1. Electrical Installation II
Wiring Enclosures & Support (Note: All the mentioned tables in this course refer to, unless otherwise specified, Low Voltage Electrical Installation Handbook, by Johnny C.F. Wong, Edition 2004)
Chapter 4
Electrical Installation II

2. Protection of cables against Electromechanical & Mechanical Stresses
Electromechanical forces may be caused by fault currents
Mechanical forces may be caused by accidental drilling of holes on a wall, the conduits own weight, etc.
Electrical Installation II

3. Electrical Installation II
Conduits
Rigid Steel Conduits
Excellent mechanical protection
Good protection against fire
Can be used as circuit protective conductor (cpc)
Resistant to ultra-violet radiation from sunlight
BS classifies conduits by:
Light gauge and heavy gauge (CoP permits only heavy gauge conduits)
Classes 1, 2, 3 & 4
Electrical Installation II

4. Electrical Installation II
Conduits
Rigid Insulating Conduits / PVC conduits
Ambient temperature is a major consideration
Overall cost saving
Separate circuit protective conductor must be used
Weight reduction compared with steel conduits
Electrical Installation II

5. Electrical Installation II
Conduits
Flexible Conduit (Metallic / PVC)
Used in applications involving constant movement, e.g. connection to a pump
Separate circuit protective conductor must be used
Where exposed to damp situation, it should have metallic pattern with PVC oversheath and IP rating not less than IP54 (Table 4.16)
Electrical Installation II

6. Electrical Installation II
Conduit
Supports:
Horizontally surface-mounted
Vertically surface-mounted
For rigid steel conduit, refer to Table 4.5
For rigid insulating conduit, refer to Table 4.7
Electrical Installation II

7. Electrical Installation II
Conduit
Cable capacity
- Selection principle: Conduit Factor ≥ ∑ (Cable Factors)
Refer to Tables 4.1 to 4.4
Solid and stranded cables
Short run (3m and no bend) and long run (>3m or with bends)
Adaptable boxes to be installed at least every 10m straight run.
Example:
How many 2.5 mm2 1-C PVC cables can be accommodated in a 20 mm conduit 10m long incorporating 2 bends?
Electrical Installation II

8. Electrical Installation II
Conduit
Solution:
From Table 4.4, conduit factor for a 20mm conduit, 10m long incorporating 2 bends is 141.
Let the max. no. of cables that can be drawn in be x, and from Table 4.3, cable factor for 2.5mm2 1/C PVC cable is 30, then
30x ≤  x ≤ 4.7
Therefore, we select x = 4
Electrical Installation II

9. Electrical Installation II
Trunkings
Construction
Steel trunking (screw-on-lid)
Electrical Installation II

10. Electrical Installation II
Trunkings
PVC (or insulating) trunking (clip-on-lid)
Electrical Installation II

11. Electrical Installation II
Trunkings
There are other trunking systems also of PVC steel construction, e.g. 2- and 3- compartment trunkings (telephone and / or data + power):-
Raised floor trunkings (flush with raised floor panels)
Underfloor trunkings
(flush with floor screed)
Skirting trunkings
Electrical Installation II

12. Electrical Installation II
Trunkings
Classification
The higher the Class, the better the mechanical protection, e.g. Class 3 steel trunking  heavy protection on both inside and outside of trunking
Electrical Installation II

13. Electrical Installation II
Trunkings
Can Be Used as Circuit Protective Conductor (cpc)
Steel trunkings may be used as cpc, whereas for PVC trunkings, separate cpc must be used.
Electrical Installation II

14. Electrical Installation II
Trunkings
Bends and supports
Support requirements for steel trunking and insulating trunking refer to Tables 4.13 and 4.15
Electrical Installation II

15. Electrical Installation II
Trunkings
Cable Capacity
- Selection principle: Trunking Factor ≥ ∑ (Cable Factors)
Refer to Tables 4.10, 4.11 and 4.12
For sizes and types of cable and sizes of trunking other than those given in the table, the no. of cables drawn into a trunking should be ≤ 45%
Space factor is the percentage occupancy of space inside the trunking
Example
What is the max. number of 10 mm2 1-C PVC cables that can be accommodated in a 100mm x 50mm trunking? What if a space factor of 0.45 is applied?
Electrical Installation II

16. Electrical Installation II
Cable Supports
Cable Clips
For surface-mounted PVC-insulated PVC-sheathed non-armoured cables  10 mm.sq.
Electrical Installation II

17. Electrical Installation II
Cable Supports
Cable Cleats / Saddles
As above but for > 10 mm.sq. cables and all armoured cables.
Electrical Installation II

18. Electrical Installation II
Cable Supports
Cable Trays and Ladders
Suitable for almost any mounting method but for heavier cables than those accommodated in a trunking.
The mechanical strength of cable ladders is better than cable trays.
Electrical Installation II