How do we begin to design?
What do we need to know? Clearly, plunging into calculations of cable size is of little value unless the type of cable and its method of installation is known. This, in turn, will depend on the installation’s environment.
At the same time, we would need to know whether the supply was single or three-phase, what the earthing arrangements (learn about this) were and so on.
Here then is our starting point and it is referred to in Part 3 of the IEE Regulations as ‘Assessment of General Characteristics’.
Having ascertained all the necessary details, we can decide on an installation method, the type of cable and how we will protect against electric shock and overcurrents. We would now be ready to begin the calculation part of the design procedure.
Basically, there are eight stages in such a procedure. These are the same whatever the type of installation, be it a cooker circuit or a steel wire armoured (SWA) cable feeding a remote garage.
Here then are the 8 steps to begin basic circuit design in a simplified form:
- Determine the design current Ib
- Select the rating of the protection In.
- Select the relevant rating factors (CFs).
- Divide In by the relevant CFs to give tabulated cable current carrying capacity It.
- Choose a cable size to suit It.
- Check the voltage drop (Use MS Excel Spreadsheet to calculate).
- Check for shock risk constraints.
- Check for thermal constraints.
Reference: Electric Wiring: Domestic – Brian Scaddan IEng, MIET