Cable sizing Example

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TECHNICAL
STUDENTTOPIC

MINIMISING VOLTAGE DIFFERENCES

Short guide to working out cable sizes
In the first of two articles, Bill Allan looks at the correct method in cross-sectional work, a job that requires a head for calculations and a steady hand Ask any electrician about the required cross-sectional areas of cables for standard circuits and the answer you receive will probably be along the lines of: “1mm2 or 1.5 mm2 for lighting circuits and 2.5 mm2 for socket-outlet circuits.”
Indeed these are the commonly used rule of thumb sizes.
However, those who undertake electrical installation work need to understand the procedure for selecting the correct cross-sectional area of a cable for a particular use.
It is the intention of this article to explain simply how to select the correct cross-sectional area of cables with particular single-phase loads in mind.
I’ll refer to the tables in Appendix 4 of BS 7671, although these tables are reproduced in Appendix 6 of the IEE On Site Guide. We’ll assume that the overcurrent protective device will be providing fault current and overload current which is the normal situation. Calculating the right size
There are five steps to calculating the right size of cable for a particular load. These are as follows:
1 Calculate the design current (Ib). This is the normal current drawn by the load. It is usually determined as follows:
Watts
Ib =
Volts
2 Select the type and current rating of the overcurrent device (In).
3 Apply the relevant correction factors to obtain the tabulated current (It).
Correction factors are applied to situations which inhibit a cable from dissipating its heat caused by the normal flow of current through it. Therefore, the following correction factors, if applicable, are applied: Ambient temperature, Ca
This factor is obtained from Table 4C1 (or Table 4C2 if a rewireable fuse to BS 3036 is used) in Appendix 4 of
BS 7671.

Fig 1

Grouping
Factor
(0.65)
Consumer
unit

Thermal insulation Ambient
temperature

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