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.

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 Z_e:
   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)

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 Z_e is 0.35 Ω. Next, find the design current I_d from the total demanded load.

Step II:

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

Where:

• C_g = Rating factor for grouping. Table 4 C1 of BS 7671
• C_a = Rating factor for ambient temperature. Table 4 B1 of BS 7671
• C_i = Rating factor for conductors surrounded by thermal insulation. Table 52.2 of BS 7671
• C_f = 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.

Where:

• V_d = 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. (V_d/230)·100% for a nominal voltage of 230V.

Step IV:

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

Where:

• R₁ + R₂ is the resistance per meter of the line and protective conductor.
• C_r is the rating factor for operating temperature
• Z_e is the value of external earth fault loop impedance.

The value of Z_s 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.

Where:

• 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:

Where:

• Z_LN is line to neutral impedance at the origin of the installation
• R₁ + R_n 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.