Download Enclosure

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
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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.
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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
BS4568-1 classifies conduits by:
Light gauge and heavy gauge (CoP permits only heavy
gauge conduits)
 Classes 1, 2, 3 & 4
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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
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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)
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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
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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?
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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 ≤ 141  x ≤ 4.7
Therefore, we select x = 4
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Trunkings

Construction
– Steel trunking (screw-on-lid)
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Trunkings

PVC (or insulating) trunking (clip-on-lid)
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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
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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
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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.
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Trunkings

Bends and supports
– Support requirements for steel trunking and insulating
trunking refer to Tables 4.13 and 4.15
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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?
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Cable Supports

Cable Clips
– For surface-mounted PVC-insulated PVC-sheathed nonarmoured cables  10 mm.sq.
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Cable Supports

Cable Cleats / Saddles
– As above but for > 10 mm.sq. cables and all armoured cables.
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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.
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