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CABLE MANAGEMENT
PERIMETER AND
DISTRIBUTION
Egatube Conduit
Technical
Conduit and fittings
Cable Capacities of Conduit
17th Edition of the IET Wiring Regulations Selection
and Erection of Wiring Systems
The 17th Edition of the Wiring Regulations, Chapter 52 ‘Selection
and Erection of Wiring Systems’, describes methods to provide a
means of compliance with Regulation 522-8.
The number of cables drawn into or laid in an enclosure of a
wiring system shall be such that no damage is caused to the
cables or to the enclosure during their installation.
The method employs a unit system, each cable size being
allocated a factor. The sum of all factors for the cables intended
to be run in the same enclosure is compared against the factors
given for conduit in order to determine the size of the conduit
necessary to accommodate those cables.
Types of run
It has been found necessary, for conduit, to distinguish between:-
1.
Straight runs not exceeding 3 metres in length, and
2.
Straight runs exceeding 3 metres, or runs of any length
incorporating bends or sets
The term “bend” signifies a British Standard 90° bend, and one
double set is equivalent to one bend.
For case 1, each conduit size is represented by only one factor.
For case 2, each conduit size has a variable factor which is
dependent on the length of run and the number of bends or sets.
For a particular size of cable the factor allocated to it for case 1 is
not the same as for case 2.
Variable factors
A number of variable factors affect any attempt to arrive at a
standard method of assessing the capacity of conduit.
Some of these are:
l
reasonable care (of drawing-in)
l
acceptable use of the space available
l
tolerance in cable sizes
l
tolerance in conduit
The following tables can only give guidance as to the maximum
number of cables which should be drawn in. The sizes should
ensure an easy pull with low risk of damage to the cables.
Only the ease of drawing-in is taken into account. The electrical
effects of grouping is not. As the number of circuits increases the
current carrying capacity of the cable decreases. Cable sizes
have to be increased with consequent increase in cost of cable
and conduit.
Single-core PVC insulated cables in straight runs of conduit not
exceeding 3 metres in length.
l
For each cable it is intended to use, obtain the term from
Table A1.
l
Add the cable terms together and compare the total with the
conduit terms given in Table A2.
l
The conduit size which will satisfactorily accommodate the
cables is that size having a factor equal to or exceeding the
sum of the cable factors
Single-core PVC insulated cables in straight runs of conduit
exceeding 3 metres in length or in runs of any length incorporating
bends or sets.
l
For each cable it is intended to use, obtain the appropriate
terms from Table A3.
l
Add all the cable terms so obtained and compare with the
conduit terms given in Table A4, taking into account the length
of run it is intended to use and the number of bends and sets in
that run.
l
The conduit size which will satisfactorily accommodate the
cables is that size having a terms equal to or exceeding the
sum of the cable terms.