CONTENT

Preface 1. Introduction 2. Scope 3. General approach 4. Energy Efficiency Requirements for Power distribution in buildings 4.1 High Voltage Distribution 4.2 Minimum Transformer Efficiency 4.3 Locations of Distribution Transformers and Main LV Switchboard 4.4 Min Circuits  4.5 Feeder Circuits 4.6 Sub-main Circuits 4.7 Final Circuits 5. Requirements for efficient Utilisation of Power 5.1 Lamps and Luminaires  5.2 Air Conditioning Installations 5.3 Vertical Transportation 5.4 Motors and Drives 5.4.1 Motor Efficiency 5.4.2 Motor Sizing 5.4.3 Variable Speed Drive 5.4.4 Power Transfer Device 5.5 Power Factor Improvement 5.6 Other Good Practice 5.6.1 Office Equipment 5.6.2 Electrical appliance 5.6.3. Demand Side management 6. Energy Efficiency Requirements for power quality 6.1 Maximum Total Harmonic Distortion (THD) of Current on LV Circuits 6.2 Balancing of Single-phase Loads 7. Requirements for metering and monitoring facilities 7.1 Main Circuits 7.2 Sub-main and Feeder Circuits 8. Energy Efficiency in Operation & Maintenance of Electrical Installations in Buildings 8.1 Emergency Maintenance 8.2 Planned Maintenance 8.3 Purpose of maintenance 8.4 Economic and Energy Efficiency of Maintenance

 

INTRODUCTION

1. Introduction

Electricity is the most common and popular form of energy used in all types of buildings including residential, commercial and industrial. However, through inappropriate design of the power distribution systems and misuse of electrical equipment in buildings, it also costs us dearly in terms of losses as far as energy efficiency is concerned. The Code of Practice for Energy Efficiency of Electrical Installations (hereinafter referred to as the Code or the Electrical Energy Code) sets out the minimum requirements of the energy efficient design on electrical installations for the guidance of engineers and other parties concerned in the electrical services design and operation of buildings. This guidebook outlines and explains the provision of those clauses in the Code in simple terms together with design examples and calculations. It aims to impress upon both electrical engineers in design and operation of buildings the importance of taking adequate energy conservation measures for compliance with the Code and to guard against unnecessary energy losses in the distribution and utilisation of electrical energy. This guide should be read in conjunction with the other Building Energy Codes in Lighting, Air Conditioning, Lift & Escalator, etc., the Code of Practice for the Electricity (Wiring) Regulations and Supply Rules published by the power companies, in which some data and information are referred and used in this guide.

2. Scope

2.1    The Electrical Energy Code shall apply to all electrical systems other than those used as emergency systems, for all new buildings except those specified in Item 2.2, 2.3 and 2.4 below. 2.2    The following types of buildings are not covered in the Code: (a) buildings with a total installed capacity of 100A or less, single or three-phase at nominal low voltage, and (b) buildings used solely for public utility services such as power stations, electrical sub-stations, and water supply pump houses etc. 2.3  Buildings designed for special industrial process may be exempted partly or wholly from the Code subject to approval of the Authority. 2 4  Equipment supplied by the public utility companies (e.g. HV/LV switchgear, transformers, cables, extract fans etc.) and installed in consumers' substations will not be covered by the Code. 2.5  In case where the requirements of the Code are in conflict with the requirements of the relevant Building Ordinance, Supply Rules, or Regulations, the requirements of this Code shall be superseded. This Code shall not be used to circumvent any safety, health or environmental requirements.

3. General Approach

3.1  The Code sets out the minimum requirements for achieving energy efficient design of electrical installations in buildings without sacrificing the power quality, safety, health, comfort or productivity of occupants or the building function. 3.2  As the Code sets out only the minimum standards, designers are encouraged to design energy efficient electrical installations and select high efficiency equipment with energy efficiency standards above those stipulated in the Code. 3.3  The requirements for energy efficient design of electrical installations in buildings are classified into the following four categories:  (a) Minimising losses in the power distribution system. (b) Reduction of losses and energy wastage in the utilisation of electrical power. (c) Reduction of losses due to power quality problems. (d) Appropriate metering and monitoring facilities.

 
 

4. Energy Efficiency Requirements for power distribution in buildings

4.1    High Voltage Distribution

The Code requires that high voltage (HV) distribution systems should be employed for high-rise buildings to suit the load centers at various locations. A high-rise building is defined as a building having more than 50 storeys or over 175m in height above ground level. The number of modern air-conditioned high-rise office buildings in Hong Kong is increasing rapidly during the past decade. Following the release of height restriction in certain areas after the opening of the new Hong Kong International Airport at Chek Lap Kok in 1998, it is expected that the growth of high-rise buildings will continue to boom. The electrical demand of a modern high-rise office building could reach well over 200 VA/m2 depending on the nature of the business type and services provided. Some of these electrical loads will be concentrated in basement, intermediate mechanical floor, or rooftop plant rooms for the accommodation of chiller plant, pump sets, air handling units, lift machinery, etc. Other loads, such as landlord/tenants lighting and small power, will be evenly distributed throughout the building floors. These high-rise buildings, with their large demand requirements, will normally have at least one HV intake, usually at 11kV, provided by the power company. The distribution (copper) losses within the building can be kept to a minimum if large block of power can be distributed at HV to load centres at various locations of the building. As the substation is sited at the centre of its load, the loss and voltage drop in the LV distribution system will be minimised. The cost may also be significantly cheaper than an all LV system due to less copper mass required. It should be noted that the HV distribution cables are defined as Category 4 circuits under The Electricity (Wiring) Regulations. Separate cable ducts and riser ducts, segregated from cables of all other circuits categories, must be provided for HV cable distribution within the buildings.  A typical SF6 gas sealed type 1500 kVA 3-phase 11 kV/380V distribution transformers used in Hong Kong have a total weight of about 5,000kg. The transportation of these distribution transformers from ground floor level to their high level substations in a high-rise building might therefore pose a major problem.

4.2     Minimum Transformer Efficiency

The Code requires that the privately owned distribution transformers should be selected to optimise the combination of no-load, part-load and full-load losses without compromising operational and reliability requirements of the electrical system. The transformer should be tested in accordance with relevant IEC standards and should have a minimum efficiency shown in Table 4.1 at the test conditions of full load, free of harmonics and at unity power factor.  Table 4.1: Minimum Transformer Efficiency Transformer Capacity Minimum Efficiency <1000kVA 98% >1000kVA 99% Transformers can be manufactured with efficiencies as high as 98% to 99%. Most transformer manufacturers offer a variety of loss designs with associate differences in cost. Transformer losses are determined at 100 load and at a winding temperature of 85oC or 75oC depending on the type of transformer (e g. SF6 gas sealed dry type and silicone fluid type). The winding (copper) loss varies approximately as the square of the load current (and varies slightly with the operating temperature). The no-load (core) loss is more or less steady (fundamental value) at constant voltage and frequency.  For privately owned distribution transformers, an efficiency of not less than 98% at full load conditions, free of harmonics and at unity power factor, is required by the Code. The transformers should be tested in accordance with IEC 76 or BS 171. Utility owned transformers are exempted from the requirement of the Code. IEEE paper C57.110-1986, entitled "IEEE Recommended  Practice Establishing Transformer Capacity When Supplying Non-sinusoidal Load Currents", details two methods for de-rating distribution transformers as a result of the additional heating effect that occurs when these transformers supply power loads that generate a specific level of harmonics. K-factor is a method of calculation, derived from the IEEE paper, used to determine the heating impact of a non-linear load on a transformer. The K-factor is defined as the sum of the squares of the per unit harmonic current times the harmonic number squared. In equation form, the K-factor is defined as:  where Ih(pu) is the harmonic current expressed in per unit and h is the harmonic number. A k-rated transformer is one that is specially designed to operate at its design temperature while supplying a load that generates a specific level of harmonics. K-rated transformers are tested in according to IEEE C57.110-1986 by the manufacturer, and then assigned a "k" rating. Typical ratings are k-4, k-9, k-13, k-15, k-20, etc. More details on transformer losses due to harmonics could be found in section 6.1 of this guide.

4.3     Locations of Distribution Transformers and Main LV Switchboard

The Code requires that the locations of distribution transformers and main LV switchboards shall preferably be sited at their load centres rather than at the periphery of the buildings, provided that all local supply rules and fire regulations etc. could also be complied. Traditional location of a transformer room in a building is normally at the ground floor level with an appropriate vehicular access for loading and unloading substation equipment. The main LV switchroom is normally located adjacent to the transformer room and all sub-main and feeder circuits including the rising mains will be fed from the main LV switchboard. Distribution losses and cost for electrical loads at roof level and far away from the main LV switchboard are usually high.  Mechanical floors are normally incorporated in the design of modern high-rise commercial buildings at intermediate levels where all major electrical and mechanical plant rooms are located. Transformers and main LV switch rooms could be provided on these floors to minimise LV distribution losses. Problems need to be considered include separate cable ducts provision for HV (11 kV) cables, vertical transportation for transformers (normally single-phase type to reduced size and weight) and switchgear, fire protection and EMI problems to adjacent floors etc. Substations sited other than at ground floor locations must be equipped with non-flammable equipment to satisfy FSD requirements, e g SF6 or vacuum circuit breaker, SF6 or silicone-fluid filled transformers and LSF/XLPE cables etc.

4.4    Main Circuits

The Code requires that the copper loss of every main circuit connecting the distribution transformer and the main incoming circuit breaker of a LV switchboard should be minimised by means of either:  (a) locating the transformer room and the main switchroom immediately adjacent to, above or below each other, or (b) restricting its copper loss to not exceeding 0.5% of the total active power transmitted along the circuit conductors at rated circuit current. The cross-sectional area of neutral conductors should not less than that of the corresponding phase conductors. In any electrical circuit some electrical energy is lost as heat which, if not kept within safe limits, may impair the performance and safety of the system. This energy (copper) loss, which also represents a financial loss over a period of time, is proportional to the effective resistance of the conductor, the square of the current flowing through it and the duration of operational time. A low conductor resistance therefore means a low energy loss; a factor of increasing importance as the energy efficiency and conservation design is concerned.  The length of the main distribution circuit conductors connecting the distribution transformer and the main incoming circuit breaker (MICB) of the LV switchboard should be as short as possible by means of locating the substation and the main LV switchroom adjacent to each other. A maximum conductor length of 20m is recommended which is based on HEC's Guide to Connection of Supply. Due to the possibility of large triplen harmonic currents existing in the neutral conductor for building loads with a large proportion of non-linear equipment, it is not recommended to use neutral conductors with a cross-sectional area less than that of phase conductors in the main circuit.  Typical sample calculations for various wiring systems used for a main circuit feeding from a 1500kVA 11kV/380V 3-phase distribution transformer to a main LV switchboard having a circuit length of 20m are provided as follows:  1. 2500A 4-wire copper insulated busduct system 2. 3x630mm2 1/C XLPE copper cables for each phase and neutral in cable trench 3. 3x960mm2 1/C XLPE aluminium cables for each phase and neutral in cable trench Assuming a balanced and undistorted full load design current of 2280A at a power factor of 0.85, the power loss in transferring the power in each case is calculated. Total active power transferred = 1500kVA x 0.85= 1275kW Case (1):    2500A 4-wire copper busduct system Resistance per conductor, r = 0.0177mW/m at 80oC (Based on data provided by a reputable busduct manufacturer) Total power losses = 3 x 22802 A2 x 0.0000177W/m x 20m=5.52kW (0.433%) Case (2):    3x630mm2 1/C XLPE copper cables for each phase and neutral in cable trench as shown below Resistance per conductor (Based on BS7671:1992, Table 4E1B) =  =0.043mW/m (at 90oC) Effective resistance per phase with 3 conductors in parallel = 0.043/3 mW/m = 0.0143 mW/m Total power losses = 3x22802 A2 x 0.0000143W/m x 20m = 4.46kW (0.35%) Case (3):     3x960mm2 1/C XLPE aluminum cables for each phase and neutral Resistance per conductor (Based on BS7671:1992, Table 4L1B) =  =0.0473mW/m (at 90oC) Effective resistance per phase with 3 conductors in parallel = 0.0473/3 mW/m = 0.0158 mW/m Total power losses = 3x22802 A2 x 0.0000158W/m x 20m = 4.93kW (0.387%) For design purpose, the examples above provide a quick guideline for main circuit design using different types of conductors up to 20m in length. All three cases above can fulfill the requirement of maximum power loss of 0.5% under full load, balanced and undistorted conditions. Designers should ensure adequate precautions have been taken in balancing the loads and harmonic reduction in the design of main circuits. Main circuits designed, supplied and installed by the utility companies are exempt from the requirement of the Code.

4.5     Feeder Circuits

A feeder circuit is defined as a circuit connected directly from the main LV switchboard to the major current-using equipment such as chiller plant, pump sets and lift system. The code requires that the maximum copper loss in every feeder circuit should not exceed 2.5% of the total active power transmitted along the circuit conductors at rated circuit current. This requirement does not apply to circuits used for compensation of reactive and distortion power.  For a 3-phase circuit with balanced and linear load, the apparent power transmitted along the circuit conductors in VA is: Active power transmitted along the circuit conductors in W is:  Total copper losses in conductors in W is:  Pcopper = 3 x Ib2 x r x L where UL   = Line to line voltage, 380V Ih   = Design current of the circuit in ampere cos q = Displacement power factor of the circuit r  = a c resistance per metre per conductor at the conductor operating temperature L    = Length of the cable in metre Percentage copper loss with respect to the total active power transmitted,  This maximum copper loss requirement is deemed to comply with for any 3-phase balanced circuit with linear characteristic, if feeder circuits are designed to the conventional safety requirement of the Electricity (Wiring) Regulations. The conventional method of cable sizing can briefly be described as follows: The relationship among circuit design current (Ib), nominal rating of protective device (In) and effective current-carrying capacity of conductor (Iz) for an electrical circuit can be expressed as: Co-ordination among Ib, In, & Iz: Ib< In< Iz Calculated minimum tabulated value of current:  Effective current-carrying capacity: Iz=It x Ca x Cg x Ci Where It = the value of current tabulated in Appendix 4 of BS7671:1992, The Requirements for Electrical Installations Ca = Correction factor for ambient temperature Cg = Correction factor for grouping Ci= = Correction factor for thermal insulation A work example on feeder cable sizing is given as below: A 380 V 3-phase feeder circuit to a 40kW sea water pump set is wired in a 4-core PVC/SWA/PVC copper cable. The cable is mounted on a perforated cable tray with 2 other similar cables touching. The steel wire armour of the cable is to be used as circuit protective conductor. HRC fuses to BS88 are lo be used for circuit protection. Assuming the ambient-air temperature is 35oC and star/delta starter is used for motor starting. The efficiency and power factor of the motor at full load are given as 0.8 and 0.85 respectively. The length of the cable is 80m from the main switchboard. The minimum cable size for compliance with the Electricity (Wiring) Regulations is determined as follows: Design current of 40kW motor circuit, Ib = 89.37 A HRC fuse rating selected, In = 100A as protective devices Correction factors    Cg = 0.94     Ca = 0.81 Minimum current-carrying capacity, It(min ) = 131 A From table 4D4A(BS7671), It= 135A for 35mm2 4/c PVC/SWA/PVC cable Voltage drop = 1.1 mV/A/m x 89.37 A x 80 m = 7.86 V (2%) Effective current-carrying capacity, Iz= 135 x 0.94 x 0.81 = 102.8 A Resistance of conductor (Table 4.2A), r = 0.625 mW/m % copper loss = (3 x 89.37 2 x 0.000625 x 80)/(40000/0.8) = 2.4% (< 2.5%) The minimum cable size selected is 35mm2, which comply with both the safety and energy efficiency requirements. This method is based on the assumption that the supply voltages and load currents are sinusoidal and balanced among the three phases in a 3-phase 4- wire power distribution system. However, extra care must be taken if the 3-phase feeder circuit is connected to non-linear load, such as Uninterruptable Power Supply (UPS) systems, Variable Voltage Variable Frequency (VVVF) lift drive systems and Variable Speed Drive (VSD) motor systems, etc. The design current used for cable sizing must take harmonic currents into account.  For a 3-phase non-linear circuit having known design current Ib or fundamental current I1 and total harmonic distortion THD, the apparent power transmitted along the circuit conductors in VA is:  where  From definition:    Therefore,  And, fundamental current  Assuming voltage distortion is small, UL = Ul, and active power transmitted along the circuit conductors in W is given by:  where  UL   = Supply line voltage at 380V Il   = Fundamental phase current of the circuit in ampere cosq = Displacement power factor of the circuit And, Total Power Factor =  Assuming the skin and proximity effects are small, total copper losses in conductors including neutral in W is given by  Pcopper = (3 x Ib2 + IN2 ) x r x L where  IN = Neutral current of the circuit in ampere  Ib  = Design rms phase current of the circuit in ampere r   = a.c. resistance per metre at the conductor operating temperature L  = Length of the cable in metre Percentage copper loss with respect to the total active power transmitted,  Using the same work example above, if the feeder circuit is designed for VSD drive instead of the conventional star/delta starter, the new feeder circuit have to be re-designed as follows. Given that THD at full-load and full-speed condition is 80% (a figure for illustrating the harmonic effect and does not comply with Table 6.1) and harmonic components are mainly 5th and 7th order.  Fundamental current of 40kW motor circuit, Il = 89.37 A Design current,  HRC fuse rating selected, In = 160 A as protective devices Correction factors    Cg = 0.94     Ca = 0.81 Minimum current-carrying capacity, It(min.) = 210 A From table 4D4A (BS7671), It = 251 A for 95mm2 4/c PVC/SWA/PVC cable Voltage drop = 0.43 mV/A/m x 126A x 80 m = 4.33V ( 1.1% ) Effective current-carrying capacity, Iz =251 x 0.94 x 0.81 = 191A Resistance per unit length of conductor (Table 4.2A), r = 0.235 mW/m,  IN=0 % copper loss = (3 x 1262  x 0.000235 x 80) / (40000/0.8) = 1.8% (<2.5%) The minimum cable size required for the new feeder circuit is 95mm2, which has much smaller voltage drop and power loss. More details on THD requirements could be found in section 6.1 of this guide.

4.6    Sub-main Circuits

A sub-main circuit can be defined as a circuit connected directly from the main LV switchboard to a sub-main distribution panel or a rising main for final connection of the minor current-using equipment. The Code requires that the maximum copper loss in every sub-main circuit should not exceed 1.5% of the total active power transmitted along the circuit conductors at rated circuit current. Similar approach could be followed for sizing conductor as feeder circuit above. However, assumption has to be made in the design for various characteristics of the sub-main circuit including design current, expected harmonic current (THD) in the circuit, degree of unbalance, etc. Alternatively, an energy efficiency method introduced by the Code could also be used for preliminary cable sizing. This energy efficiency method for cable sizing requires the calculation of the maximum allowable conductor resistance based on the maximum copper loss requirement as stipulated in the code. For a 3-phase 4-wire circuit (assuming balanced, linear or non-linear):  Active power transmitted via the circuit conductors,  Total copper losses in conductors, Pcopper = (3 x Ib2 + IN2) x r x L where  UL    = Line to line voltage, 380V Ib     = Design current of the circuit in ampere I1    = Fundamental current of the circuit in ampere IN     = Neutral current of the circuit in ampere cos q = Displacement power factor of the circuit r      = a.c. resistance / conductor / metre at the conductor operating temperature L     = Length of the cable in metre Percentage copper loss with respect to the total active power transmitted,  % copper loss =  Therefore, max.  Table 4.2A and 4.2B in the Code provide a quick initial assessment of cable size required for the common cable types and installation methods used in Hong Kong. The tabulated current rating of the selected cable could then be corrected by applying the correction factors accordingly. The effective-current carrying capacity of the selected cable must be checked so that its value is larger than or equal to the nominal rating of the circuit protective device. A work example on sub-main cable sizing under different loading characteristics is given below: A 3-phase sub-main circuit having a design fundamental current of 100A is to be wired with 4/C PVC/SWA/PVC cable on a dedicated cable tray. Assuming an ambient temperature of 30oC and a circuit length of 40m, calculate an appropriate cable size at the following conditions: (a) Undistorted balanced condition using conventional method (cos q = 0.85); (b) Undistorted balanced condition with a maximum copper loss of 1.5% (cos q = 0.85); (c) Distorted balanced condition with I3=33A & I5=20A (THD 38.6%) and a maximum copper loss of 1.5% (cos q = 0.85); (d) Circuit lo feed VSD loads with harmonic current I5=70A, I7=50A & I11=15A (THD 87%) and a maximum copper loss of 1.5% (cos q =1), and (e) Circuit to feed 3 VSD loads as in (d). Case ( a): Undistorted balanced condition using conventional method: Ib= 100A    In= 100A Assume the correction factors Ca, Cp, Cg & Ci are all unity.  Refer to BS7671:1992, The Requirements for Electrical Installations. Table 4D4A 25mm2 4/C PVC/SWA/PVC cable   It =110A Conductor operating temperature tl = 30+ 1002/ 1102 x (70-30) = 63oC Ratio of conductor resistance at 63oC to 70oC = (230+63)/(230+70)= 0.98 Voltage drop = 1.5mV/A/m x 0.98 x 100A x 40m=5.88V(1.55%) Active power transferred  Total copper losses in conductors =3 x 1002 A2 x 0.0015W/m /  x 0.98 x 40m = 1.02kW ( 1.82%) Cable size of 25mm2 selected can comply with the safety requirement but is not acceptable if the maximum allowable copper loss is limited to 1.5%. Case (b)     Maximum copper loss method using Table 4.2A in the Code for initial assessment of an approximate conductor size required by calculating the maximum conductor resistance at 1.5% power loss: max. r (mW/m)  From Table 4.2A  35 mm2 4/C PVC/SWA/PVC cable having a conductor resistance of 0.625 mW/m is required.  Refer to BS7671:1992, The Requirements for Electrical Installations, Table 4D4A  35mm2 4/C PVC/SWA/PVC cable   It=135A Conductor operating temperature tl = 30 + 1002 / 1352 x (70-30) = 52oC Ratio of conductor resistance at 52oC to 70oC = (230+52) / (230+70) = 0.94 Voltage drop= 1.1mV/A/m x 0.94 x 100A x 40m = 4.14V(1.09%) Total copper losses in conductors = 3 x1002 x 0.625 x 094 x 40 = 716W(1.28%) Cable size of 35mm2 selected is acceptable for both safety and energy requirements, i.e power loss < 1.5%, under undistorted and balanced conditions. Case (c)  Distorted balanced condition with I3=33A & I5=20A (THD 38.6%) and a maximum copper loss of 1.5% Fundamental current Il = 100A, harmonic currents I3= 33A & I5= 20A Neutral current (rms) IN = 3 x 33A = 99A Ler In = 125A Fig. 4.1: Current Waveforms for case (c) From case (b) above 35mm2 4/C PVC/SWA/PVC cable was selected Refer to BS7671 1992. The Requirements for Electrical Installations, Table 4D4A  35mm2 4/C PVC/SWA/PVC cable  It=135A Conductor operating temperature, tl = 30+ (3x107.2+99)2 / (3x135)2 x (70-30)=73oC (Note: conductor operating temperature would be 73oC at this condition which is over the maximum of 70oC for PVC insulated cable) Ratio of conductor resistance at 73oC to 70oC =(230+73)/(230+70) =1.01 (over temperature) Total copper losses in conductors (assuming skin & proximity effects are negligible for harmonic currents)  = (3 x 107.22 + 992) x 0.000625 x 1.01 x 40= 1.14kW Active power,  % copper loss = 1.14kW / 56kW x 100 = 2% (over 1.5max.) Try next cable size: 50mm2 4/C PVC/SWA/PVC cable Refer to BS7671:1992, The Requirements for Electrical Installations,  Table 4D4A 50mm2 4/C PVC/SWA/PVC cable  It=163A Conductor operating temperature, tl =30 +(3x107.2+99)2 / (3x163)2 x (70-30) = 59.6oC Ratio of conductor resistance at 59.6oC to 70oC = (230+59.6) / (230+70) = 0.965 Total copper losses in conductors = (3 x 107.22 + 992) x 0.000465 x 0.965 x 40 = 789W % copper loss = 0.789kW / 56kW x 100 = 1.4%(<1.5%OK) A cable size of 50mm2 is selected for compliance with both safety and energy requirements under this condition. Case (d): Circuit to feed VSD loads with full load and full speed harmonic current I5=70A, I7=50A & I11=15A (THD 87%) and a maximum copper loss of 1.5% (cosq = 1) Fig. 4.2: Current Waveforms for case (d) Fundamental current, I1 = 100A Harmonic current, I5=70A, I7=50A & I11=15A New design current, Ib=Irms=133A New rating of protective device,      In= 160A Minimum current-carrying capacity of conductors, It(min) = 160A Max. conductor resistance, r From Table 4.2A 50mm2 4/C PVC/SWA/PVC cable having a conductor resistance of 0.465 mW/m is required. Refer to BS7671 1992. The Requirements for Electrical Installations,  Table 4D4A 50mm2  4/C PVC/SWA/PVC cable  It= 163A Table 4D4B  r = 0.8mV/A/m,      x = 0.14mV/A/m,    z = 0.81 mV/A/m Conductor operating temperature tl = 30+ 1332/1632 x (70-30) = 57oC Ratio of conductor resistance at 57oC to 70oC = (230+57) / (230+70) = 0.956 Voltage drop  Active power drawn  Total copper losses in conductors (assuming skin & proximity effects are negligible) = 3 x 1332 A2 x 0.000465 W/m x 0.956 x 40m = 0.94kW(1.4%)(<1.5% OK) A cable size of 50mm2 is selected for compliance with both safety and energy requirements under this condition. Case (e) A riser is going to supply 3 nos. of VSDs as described in Case (d) on 13/F, 14/F and 15/F of a building. No diversity factor is to be applied. Fundamental current, Il = 300A 5th harmonic current, I5 = 210A 7th harmonic current, I7 = 150A 11th harmonic current. I11 = 45A Design current Ib = 398A Rating of protective device, In = 400A Min. cable current carrying capacity It(min) = 400A Assume the floor-to-floor height is 3m and the cable is with a horizontal run of 10m. Actual cable length, L' = 10m + 15 x 3m = 55m Effective cable length, L = 10m + 13 x 3m + 2/3 x 3m +1/3 x 3m = 52m Max. conductor resistance From Table 4.2A, 240mm2 4/C PVC/SWA/PVC cable having a conductor resistance per unit length of 0.095 mW/m is required. Refer to BS7671:1992 The requirements For Electrical Installations. Table 4D4A 240mm2 4/C PVC/SWA/PVC cable It = 445A Table 4D4B r = 0.165 mV/A/m; x = 0.130 mV/A/m and z = 0.21 mV/A/m Conductor operating temperature  Ratio of conductor resistance at 62oC to 70oC Voltage drop  Active power drawn =  Total copper losses in conductors (assuming skin & proximity effects are negligible)  = 3 x 3982A2 x 0.000095mV/A/m x 0.973 x 52m = 2284W (1.16%<1.5%) A cable size of 240 mm2 is selected for compliance with both safety and energy efficiency requirements under this condition.

4.7     Final Circuits

A final circuit is defined as a circuit connected directly from a sub-main panel (distribution board ) to current using equipment, or to a socket-outlet or socket-outlets or other outlet points for the connection of such equipment. The Code requires that the maximum copper loss for every single-phase or three-phase final circuit over 32A should not exceed 1% of the total active power transmitted along the circuit conductors at rated circuit current. This requirement excludes most standard final circuits below 32A rating for lighting, socket outlet and small power distribution in buildings in which minimum conductor size is specified in the Electricity (Wiring) Regulation. However, designers are required to ensure that the standard final circuits (A1 ring, A2 radial and A3 radial) using 13A socket outlets, as slated in Clause 6C of the Code of Practice for the Electricity (Wiring) Regulations, should be as short as possible by locating the MCB distribution board at the proximity of the areas served by the circuit. Table 4.2A & 4.2B in the following pages are given to provide guidance for preliminary selection of appropriate cable size for main, feeder, sub-main and final circuits based on the maximum allowable resistance value for a certain percentage copper loss. TABLE 4.2A Multicore Armoured and Non-armoured Cables (Copper Conductor) Conductor Resistance at 50 Hz Single-phase or Three-phase a.c. (Based on BS7671:1992 The Regulations for Electrical Installations, Table 4D2B, 4D4B, 4E2B & 4E4B)   Conductor  cross-sectional area (mm2) Conductor resistance for PVC and XLPE cable in milliohm per metre  (mW/m) PVC cable at max. conductor operating temperature of 70oC XLPE cable at max. conductor operating temperature of 90oC 1.5 14.5 15.5 2.5 9 9.5 4 5.5 6 6 3.65 3.95 10 2.2 2.35 16 1.4 1.45 25 0.875 0.925 35 0.625 0.675 50 0.465 0.495 70 0.315 0.335 95 0.235 0.25 120 0.19 0.2 150 0.15 0.16 185 0.125 0.13 240 0.095 0.1 300 0.0775 0.08 400 0.0575 0.065 TABLE 4.2B Single-core PVC/XLPE Non-armoured Cables, with or without sheath (Copper Conductor)  Conductor Resistance at 50 Hz Single-phase or Three-phase a.c. (Based on BS7671:1992, Table 4DIB & 4EIB)   Conductor  cross- sectional area (mm2) Conductor resistance for PVC and XLPE cable in milliohm per metre  (mW/m) PVC cable at max. conductor operating temperature of 70oC XLPE cable at max. conductor operating temperature of 90oC Enclosed in conduit/trunking Clipped direct or on tray, touching Enclosed in conduit/trunking Clipped direct or on tray, touching 1.5 14.5 14.5 15.5 15.5 2.5 9 9 9.5 9.5 4 5.5 5.5 6 6 6 3.65 3.65 3.95 3.95 10 2.2 2.2 2.35 2.35 16 1.4 1.4 1.45 1.45 25 0.9 0.875 0.925 0.925 35 0.65 0.625 0.675 0.675 50 0.475 0.465 0.5 0.495 70 0.325 0.315 0.35 0.34 92 0.245 0.235 0.255 0.245 120 0.195 0.185 0.205 0.195 150 0.155 0.15 0.165 0.16 185 0.125 0.12 0.135 0.13 240 0.0975 0.0925 0.105 0.1 300 0.08 0.075 0.0875 0.08 400 0.065 0.06 0.07 0.065 500 0.055 0.049 0.06 0.0525 630 0.047 0.0405 0.05 0.043 800 - 0.034 - 0.036 1000 - 0.0295 - 0.0315