Handbook of Electrical Installation Practice - BeKnowledge

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Selection of Wiring Systems 545 individual cables are thermally adequate and that the total power loss is within the capability of the enclosure. An optimisation technique is described which augments the current-carrying capacity of certain circuits, thus improving the economy of the installation. The procedure for conduit and trunking is outlined in Fig. 20.2 and examples are provided in Tables 20.3 and 20.5. Fig. 20.2 Cable size selection for enclosures.
546 Handbook of Electrical Installation Practice The cable type used in the examples, is single-core PVC insulated to BS 6004, in steel conduit to BS 4568 or steel trunking to BS 4678. The relevant cable and trunking data required to effect the designs are given in Tables 20.6 and 20.7. Example 1 Group of cables in conduit Table 20.3 Group of cables in conduit, first selection. Ambient temperature 30°C, C a = 1.00. (Note:For any other ambient temperature C a = Ê 70 - Ambˆ Ë 40 ¯ where Amb = ambient temperature, °C.) Voltage-drop limit = 6000mV single-phase. Suitable conduit size could be 32mm, for which C = 5.3W/m (Table 20.7). Conduit internal area = 615mm 2 . Fill-factor = 223.5 ¥ 100/615 = 36.8%. Maximum permitted total power loss = 5.3 ¥ 36.8 0.36 = 19.41W/m. The sizes determined by voltage drop are adequate, except for circuit No. 2, marked (*), where the power loss is greater than that permitted for uniform heating. Further, the total power loss W t is higher than the permissible value W p.This would suggest that either larger cables or a larger size of conduit is needed. In this example, the fill-factor is below the maximum recommended so an increase in the size of one circuit could be acceptable. This would reduce the power loss and increase the permitted power capacity of the group. Examine the effect of increasing the size of cables for circuit No. 2 by one size. The result is shown in Table 20.4.
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HANDBOOK OF ELECTRICAL INSTALLATION
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© 2003 by Blackwell Science Ltd, a
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vi Contents System operation 79 Ide
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viii Contents Training and systems
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x Contents 18 Lighting 475 h.r. kin
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Preface My first task as editor is
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2 Handbook of Electrical Installati
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10 Handbook of Electrical Installat
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INTRODUCTION CHAPTER 4 Cable Manage
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CHAPTER 5 Electricity on Constructi
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CHAPTER 6 Standby Power Supplies D.
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INTRODUCTION CHAPTER 7 Ground Earth
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INTRODUCTION CHAPTER 8 Cathodic Pro
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CHAPTER 10 Special Installations or
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CHAPTER 12 Standards, Specification
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CHAPTER 13 Distribution Transformer
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DEFINITIONS CHAPTER 14 Switchgear A
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Table 14.1 (contd) 374 Handbook of
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CHAPTER 15 Rotating Machines D.B. M
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APPENDIX I Main Authoritative Docum
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670 Index oil, 364, 366 oil filling
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672 Index earthing, 234 earth termi
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674 Index swimming pools, 252 switc
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