Guide to the Wiring Regulations
Table C 4.5 Appendix12 voltage drop specification. Type of supply
Voltage drop lighting
Voltage drop other
From DNO
3%
5%
Private supply
6%
8%
A new Appendix (12) is included in BS7671: 2008 for consumers’ installations which, if followed, is ‘deemed to comply’ with voltage drop design requirements of the Regulations. This is summarized in Table C 4.5. It is recognized in the body of Section 525 and Appendix 12 that high inrush currents may cause higher voltage drop levels, and reference to equipment product standards is made. In essence this does not change the basic requirement that equipment must work! C 4.5.2 System design values For domestic installations, voltage drop is quite simple and rarely needs a design or further consideration. In commercial and industrial installations, voltage drop design will depend upon the nature of the supply. If the supply is to the Electricity Safety, Quality and Continuity Regulations 2002, the supply voltage can vary between statutory limits of +10% to –6%. As such, the voltage drops in Table C 4.5 are recommended with the caveat that voltage can be lower if the equipment standard allows. For private transformers the subject is more complicated, as the voltage regulation of the transformer is now under the control of you, the designer. The secondary terminal voltage of the transformer will largely depend upon the magnitude of the total load at any instant. Generally it is good practice to simulate the values of statutory voltage limits found in the Electricity, Safety, Quality and Continuity Regulations. The current limits are nominal voltage +10% to –6%. Thus for a large installation, a system of main and sub voltage drops may be required to be set, to achieve a ‘system’ design. A simple system view of this is given in Figure C 4.4. A guide to installation voltage drop limits is 6% with an 8% maximum. Often it can be useful to apply values in stages in a system, and popular values are 2% to 3% for submains coupled with 4% or 3% for final circuits.
C
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