Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This one, concerning five new circuits of light loads and the right cable size to choose, is answered by SELECT:
Question: Fitting a new 24-way three-phase distribution board with a 200A main switch, cable is SWA run on cable tray for 10m with an ambient temperature of 25oC. The new board is only to supply approximately five new circuits of light loads initially. With no idea what it will supply in the future, what cable size would you suggest I use?
Answer: Guidance on determining the size of a cable to be used is given in Appendix 4 of BS 7671:2008.
You will first need to determine the design current from the estimated load. You will be unable to follow the design sequence until you have this information, together with the supply characteristics.
As voltage drop may be the overriding factor, this may then be obtained by consulting the relevant table - probably Table 4E4, in this case, for the sub-main cable. Remember, however, to add the voltage drop for the final circuits.
A protective device can then be chosen which has a rating not lower than the design current. The cable size can then be determined taking into account any correction factors and consulting Table 4E4. You will then need to check that Zs values will be satisfactory for the protective devices and the adequacy of earthing arrangements.
Further guidance can be found in the IEE Electrical Installation Design Guide.
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