Electrical Installation Calculations
New sections on buried cables that take into account soil thermal conductivity, trenches and grouping, allowing readers to carry out accurate cables sizing
Information and examples of steel wired armour cables, new to this edition. This includes sufficiency during short circuits and, for cables with externally run CPCs, gives unique fault conditions.
Covers calculations of cross-sectional areas of circuit live conductors
Earth fault loop impedances
Protective conductor cross-sectional areas and short circuit conditions
Short circuit protection.
The last chapter combines all of the calculations of the previous chapters to enable the reader to complete an accurate design of an installation circuit under all conditions. A unique tool for detailed electrical installation trade, Electrical Installation Calculations, Fourth Edition is invaluable to electricians, electrical designers, installers, technicians, contractors, and plant engineers. Senior electrical engineering students and technical colleges, junior engineers, and contracts managers will also find this text useful.
Electrical Installation Calculations
Calculation of the cross-sectional areas of circuit live conductors
The first stage in designing an installation after having carried out the assessment of general characteristics demanded in Part 3 of BS 7671 is the choice of the type of cable and the method of installation of that cable for each circuit. In some cases these choices are closely interrelated, e.g. non-sheathed cables are required to be enclosed in conduit, duct, ducting or trunking (Regulation 521.10.1).
Where there are several options open to the installation designer from purely technical considerations, the final choice will depend on commercial aspects or the designer's (or client's) personal preferences. Here it is assumed that the designer, after having taken into account the relevant external influences to which the circuit concerned is expected to be subjected, has already decided on the type of cable and the installation method to use. The appropriate table of current-carrying capacity, in Appendix 4 of BS 7671, and the appropriate column within that table are therefore known.
To determine the minimum conductor cross-sectional area of the live conductors of a particular circuit that can be tolerated the designer must:
(a) Establish what is the expected ambient temperature (ta °C). This gives the relevant value of Ca. Note that more than one value of ta°C may be encountered in some installations. Where there is more than one value the designer may opt to base all his calculations on the highest value or, alternatively, base his calculations for a particular part of the installation on the value of ta°C pertinent to that part.
(b) Decide whether the circuit is to be run singly or be bunched or grouped with other circuits and, if the latter, how many other circuits. The decision taken gives the relevant value of Cg.
(c) Decide whether the circuit is likely to be totally surrounded by thermally insulating material for any part of its length (Regulation 523.7). If the length surrounded by thermal insulation is more than 0.5 m, Ci is taken to be 0.5. For shorter lengths surroundedby thermal insulation the factors given in Table 52.2 are applied.
(d) Determine the design current (Ib) of the circuit, taking into account diversity where appropriate (Regulation 311.1), and any special characteristics of the load, e.g. motors subject to frequent stopping and starting (Regulation 552.1.1).
(e) Choose the type and nominal current rating (In) of the associated overcurrent protective device. For all cases In must be equal to or greater than Ib. Remember that overcurrent protective devices must comply with Chapter 43 of BS 7671 as regards their breaking capacity, but for the present let it be assumed the chosen devices do so comply.
(f) Establish whether it is intended the overcurrent protective device is to give:
(i) overload protection only, or
(ii) short circuit protection only, or
(iii) overload and short circuit protection.
The intended function of the overcurrent protective device not only determines whether Ib or In is used as the basis for calculating the minimum cross-sectional area of the live conductors, but also influences the value of Ca that is to be used in the calculations.
(g) Establish the maximum voltage drop that can be tolerated, (h) Estimate the route length of the circuit.
If the cable circuit is to be buried direct in the ground or in buried ducts there are further factors the designer must consider. These factors include:
(a) The thermal resistivity of the ground. The tabulated ratings given for buried cables in BS 7671 are for cables buried 'in or around buildings'. These ratings assume that the cables are buried in dry made-up ground that is likely to contain rubble, clinker and similar materials having poor thermal properties. Because of this the rat