A Voltimum user is curious about the calculations involved and cable sizes used in an installation by one of their clients.
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Question:
A client has asked me what size SWA he should obtain to run a line 1000m for a ring main supply capable of running a high-powered jet washer (load unknown) at the far end. I'm not an electrician, neither am I intending to advise him on this matter as I feel he should actually go to an electrician for this. But just out of curiosity, what is the calculation involved? And what minimum size cable should he be using?
Answer:
Gary Parker, ECA
Hi there, thanks for this. I probably can't be of much help here as there is not enough information to do a reasonable design, also we don't have the indemnity insurance to do 3rd party designs.
If you are interested in the calculations, the IET do some very good design books, I am sure if you do a search online for IET Design Guide you will find a seller.
In terms of the calculation itself, you would first need to know the load in either Watts, Amps or KVa and you would pick a suitable protective device that is greater than the load. Once you have this information you would need to use the IET Wiring Regulations to determine the current carrying capacity of the cable taking the installation method and other reduction factors (grouping, ambient temperature, insulation, buried etc) and ensure that you have a cable with a greater CCC than that of the protective device. Finally, you would have to ensure that you have a suitable voltage drop based on the load, the length and mV/A/m of the cable you have chosen. If the drop is too high, you would increase the cable size. You would also have to ensure that you have the adequate fault current at the origin and the load to operate the protective device in the event of a fault.
I have probably trimmed a few stages off here, but as you can see there is quite a lot to consider.
One thing, you mention a ring circuit? Typically these loads are fed with a final circuit, ring final circuits and used for sockets and ring mains are used on HV installations.
I would imagine that this design should be fine for most electrical contractors to do, either by hand or by using software, so perhaps speaking to a contractor or designer/consultant when you have the data would be the way forward.
Thanks, hope this helps