Load Estimation for Residential Building Electrical Installation

Load Estimation for Residential Building

Load Estimation for Residential Building Electrical Installation- To accurately determine the correct rating of the main cable, sub-main cable, fuse and circuit breakers, etc., being used for the electrical installation of every building, an estimation of the total load likely to be used in this building is done. The total load is estimated through the number of lamps, the number of sockets or outlets, and a list of electrical permanently or temporarily installed electrical appliances in any building. Then the rating of the main cable, sub-main cable, fuses, and circuit breakers is determined according to this total load. As a consumer of any residential building does not turn ON or use the entire load simultaneously, therefore only 70% load of any residential building is used at a time. For example, a consumer does not connect all lights, fans, tubes, tv, refrigerator, air conditioners, electrical iron, motors, etc. at one time, rather some of the loads remain ON while some others remain OFF. Besides certain extraordinary circumstances or conditions, the entire household load is rarely used at a time. Suppose that the total household load is 100 amperes, so the consumer of this house only uses a maximum of up to 70% of this load at one time. As such, to accurately estimate the required load for the installation of any building, it is very necessary to keep the diversity factor in mind. The definition of diversity factor is as under;

The ratio of the total connected load and the actual maximum load of any installation is called the diversity factor, which is expressed as a percentage. 

Thus, after estimating the load of the main cable, sub-main cable, fuses, and circuit breakers through the diversity factor, expected costs on any installation can be mitigated to a reasonable extent via the selection of a correct rating. The procedure to ascertain the load estimation of any residential building is as follows;

Number of Light Points or Incandescent Lamps

If the current rating of a sub-circuit in any given residential building has been decided, and cables also installed according to that current rating, and under such a situation we have to find out how many points can be installed along with this circuit, its procedure is as follows;

Suppose that 100 watts lamps are used for the acquisition of normal light in the house, whereas the rating of the final sub-circuit is 5 amperes and the supply voltage 230 volts. In such a situation, the maximum number of lamps can be determined with the help of the following formula;

No. of lamps or light points = Current rating of sub-circuit x Supply voltage / Wattage of lamps to be connected

= 5 x 230 / 100 = 11.5 

As such, practically incandescent lamps above 10 should not be used along with the final sub-circuit for the acquisition of normal light. As the power (watts) of lamps used as light points differ (i.e., lamps available in the market are of different power), therefore if high-powered special lamps are required to be used at certain places, the number of lamps can also be determined through the wattage of high-powered lamps, the method of which is as follows;

For example, suppose that the current rating of any circuit is 5 amperes, the supply voltage is 230 volts and 150 watts incandescent lamps are to be used in the circuit. Thus, the maximum number of lamps, that can be installed on the circuit, will be as follows;

Maximum number of lamps/circuit = 5 x 230 / 150 = 7.6

Thus, a maximum of 7 or 8 lamps with a power of 150 watts can be used on this circuit.

Number of Fluorescent Lamps Per Circuit

The number of fluorescent lamps or tubes fitted along with a final sub-circuit does not match the number of same-powered incandescent lamps. Its reason is the existence of a choke along with every fluorescent lamp, which plays a vital role in the circuit. This choke fitted along with a fluorescent tube function as a control instrument in the circuit. Due to the inductance of the choke, the power factor of the circuit reduces. Though a capacitor is also used to keep the power factorbetter,however, despite this total current of a fluorescent lamp is higher compared to the same wattage of an incandescent lamp. The following formula is normally used to determine the wattage of a fluorescent lamp;

Wattage of a fluorescent lamp = Current x Voltage / 2

Due to this reason, the wattage of the fluorescent lamps must be confirmed by the consumer, so that apart from the number of points per circuit, the number of fitting tubes (which have different lengths for different wattages) can also be estimated. 

Suppose that 80 watts of fluorescent lamps are required to be installed in a house instead of incandescent lamps, and the rating of the sub-circuit is 5 amperes while its supply voltage is 230 volts, then the formula for determining the number of fluorescent lamps will be as follows;

No. of fluorescent lamps to be connected to one circuit = Current rating of sub-circuit x supply voltage / 2 x Wattage of lamps to be connected

= 5 x 230/ 2 x 80 = 7

Thus, the maximum number of 80 watts of fluorescent lamps that can be installed on a lighting circuit, is 7. Similarly, calculations for other wattages can also be done exactly in the same way.

For example, on a 5-ampere sub-circuit, which has a supply voltage of 230, 14 tubes of 40 watts can be installed i.e., 

No. of tubes = 5 x 230 / 2 x 40 = 14

Number of Socket Outlets

The electrical appliances, which easily be shifted to different places when needed, are known as portable or travel appliances e.g., radio, tv, iron, hairdryer, heater, refrigerator, etc. The sockets, which are installed on the wiring to connect these portable appliances with the electrical wiringthose sockets are called socket outlets. In other words, sockets that are temporarily connected with electrical wiring to provide electric supply to the electrical appliances, are called socket outlets, or sockets installed on electrical wiring to provide supply to the electric appliances are called socket outlets. Sockets outlets are available in several forms, out of which 2 pin 5-ampere, 3 pin 15-ampere socket outlets are most commonly used. All socket outlets should be controlled through separate switches from the security point of view. The two-pin sockets are never advised to be installed (i.e., installation of two-pin sockets is not considered unapproved or unacceptable) because due to the non-existence of earth points on such sockets, electric shock can be feared at any time, which may even prove fatal to human life. Therefore, instead of two-pin sockets, the use of three-pin sockets (3 pins, 15 amperes) is preferred. From the security point of view, the use of shutter type sockets is always recommended in all those places, where children are present. 

Normally, a maximum of two-socket points can be kept in a room, which is considered insufficient due to the existence and application of several types of electric appliances nowadays, e.g., radio, tv, air conditioner, electric iron, heater, hairdryer, and refrigerator, etc. Therefore, keeping in mind the present-day needs, it is recommended that along with the rest of the world, at least three socket outlets of 5-ampere, and two-socket outlets of 15-ampere, be installed in a room. Every socket is generally provided electricity from a final sub-circuit. 

According to IEE regulation A-26, a 15-ampere socket should be treated as a separate sub-circuit, and for all such sub-circuits, which have a current rating above 15 amperes, only one socket-outlet be provided on them, and they must be supplied electricity through a 7/.029 size wire (4.0mm2). Moreover, more than three 5-amperes socket outlets should never be installed on a single sub-circuit. Further, a separate circuit should be reserved for each 15-ampere socket outlet, wherein a minimum 3/ .036 size wire should be used. Thus, the capacity of the final sub-circuit for a socket outlet is always taken as 15 amperes.  

To connect a three-pin socket with the supply, red color wire or the phase conductor is connected to the “L” terminal of the socket, the black colored or neutral wire is connected to the “N” terminal, whereas the earth wire is connected to the “E” terminal of the battery. When a conduit (pipe) is used along with the socket outlets, then a single core V.R.I or P.V.C insulated cables passing through the pipe should be connected to the outlet sockets, and the size of bare copper earth continuity conductor should in no way be less than 14 S.W.G. However, if cables are desired to be passed on the batten, or cables are passed within the plaster and connected to the socket outlets, then a two core sheathed cables should be used (such type of a cable which has a thick rubber or plastic sheath on it besides conductor insulation, is called sheathed cable), wherein the earth continuity conductor also exists within its sheath. 

Individual or separate fuses should not be installed on the same board or panel for the socket outlets in homes, (i.e., separate fuse for every sub-circuit), that’s installing a fuse along with one socket-outlet should be avoided in homes, because children present in the home can unplug the fuse installed on a single board and as such, they may be vulnerable to an electric shock. Thus, fuses of the socket outlets which are within the reach of children should be installed in some secure places. In commercial buildings too, where there is a danger of heavy items colliding with the porcelain-made fuse at a low height, fuses should not be installed. Remember that all sockets’ fuses should be safely protected within a metal container, switch fuse casing, or distribution fuse boards at some secure and suitable central place. 

The socket outlets installed within a kitchen should be at a low level according to needs or should be slightly above the working surface. They should never be installed near the sink. Sockets are not advised to be installed in the bathrooms, and socket-outlets for the running of essential electric appliances e.g., hair drier, shaver, etc., should be at such places inside a bathroom, where moisture and water, etc., cannot be accessed. However, according to IEE (Institute of Electrical Engineering) regulation 19, a separate point should always be reserved for a shaver, and “for shaver only” should be written above it. The socket outlets of bedrooms consisting of fixed baths or showers should be installed away from the bathroom shower or at some safe place in the room. 

Determine the Number of Ceiling Fans

The power rating of every ceiling fan is considered 100 watts for the estimation of current demand. More than 6 fans should never be installed along with a single final sub-circuit. As lights and fans are installed on the same circuit, therefore its total current rating should never exceed the total current rating of the fuses providing safety to this circuit. 

All ceiling fans should be hanged with double nut bolts clamps or hooks, and the size of the clamp rod should not be less than 5/8 inches at all. A separate switch and a separate speed regulator should be installed with every fan, through which a fan may be turned ON, or OFF apart from controlling its speed. A speed regulator should be such that a fan’s speed could be reduced through it in different stages up to 40 percent of the total speed. A fan should be hung at a minimum height of 9 feet (2.75 meters) and a maximum of 10 feet (3 meters) from the floor, whereas the height of a speed regulator from the floor should be approx. 5 feet. 

Ceiling fans are available in the market in different sizes (e.g., 36 inches, 48 inches, 56 inches). The size of a fan is selected according to the size of the room or the place where it is going to be installed. However, at the time of installation of a fan, it is also previewed whether the place where it going to be installed, is ventilated or non-ventilated. That’s a room where the flow of air in a room is common due to the presence of windows and doors on the front, a small size fan will be installed within it as compared to a non-ventilated room. As such, the total number of fans that can be installed in a large room or hall can be determined by dividing the total area of that hall or a large room by the area which is covered by a fan, i.e.,

No. of fans = Total area of hall/room Area covered by the selected fan

The detail about the sizes of fans according to the area has been provided in the following table. However, room sizes don’t need to be according to the sizes given in this table. For example, if the size of a ventilated room is 16’ x 14’, then in such a condition, the size of the room should be considered 16’ x 16’. Thus, the size of the fan which is going to be installed in a room of this size will be 56”. Look at the table;

Fan size Area of Ventilated Room Area of Non-Ventilated room
In Meters In Feet In Meters In Feet
36” 3.67 x 3.67 12 x 12 3.05 x 3.05 10 x 10
48” 4.27 x 4.27 14 x 14 3.67 x 3.67 12 x 12
56” 4.88 x 4.88 16 x 16 4.27 x 4.27 14 x 14

Example: Calculate the number of 56” ceiling fans for the ventilated room having dimensions of 15-meter x 10 meters shown in the attached building plan, so as to supply an equal amount of air in every corner of the room. No wiring is required. (DAE/I-A/2006)


Area of ventilated room = 15 m x 10 m, size of fan required = 56”

No. of fan = Total area of the room/ Area covered by a 56” fan

15 x 10 / 4.88 x 4.88 = 6.3 = 6 No.

No. of row = 2, No. of fan in each row = 3

Distance between fan (length wise) = Total length / Fan in a row 

= 15/3 = 5 m

Distance between fan and wall (length wise) = 5/2 = 2.5 m

Distance between fan 9width wise) = Total width / No. of row = 10/2 = 5 m

Distance between fan and wall (width wise) = Total width / No. of row = 10/2 = 5 m

Distance between fan and wall (width wise) = 5/2 = 2.5 m

Figure 5.1 – Ventilated room

Estimating the Load of a Residential Building

Number of Circuits Required for a Residential Building

The number of total circuits going to be installed concerning the main circuit and sub-main circuit in any given building depends on the number of points existing on the wiring and the total load going to be connected to the supply. An end of a wiring cable, which provides supply to a light fitting, socket outlet, or any electrical appliance, is known as a point. If the sub-circuit consists of one light and one fan, a maximum of 800-watt load can be connected to this sub-circuit, whereas the total number of points in this sub-circuit should be 10. However, if any sub-circuit consists of lights only, in such a situation a maximum of 1000 watts load can be connected to this sub-circuit. However, the maximum number of points in this type of sub-circuit will also be 10. The maximum load for a power sub-circuit tends to be 3000 watts, whereas the number of outlets on it should be two. 

If more than one final sub-circuits exist on any installation, every sub-circuit should be installed separately along with the distribution board through a fuse or miniature circuit breaker. Its advantage is that any final sub-circuit can be detached from a distribution board and connected again as and when required (i.e., for testing, or finding a fault and removing it). For testing purposes, to remove a final sub-circuit from the distribution board easily, neutral conductors should also be installed along with the distribution boards exactly in the same fashion, the way live or phase conductors are connected to the distribution boards (i.e., all neutral conductors of the final sub-circuit should also be connected to the distribution board through fuses or miniature circuit breakers just similar to the phase conductors). 

It should be remembered that the current rating of every final sub-circuit should not exceed 15 amperes. Moreover, according to the electrical regulations, the number of points on a circuit should be limited keeping in view the total load requirements. In figure 5.2, a final sub-circuit has been illustrated. It is clear from this figure that every sub-circuit is separately connected to the distribution board through fuses. If the rating of any final sub-circuit is 15 amperes or above it, it is not allowed to supply more than one point from this circuit. 

Figure 5.2

Estimating the Load of a Residential Building

Distribution of Electricity in Various Residential Buildings

As residential buildings can either be a single-story or multiple storied, therefore the number of circuits required for different buildings and the process of distribution of electricity in them, also differs, the explanation of which is as follows;

Distribution of Electricity in Single Story Building

A single-story residential building normally consists of a single-phase energy meter, along with two terminals to which a single-phase supply is connected through a two-core cable provided by the electric company. Whereas, the consumer receives electric supply through its remaining two terminals. Two wires ejecting from the main switch, lead to the distribution fuse board, where electric supply is distributed as required between all rooms and other necessary spots through various circuits. This has been illustrated in figure 5.3. The detail of all the sections ranging from the home’s energy meter to the final sub-circuit is as follows;

Figure 5.3

Estimating the Load of a Residential Building

Estimating the Load of a Residential Building

Main Circuit or Main Cable

The cable being used from a consumer’s energy meter up to the main switchboard or main circuit breaker in a consumer’s area is called the main circuit or main cable. In other words, the cable between an energy meter and the main switch is known as the main cable. As the entire household wiring current passes through this cable, therefore this cable tends to be large (i.e., thick). 

Sub main Circuit or Sub main Cable

The cable which connects a distribution fuse board to sub-distribution boards or switchboards is called a sub-main circuit or sub-main cable. In other words, a circuit, or cable, which ejects from the distribution fuse board and goes up to a switchboard in a room, is known as a sub-main circuit or sub-main cable. The thickness of this cable tends to be less than the main cable. 

Final Sub-Circuit or Final Cable

The circuit, which ejects from a sub-distribution board or switchboard and goes up to any electric point or electric appliance e.g., electric fan, bulb, socket, etc., is known as the final sub-circuit. In other words, a cable, which connects any electric point or electric instrument to the switchboard, is called a final cable, and this type of circuit is known as a final sub-circuit. The final sub-circuit is the last part of any wiring system, and generally, a 3/.029” size wire is used in it. 

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My name is Shahzada Fahad and I am an Electrical Engineer. I have been doing Job in UAE as a site engineer in an Electrical Construction Company. Currently, I am running my own YouTube channel "Electronic Clinic", and managing this Website. My Hobbies are * Watching Movies * Music * Martial Arts * Photography * Travelling * Make Sketches and so on...

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