0.92 |
0.89 |
1.50 |
0.93 |
0.90 |
0.87 |
1.75 |
0.92 |
0.89 |
0.86 |
2.00 |
0.91 |
0.88 |
0.85 |
2.50 |
0.90 |
0.87 |
0.84 |
3.00 or more |
0.89 |
0.85 |
0.82 |
4- Group Rating Factors for Circuits of Three Single Core Cables in Trefoil or Laid Flat Touching in Horizontal Formation
Number of Circuits
|
|
Spacing of Circuits |
Touching + |
NOT Touching |
Trefoil |
Laid Flat |
0.15m |
0.30 m |
0.45 m |
0.60 m |
600/1000 V |
|
|
|
|
|
|
2 |
0.78 |
0.81 |
0.83 |
0.88 |
0.91 |
0.93 |
3 |
0.66 |
0.70 |
0.73 |
0.79 |
0.84 |
0.87 |
4 |
0.61 |
0.64 |
0.68 |
0.73 |
0.81 |
0.85 |
5 |
0.56 |
0.60 |
0.64 |
0.73 |
0.79 |
0.85 |
6 |
0.53 |
0.57 |
0.61 |
0.71 |
0.78 |
0.82 |
5- Rating Factors for Multicore Cables in Horizontal
Number of Cables in Group
|
|
Spacing |
Touching + |
0.15 m |
0.30 m |
0.45 m |
0.60 m |
600/1000 V |
|
2 |
0.81 |
0.87 |
0.91 |
0.95 |
0.95 |
3 |
0.70 |
0.78 |
0.84 |
0.88 |
0.90 |
4 |
0.63 |
0.74 |
0.81 |
0.86 |
0.89 |
5 |
0.59 |
0.70 |
0.78 |
0.84 |
0.87 |
6 |
0.55 |
0.68 |
0.77 |
0.83 |
0.87 |
b) Cables Laid in Ducts:
The term ducts simply applies to single way earthenware fibre or ferrous pipe
Overall Cable Diameter (mm) |
Duct |
Inside Diameter (mm) |
Outside Diameter (mm) |
Up to and including 65 |
100 |
130 |
Above 65 up to & including 90 |
125 |
160 |
1- Rating Factors for Ground Temp
like factors in a – 1 (cables laid directly in ground)
2- Rating Factors for Variation in Thermal Resistivity of Soil
(Average Values)
Size of Cable (mm 2) |
Soil Thermal Resistivity in (°C m/W) |
0.8 |
0.9 |
1.0 |
1.2 |
1.5 |
2.0 |
2.5 |
3.0 |
Single core cables |
|
Up to 150 |
1.10 |
1.07 |
1.04 |
1.0 |
0.94 |
0.86 |
0.80 |
0.76 |
Multicore cables |
|
Up to 16 |
1.04 |
1.03 |
1.02 |
1.0 |
0.97 |
0.92 |
0.88 |
0.86 |
From 25 to 150 |
1.06 |
1.04 |
1.03 |
1.0 |
0.95 |
0.90 |
0.85 |
0.81 |
3- Rating Factors for Depth of Laying (to center of duct of trefoil group of ducts)
Depth of Laying (m) |
600/1000 Volt |
Single Core |
Multicore |
0.50 |
1.00 |
1.00 |
0.60 |
0.98 |
0.99 |
0.80 |
0.95 |
0.98 |
1.00 |
0.93 |
0.96 |
1.25 |
0.91 |
0.95 |
1.50 |
0.89 |
0.94 |
1.75 |
0.88 |
0.94 |
2.00 |
0.87 |
0.93 |
2.50 |
0.86 |
0.92 |
3.0 or more |
0.85 |
0.91 |
4- Rating Factors for Depth of Laying (to center of duct of trefoil group of ducts)
Number of Circuits
|
 Spacing 
|
Spacing |
Touching |
0.45 m |
0.60 m |
600/1000 V |
|
2 |
0.87 |
0.91 |
0.93 |
3 |
0.78 |
0.84 |
0.87 |
4 |
0.74 |
0.81 |
0.85 |
5 |
0.70 |
0.79 |
0.83 |
6 |
0.69 |
0.78 |
0.82 |
- Rating Factors for Multicore Cables in Single Way ducts, Horizontal Formation (average values)
Number of Circuits
|
  Spacing  
|
Spacing |
Touching |
0.30 m |
0.45 m |
0.60 m |
600/1000 V |
|
2 |
0.90 |
0.93 |
0.95 |
0.96 |
3 |
0.83 |
0.88 |
0.91 |
0.93 |
4 |
0.79 |
0.85 |
0.89 |
0.92 |
5 |
0.75 |
0.83 |
0.88 |
0.91 |
6 |
0.73 |
0.82 |
0.87 |
0.90 |
c) Cables Installed in Air:
It should be noted that all ratings for cables run in free air have been on the assumption that they are shielded from the direct rays of the sun without restriction of ventilation.
* Rating Factors for Single Core Cable in Air:
Insulation |
Air Temperature |
25 °C |
30 °C |
35 °C |
40 °C |
45 °C |
50 °C |
55 °C |
PVC |
1.22 |
1.15 |
1.08 |
1.00 |
0.91 |
0.80 |
0.70 |
* 1- Single Core Cables:
- Two single core cables are installed one above the other, fixed to the vertical surface of wall or open cable trench. The distance between the wall and the surface of the cable is not less than 20 mm.
- Cables are installed at a distance between centers of twice the overall diameter of the cable i.e. 2D where D is the overall diameter of the cable.
- Three single core cables are installed in trefoil formation, fixed to the vertical surface of a wall or open cable trench. The cable touching through out and the distance between the wall and the surface of the cable is not less than 20 mm.
- The cable is assumed to be remote from iron, steel or ferro-concrete, other than the cable supports. Single core armoured cables to be electrically bonded at each end of the run.
* 2- Multicore Cables:
Cables of all types other than single core cable are installed singly and fixed to the vertical surface of a wall or open cable trench. The distance between the surface of the cables and the wall is not less than 20 mm in every instance.
If it is necessary for the cables to be installed at distance less than those described above, clipped direct to surface, regulations should be employed.
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