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CHAPTER 4Electrical Installation Circuit Design Terminology and definitions:Ampacity: current carrying capacity of electric conductors expressed in amperes. Appliance: utilization equipment. Branch circuit: the circuit conductor between the final over current device protecting the circuit and the outlet(s). Demand factor: the ratio of the maximum demand of a system, or part of a system, to the total connected load of a system or the part of the system under consideration. Feeder: all circuit conductors between the service equipment, or the generator switchboard of an isolated plant, and the final branch circuit over current device. Ground: a conductor connection, whether intentional or accidental, between an electric circuit or equipment and the earth, or to some conducting body that serves in place of the earth. Lighting outlet: an outlet intended for direct connection of a lamp holder, a light fixture, or a pendant cord terminating in a lamp holder. Outlet: a point on the wiring system at which current is taken to the utilization equipment Receptacle: a contact device installed at the outlet for the connection of a single attachment plug. Service: the conductor and equipment for delivering energy from electric supply system to the wiring system of the premises served. Switch board: a large panel, frame or assembly of panels on which are mounted, on the face or back or both, switches, over current and other protective devices, buses, any usual instruments.

Wiring Design CriteriaFlexibility: every wiring system should incorporate sufficient flexibility of design in branch circuitry, feeders, and panels to accommodate all portable, patterns, arrangements and locations of electric loads. The degree of flexibility to be incorporated depends in large measure on the type of facility. As part of the design for flexibility, provision for expansion must be provided. It must, however, be emphasized that over design is as bad as under design. Reliability: the reliability of electrical power within a facility is determined by two factors: the utilitys service and the buildings electrical system. Safety: the designer must be constantly alert to an initial safe electrical installation and such factors as electrical hazards caused by misuse of equipment or by equipment failure after installation. Energy consideration: includes limiting voltage drops, power factor correction, use of switches for control, etc Economic cost: includes initial cost and operating cost Space allocation: concerned with maintenance ease, ventilation, expandability, centrality, limitation of access, and noise, in addition to the basic item of space adequacy.

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Design procedureThe steps involved in the electrical wiring design of any facility are outlined below. These may in some instances be performed in different order, or two or more steps may be combined, but the procedure normally used is that listed below. a) b) c) Determine with the client the usage of all areas, and type and rating of all client furnished equipments including their specific electric ratings. If the designer could not get the exact electrical rating of all the equipment that are going to be installed in the building such as plumbing, elevators, kitchen, motors etc, determine their ratings from other consultants. Make an electrical load estimate based on the above collected data, areas involved, previously installed similar installation data and any other pertinent data. Load Estimation: when initiating the wiring design of a building, it is important to be able to estimate the total building load in order to plan such spaces as transformer rooms, chases, and closet. This information is also required by the local power company well in advance of the start of construction. Of course, an exact load total can be made after completing the design. But such estimation can be made from the knowledge of the loads the building uses. The electrical loads in any facility can be categorized as: (i) Lighting. (ii) Miscellaneous power, which includes convenience outlets and small motors. (iii) Heating, ventilating, and air conditioning. (iv) Plumbing or sanitary equipment: house water pump, air compressors, and vacuum pumps etc. (v) Vertical transportation equipment: elevators, moving stairs, and dumbwaiters. (vi) Kitchen equipments. (vii) Special equipments. In cooperation with the local electric utility, decide upon the point of service entrance, type of service run, service voltage, metering location, and building utilization voltage. The above considerations and general rules affecting service equipment are listed below: i. A building may be supplied at one point by either a single set or parallel sets of service conductors. ii. All equipment used for service including cable, switches, meters, and so on, shall be approved for that purpose. iii. It is recommended that a minimum of 100-amp, 3-wire, 220/380V service be provided for all individual residences. iv. No service switch smaller than 60 amp or circuit breaker frame smaller than 50 amp shall be used. v. In multiple occupancy buildings tenants must have access to their own disconnect means. vi. All building equipment shall be connected on the load side of the service equipment except that service fuses, metering, fire alarm, and signal equipment and equipment serving emergency systems may be connected ahead of the main disconnect. In computing the size of the service equipment bus, a total is taken of the various feeder loads. Although application of a Diversity Factor to this total is permissible, good __________________________________________________________________________ 37 Electrical Installation Circuit Design

d)

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practice dictates the use of a unity Diversity Factor in order to provide a measure of spare capacity in the service equipment. (e) Determine the location and estimate the size of all required electric equipment spaces including switchboard rooms, emergency equipment spaces, electric closets, and so forth. NOTE: - Panel boards are normally located in closets but may be located in corridor walls or elsewhere. This work is necessary at this point to enable the architect to reserve these spaces for the electrical equipment. Once the design is accomplished in detail, the estimated space requirements can be checked and necessary adjustments made. (f) (g) Design the lighting for the facility. This step is complex and involves a continued interaction between the architect and the lighting designer. On the same plan, or on a separate plan, as decided, locate all electrical apparatus including receptacles, switches, motors, and other power consuming apparatus. Under floor duct and ceiling track systems would be shown at this stage. If extensive, a separate plan is made. On the plan, locate signal apparatus such as phone outlets, speakers, microphones, TV outlets, fire and smoke detectors, and so on. Make drawing showing all lightings, devices, and power equipments circuit connection to the appropriate panel board. Prepare the panel schedule (table). This table shows the load distribution over the three phases and the type of load which is connected on each circuit. At this step, include the separate circuitry for emergency equipments and for spare circuit. From the panel schedule (table) compute panel loads, And make connection rearrangement so that you will be able to an optimum power balance over the three phases R, S and T. Prepare the riser diagram. This includes design of distribution panels, switchboards, and service equipment. Compute feeder sizes and all protective equipment ratings. Cheek the preceding work.

(h) (i) (j)

(k)

(l) (m) (n)

Branch Circuit Design Guidelines for Residential (a) (b) The NEC requires for residences sufficient circuitry to supply a load of 3w/sq ft in the building, excluding unfinished spaces such as porches, garages, and basements. The NEC requires a minimum of two 20-amp appliance branch circuits to feed all the small appliance outlets in the kitchen, pantry, dining room, family room etc. Furthermore, all kitchen outlets must be fed from at least two of these circuits (Avoid placing all the lighting in a building on a single circuit). Also receptacles should be circuited with preferably two, but not more than four on a 20-amp circuit.

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(c)

The NEC requires that at least one 20-amp circuit supply to be set for laundry outlets. This requirement satisfies good practice. If electric clothes dryer is anticipated an individual branch circuit should be supplied to serve this load, via a heavy-duty receptacle. Do not combine receptacles and switches into a single outlet except where convenience of use dictates high mounting of receptacles. Circuit the lighting and receptacles so that each room has parts of at least two circuits. This includes basements and garages. Supply at least one receptacle in the bathroom and one outside the house Provide switch control for closet lights. In bedrooms supply two duplex outlets at each side of the bed location to accommodate electric blanket, clocks, radios, lamps, and other such appliances. Since receptacles are counted as part of general lighting and no additional load is included for them, no limit is placed on the number of recepta