Electrical Installation Design and Specification
Electrical Installation Design and Specification
This domain tests whether you can design to BS 7671:2018+A3:2024 Chapter 34 and its appendices: assessing the characteristics of the installation, sizing conductors, co-ordinating protection, and proving voltage drop and disconnection times before anything is specified.
Design sequence
- Establish characteristics: maximum demand and diversity, earthing arrangement (TN-S, TN-C-S, TT), prospective fault current, external influences (Ch 32), compatibility and EMC (Ch 33), maintainability (Ch 34) and safety services (Ch 56).
- Select conductors so that Ib <= In <= Iz, where Iz is the tabulated It multiplied by Ca, Cg, Ci and Cd. Applying the correction factors is not optional.
- Voltage drop limits from Table 4Ab: 3 % for lighting and 5 % for other uses on public low-voltage supplies.
- Disconnection: Zs must not exceed the Table 41.3 or 41.4 value, or be calculated; confirm the CPC using k2S2 >= I2t.
Worked example
A 6 kW shower on a 230 V TN-C-S supply runs 18 m in 6 mm2 twin-and-earth clipped direct. Ib = 6000/230 = 26.1 A. A 32 A Type B MCB satisfies In >= Ib, and Table 4D5 gives It = 47 A, so with no derating Iz = 47 A, comfortably above 32 A. Voltage drop = 7.3 mV/A/m x 26.1 A x 18 m = 3.43 V = 1.49 %, inside the 5 % limit. Table 41.3 gives a maximum Zs of 1.37 ohm for a B32, which the measured loop impedance must not exceed.
Traps
Using It without ambient temperature or grouping correction; applying the 3 % lighting limit to a mixed circuit; assuming a 30 mA RCD removes the need to check Zs; falling back on a remembered 4 % voltage drop figure.