Electrical Design Project

Electrical Design Project 01


Choice of room utilization, decor, hobbies and the activities of the various residents are now critical to electrical design.

Electrical installation cannot make universal provision for every conceivable arrangement.

Customer is the key person in determining the final arrangement. BS 7671 (the Wiring Regulations) will often recommends that a project should be discussed with the client.

Flexible design approach is now often practiced. Standards for the house industry are determined by the National House Building Council (NHBC).


 The minimum requirements


The following is an outline of basic requirements for a three-bedroom house with 120m2 floor area. This would be a typical speculative estate development (Figure 2).

  1. Rooms: small kitchen; dining room; lounge; downstairs cloakroom; main bedroom with en-suite bathroom; second double bedroom; small single bedroom; landing bathroom or shower room;

  2. Heating: central heating by gas or oil;

  3. Garden: small garden at both front and rear.


Electrical Design Project 02

Figure 1. Bathroom zones





The National House Building Council (NHBC) give minimum standards for living accommodation and services, which are shown in Table 1. It should be noted that houses vary in size and use, so the values given should be considered as a minimum.


Table 1. NHBC recommendations for a house.

13 A socket-outlets (twin socket count as two outlets)

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Relevant wiring regulations 13 A socket-outlets

  1. Building Regulations now require socket-outlets, wall switches and other similar accessories to be sited in habitable rooms at appropriate heights of between 450 and 1200mm from the finished floor levels.

  2. Any socket-outlet with a rated current not exceeding 20 A, which is used by ordinary persons’ equipment for outdoors use must be provided with additional protection by means of a 30mA RCD.

  3. No 230V sockets, except shaver sockets complying with BS EN 61558-2 are permissible in bathrooms and shower rooms.

  4. 230V socket-outlets must be located at least 3m outside the boundary of zone 1 and provided with 30mA RCD protection.


  1. To avoid danger and inconvenience, there should be more than one lighting circuit.

  2. In a bathroom, all equipment must be suitable for the zone in which it is installed.

  3. Wall switches and other accessories must not be installed in zones 0, 1 and 2.

  4. Cord switches must be installed outside of zones 0, 1 and 2, but the cord may hang within zones 1 and 2.

  5. There are restrictions on the current-using equipment in zones 0, 1 and 2.

  6. Equipment that is installed in zones 1 and 2 must have water penetration protection of at least IPX4.

  7. A careful study of BS 7671: (Most recent version) Section 701 is recommended.


Electrical Design Project 03

Figure 2. Typical three bedroom house


Building regulations

Smoke detectors

All new domestic dwellings, including conversions, must either have:

  1. A fire detection and fire alarm system in accordance with the relevant recommendations of British Standard BS 5839-6 system; or

  2. Mains operated smoke alarms, with at least one on each floor and interconnected.

Load assessment

The Regulations require that the characteristics of the supply, including an assessment of the maximum demand, should be determined by calculation, measurement, enquiry or inspection.

It is assumed that this house will be connected to an underground 230VAC supply and that the maximum demand load will be less than 100 A.

The other characteristics may be determined by enquiry to the electricity company (Distributor). Some of the Distributors publish guidance on the electricity supply characteristics and requirements for systems.

As a rule of thumb follow these steps to calculate the total demanded load:

  • Step I: indicate buildings (rooms) in project

  • Step II: find total connected load of a building in kVA for example

  • Step III: apply appropriately the demand factor, load factor and diversity factor.

  • Step IV: consider the future expansion of the service (25%).

A typical domestic supply

Except in unusual circumstances, supply characteristics are:

  1. Prospective short-circuit current at the origin:
    Never more than
    16 kA and more likely to be less than 2 kA;

  2. External earth fault loop impedance Ze:
    Not exceeding
    0.35 Ω (for TN-C-S) and more likely to be less than 0.2 Ω;

  3. Main fuse:
    This will be to the usual Distributor standard,
    BS 88-3 (previously BS 1361, Part 2) 100 A. Provided that these figures apply, there should be no problems in applying a standardized electrical design.


Project specification

It is necessary to produce a Project Specification, as in Table 2 below. This will be used initially for pricing and technical purposes. It will eventually be updated to form the basis of a user manual.


Table 2 – Project Specification (acc. to BS 7671)

Electrical Design Project 04

Wiring systems and cable sizes

Circuits and circuit design are two important discussions that every electrical engineer should know it. Traditionally, domestic installations have been wired using the ‘three plate rose’ lighting system and ring circuits for the socket-outlets.

As a rule of thumb follow these steps to design your cable circuits:


Step I:

Determine the supply characteristic including earthing arrangement. I prefer TN-C-S. Adopted value for Ze is 0.35 Ω. Next, find the design current Id from the total demanded load.


Step II:

Select cable and protective device (PD) size. Always make sure that Ipd ≥ Id. Next, select a cable size from Appendix 4 of BS 7671 which can carry the load It. Always make sure that:


Electrical Design Project 05



Cg = Rating factor for grouping. Table 4 C1 of BS 7671

Ca = Rating factor for ambient temperature. Table 4 B1 of BS 7671

Ci = Rating factor for conductors surrounded by thermal insulation. Table 52.2 of BS 7671

Cf = Rating factor for semi-enclosed fuse. BS 3036


Step III:

Check for voltage drop. Use this equation to make sure the minimum cross section is OK.


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Vd = Voltage drop in volts.

L = Cable length in meters

mV/A/m = The value from appropriate table. Appendix 4 of BS 7671

This voltage drop should be equal to or lower than the appropriate voltage drop percentages given in Table 4Ab of Appendix 4 of BS 7671. (Vd/230)·100% for a nominal voltage of 230V.


Step IV:

Check for electrical shock. The earth fault loop impedance Zs at the furthest point of the circuit must be determined by using the following formula:


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R1 + R2 is the resistance per meter of the line and protective conductor.

Cr is the rating factor for operating temperature

Ze is the value of external earth fault loop impedance.

The value of Zs must be less than the value given in Tables 41.2, 41.3 or 41.4 of BS 7671.


Step V:

Check for prospective fault current. Live conductor are protected against short circuit currents if their value of k2S2 must be greater than the value of let-through energy I2t for the protective device.


  • S is the cross sectional area of conductors

  • k is the value taken from Table 43.1 of BS 7671.

First find the short circuit current by:


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ZLN is line to neutral impedance at the origin of the installation

R1 + Rn is the resistance per meter of the line conductor and neutral conductor

t is the operating time of the protective device determined from time/current characteristic given in Appendix 3 of BS 7671.