Premium Membership ♕

CYBER MONDAY OFFER 💥 Save 20% on PRO Membership Plan and Video Courses with the coupon CYM23 and learn from experienced engineers.

Home / Technical Articles / Electrical Design Project of a Three Bed Room House (Part 2)
Electrical Design Project of a Three Bed Room House (Part 2)
Electrical Design Project of a Three Bed Room House (Part 2)

Continued from 1st part: Electrical Design Project of a Three Bed Room House (Part 1)

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)

Project specification (acc. to BS 7671)
Project specification (acc. to BS 7671)

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:

Load current It


  • 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.

Voltage drop


  • 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:

Earth fault loop impedance


  • 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:

Short circuit current


  • 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.

Premium Membership

Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! It helps you to shape up your technical skills in your everyday life as an electrical engineer.
More Information

Sufi Shah Hamid Jalali

B.Sc. in Electrical Engineering. Designer of websites. Currently working as Electrical Engineer at Aga Khan Foundation, Afghanistan. Likes to write and share knowledge of Electrical Engineering. Knows Primavera P6, Graphics, Professional AutoCAD and Project Management.


  1. musa
    Nov 18, 2016


  2. Muralikrishnan
    Jul 23, 2014

    Like super simple

  3. kecy
    Apr 06, 2014

    I need electrical description on domestic building

Leave a Comment

Tell us what you're thinking. We care about your opinion! Please keep in mind that comments are moderated and rel="nofollow" is in use. So, please do not use a spammy keyword or a domain as your name, or it will be deleted. Let's have a professional and meaningful conversation instead. Thanks for dropping by!

  −  7  =  three

Learn How to Design Power Systems

Learn to design LV/MV/HV power systems through professional video courses. Lifetime access. Enjoy learning!

Subscribe to Weekly Newsletter

Subscribe to our Weekly Digest newsletter and receive free updates on new technical articles, video courses and guides (PDF).
EEP Academy Courses - A hand crafted cutting-edge electrical engineering knowledge