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. An example of system design of a 61 ft-6 in by 36 ft, single story, single-family residence is used in the explanation. This residence contains general lighting (lights and receptacle outlets), equipment, and appliances, including what follows: water heater (3800 VA), clothes dryer (4400 VA), dishwasher (1000 VA), range (11 700 VA), waste disposal (1000 VA), air conditioner (6300 VA), and garage door opener (1000 VA). Loads used are from manufacturers' data. Fig.12 Layout of locations of convenience receptacles and other outlets for design example. Fig.13 Layout of lighting outlets and related switches for lights and outlets for design example. ======= Fig.14 An example of a light fixture (luminaire) schedule. Fixture Fixture Designation Type Description Lamp Type Mounting A 75W R30 A Recessed downlight or similar Halo's H7UICT, with No. 310W 75W R30 Ceiling White Coilex Baffle cone and white trim ring. A1 75W R30 A1 Recessed downlight or similar Halo's H7UICT, with No. 310 75 W R30 Ceiling Black Coilex Baffle cone and white trim ring. B 75W R30 B Recessed downlight or similar Halo's H7T, with No. 310W 75W R30 Ceiling White Coilex Baffle cone and white trim ring. C Owner furnished hung chandelier/ceiling mounted light fixture. Ceiling D 90W PAR38 D Recessed downlight designed for sloped ceilings, or similar 90W PAR38 Sloped ceiling Halo's H47ICT, with No. 14199 Coilex Baffle and white trim ring. E1 75W E1 Recessed combination light fixture/exhaust fan unit similar to 75W Ceiling Nutone's "QuieTTest" Model QT140L. Fan to be capable of delivering a minimum of 150 cfm and must have a sound rating of 2.0 sones or better. Provide new exhaust ductwork to exterior in accordance with manufacturer's specifications. E2 75W E2 Recessed exhaust fan unit similar to Nutone's "QuieTTest" Model 75W Ceiling QT200. Fan to be capable of delivering a minimum of 150 cfm and must have a sound rating of 2.0 sones or better. Provide new exhaust ductwork to exterior in accordance with manufacturer's specifications. F 40RS F Surface mounted fluorescent fixture with prismatic lens, 48" long, 40RS Ceiling sim. to KLP's Model MWB140A. G Owner furnished exterior wall mounted fixture. Wall H 20W 12V MR16 H Recessed downlight or similar to Halo's H1499T, with No. 1493P 20W 12V MR16 Ceiling white Coilex baffle and white trim ring. L L Owner furnished exterior lantern. Ceiling/wall M 40W M Bare bulb porcelain fixtures. 40W Ceiling N N Owner furnished incandescent fixture to be mounted to wall Wall/Mirror over mirror or to mirror face, as directed by owner. P P Owner furnished paddle fan. Ceiling S 60W A19 S Recessed downlight or similar Halo's H7UICT, with No. 170PS 60W A19 Ceiling Showerlite Albalite Lens. T 10H2 Halogen T Low voltage halogen under-cabinet task lights, or similar and 10H2 Halogen Under cabinet equal to Ardee Lighting's Diskus Series mini-halogen downlights, Model DULWF10. W W Owner furnished wall sconce. Wall Light Fixture Notes 1. Light fixture products specified above are for contractor reference only. Equivalent fixtures, subject to owner and architect review and approval may be accepted. Contractor to submit cuts on all light fixtures to architect and owner for approval prior to ordering any light fixtures. 2. Fixtures to be recessed in direct contact with insulated ceilings or installed in damp locations are to be rated for such applications. 3. Contractor to coordinate with owner regarding method of installation of rough wiring for "J" type fixtures (owner's cabinetry subcontractor may design method of wiring concealment into casework). 4. Contractor to review exterior lighting control concept with owner prior installation of any materials. ========= Circuit Design 1. Locate the convenience receptacles on the floor plan with symbols used to represent each. Receptacles should be located conveniently following the guidelines provided. All switching, sizing of conductors, and circuit layout will be done later. GFCI- and AFCI-protected receptacles should be located following the requirements specified. See Fig.12. 2. Locate all appliance and equipment outlets on the floor plan using the appropriate symbols for the various receptacles required. See Fig.12. This process calls for coordination with the architectural designer because each appliance and piece of equipment must be known to locate the outlets properly. Many times, the architectural plans don’t convey each appliance and piece of equipment that must be connected. For example, a kitchen waste disposal must be connected with a switch on the wall, yet the waste disposal and switch are seldom shown on the floor plan. The electrical designer should request that the architectural designer list all appliances and equipment in writing or on the drawing for complete coordination. The electrical designer should also check the number of appliances or equipment required. For example, two furnaces may be used in large residences and both will need connections. Also, in commercial buildings, typically more than one heating and/or cooling unit may be used. The electrical designer will need to know the specified voltage and amperage requirements for equipment and appliances. A list of typical appliances that require electrical connections and their requirements are listed in Tables **12 through **17. These requirements will vary among different manufacturers. Manufacturer nameplate ratings should be confirmed and used in computations. 3. Locate the lighting fixtures, using the appropriate symbols to represent each. See Fig.** The electrical designer should make a list of types of luminaires and the load requirements. This list will be used later when grouping circuits. It may also be included in the specifications or on the drawings as a luminaire (light fixture) schedule. The fixture schedule will be used by the electrical estimator when determining the cost to be charged for the work; next by the electrical purchasing agent when the fixtures and materials are ordered for the project; and then by the electrician who actually installs the work. The architectural designer or even the building owner may decide the fixtures that are to be used, often with assistance of the electrical designer. See Fig.14. On commercial, industrial, and institutional projects, the electrical designer will need to determine the number and types of lighting fixtures required to provide adequate lighting levels. A discussion of lighting system design is provided in Section 20. 4. Lay out the switches required to control the lights, appliances, equipment, and any desired receptacles. The discussion of switches outlines where they are most commonly used and the symbols used. See Fig.13 5. Locate the panelboard in a convenient location. The location must be accessible (e.g., not in a closet or storage room, unless dedicated). Follow the requirements out lined earlier. See Fig.12. 6. Layout circuiting for large appliances and equipment served by an individual branch circuit. Examples of individual branch circuits would be circuits from the panel board to a dishwasher (120 V), electric clothes dryer (120/240 V), an electric oven or range (120/240 V), or an electric water heater (240 V). 7. Layout circuiting for lighting and convenience receptacles on general purpose (lighting) branch circuits. Usually 15 A and 20 A general purpose branch circuits are used for convenience receptacles, luminaires, and small appliances. See Fig.15. The designer must comply with code requirements. In practice, a good designer tends to be a little more conservative, generally limiting a 15 A branch circuit to 1000 to 1200 VA and a 20 A branch circuit to 1300 to 1600 VA. Convenience receptacles are generally limited to about 6 to 8 on a circuit. This allows the circuit to take additional loads, such as when higher wattage lamps are used to replace those originally installed and calculated. Because more and more small appliances and equipment are being purchased and connected to receptacles, the designer must anticipate future requirements. Such a layout allows for the extension of a circuit if it’s necessary to add a light or a convenience receptacle instead of adding a whole new circuit from the panelboard. If it’s possible that the occupant will desire to install individual air conditioners, an individual branch circuit may be desired. 8. Lay out the panel circuits, either on the drawing or in a table as shown in Fig.16. In large designs, with more than one panel, this provides the electrician with a schedule of what circuits will be served from what box. Although a panelboard layout is not often done for a residence, it’s helpful to both the electrician and the designer if one is included. For commercial projects, a panel layout is almost always included. Load Computations Load computations involve computing the demand load for a building system or a distribution system extending from a panelboard. This load includes the total of all general lighting, appliance, and equipment loads in the building. The demand load allowed by the code takes into account that all of the electrical connections won’t be in use at one time. As a result, the demand load is not a total of all connected loads, but rather a fraction of the connected loads. Code prescribes methods of computing the demand load, which trims down the total connected load to a safe, more reasonable level. Demand load computation and panelboard layout work sheets such as the sheets provided in Fig.17 assist the de signer in these computations. General considerations in demand load computations follow. 1. Compute the general lighting load. This is calculated for all types of occupancies based on the unit load given in the table (in watts) times the square footage of the building. (For this exercise it’s based upon Code specified 3 VA/ft^2 for residences). The floor area is determined using the outside dimensions of the building involved and the number of stories. For dwellings, don’t include any open porches, garages, or carports. Any unfinished or un used spaces don’t have to be included in the square footage unless they are adaptable for future use. For the (hypothetical) residential system example, the outside dimensions (excluding garage) of the garage are: The minimum general lighting load, based on the specified 3 VA/ft^2 for residences: 2. Compute the appliance and laundry circuit load. Code requires at least two 20 A appliance branch circuits for the kitchen. The load is based on 1500 VA (from Code) for each appliance branch circuit in the kitchen. In addition, one 20 A circuit is required for laundry room appliances. For the residential system example, this results in a total of three 20 A branch circuits for appliances: 3. Subtotal the general lighting, appliance, and laundry branch circuit loads. For the residential system example: General lighting 6642 VA Appliance and laundry circuits 4500 VA Subtotal 11 142 VA 4. The demand load allowed by the Code takes into account that all of the electrical connections won’t be in use at one time. Although there are limits to this reduction for certain types of occupancies, in a dwelling the first 3000 VA are taken as 100%, and from 3000 to 120 000 VA, only 35% of the load is calculated. Fig.15 Circuit layout and numbering for design example. For the residential system example, the load subtotal is 11 142 VA, so: First 3000 VA at 100% 3000 VA Remaining 8142 VA at 35% 2850 VA Total demand load 5850 VA _ 4500 VA Appliance and laundry load _ 3 circuits _ 1500 VA 3 VA ft^2 _ 2214 ft^2 _ 6642 VA 61 ft 6 in _ 36 ft, there are 2214 ft^2 of floor area The loads of all other appliances and equipment (motors) must be added to this demand load to determine the total service load on the system. 5. To determine the appliance and equipment load, all appliances and equipment that won’t be on the lines discussed above must be listed along with their electrical requirements. Although typical ratings are given in Tables **12 through **16, nameplate ratings from manufacturers' data should be used in the design. For the residential system example, the following is a list of fixed appliances and equipment and their ratings from manufacturer's data: Water heater 3800 VA Clothes dryer 4400 VA Dishwasher 1000 VA Range 11 700 VA Waste disposal 1000 VA Air conditioner 6300 VA Garage door opener 1000 VA The demand load for an electric range, consisting of an oven and a cooktop unit, is taken from manufacturer's data. For the residential system example, an electric range with a rating of 11.7 kW, the demand load would be 8 kW (or 8000 VA): Electric range demand load _ 8000 VA The demand load for the clothes dryer is the total amount of power required according to the manufacturer's data. For the residential system example, the full 4400 VA must be used in the calculation: The demand for fixed appliances (other than the range, clothes dryer, and air conditioning and space heating equipment) is taken as 100% of the total amount required, except that when there are four or more of these fixed appliances (other than those omitted), the demand load can be taken as 75% of the fixed appliance load. For the residential system example, there are three fixed appliances: the water heater at 3800 VA, the dishwasher at 1000 VA, and the waste disposal at 1000 VA. The total of the ratings is 4800 VA, also the demand load. Motors, such as those used in central air conditioners, have their demand loads calculated as 125% of the motor rating. For the residential system example, the air conditioner is rated at 6300 VA and the garage door opener is at 1000 VA. The total demand load will be 7300 VA _ 125% _ 9125 VA. The total demand load for all of the lighting and appliances is then tabulated. For the residential system example: General lighting, appliances, and laundry 5850 VA Electric range 8000 VA Clothes dryer 4400 VA Fixed appliances 5800 VA Motor (air conditioner/opener) 9125 VA Total demand load 33 175 VA Service Entrance Design The service entrance conductors and equipment are designed based on the computed total demand load. 1. The minimum service entrance size is found by dividing the demand load for the building by the voltage serving the building. Most commonly, 240 V service is used. For the residential system example, the total demand load of 33 175 VA is divided by the 240 V service for a mini mum service entrance of 143 A: The 138 A computed demand load would be rounded up to the nearest commercially available panelboard rating. From Tbl. 4 ( Section 18), a panelboard with a 150 A _ 138.2 or 138 A minimum service entrance _ 33 175 VA>240 V _ 9125 VA Motor 1: air conditioner opener 2 demand load Fixed appliances demand load _ 5800 VA Clothes dryer demand load _ 4400 VA rating would be selected. A 200 A panelboard would be a better choice if expansion for additional future circuits for appliances and other equipment was anticipated. Assume a 150 A panel is deemed adequate. 2. The next step is sizing the service entrance conductors and any conductors between the service equipment and the branch overcurrent device (circuit breaker or fuse). The feeder size is based on the computed demand load. The size is then selected from Tbl. 12 (in Section 18). For the residential system example, using XHHW (75°C, 167°F) aluminum service entrance conductors and a 150 A load, as calculated: 3. The size of the neutral conductor may be determined as 70% of the demand load calculated for the range plus all other demand loads on the system. For the residential system example, the neutral feeder demand load would be: Select the size of the neutral conductor for a 128 A neutral feeder demand load from Tbl. 12 (in Section 18): 2/0 AWG, XHHW, aluminum conductor For the residential system example, the three service conductors are: Two No. 3/0 AWG and one 2/0 AWG aluminum conductors 6642 VA 120 V _ 56 A _ 128.2 A Neutral net computed demand load _ 30 778 VA>240 V Neutral demand load 30 778 VA All other demand loads 25 175 VA Range load (8000 W _ 70%) 5600 VA aluminum 150 A feeder demand load, 30 AWG, XHHW, Circuit Design Circuit design involves ascertaining the number and rating of circuits needed in the panelboard. It generally involves the following steps: 1. Determine the minimum number of lighting circuits by dividing the general lighting load by the voltage, finding the amperage required and dividing the amperage into circuits. For the residential system example, the general lighting load was calculated above as 6642 VA and the voltage used for the lighting is 120 V. Therefore: Because the general purpose branch circuit size is limited to 80% of the rating, four 20 A branch circuits for a total of 64 A, or five 15 A branches for a total of 60 A, are needed. This is the minimum number of branch circuits required to serve the residence. In laying out the circuits, almost all designs will have more circuits than the minimum number required. This is because most designers will limit each circuit to fewer receptacles, lights, or a combi nation of receptacles and lights. 2. Lay out and number all branch circuits on the drawing. Bear in mind that, in most cases, all of these general use receptacles and all lighting will use 120 V service. In large commercial applications, 277 V may be used for interior lighting. Note that each circuit is numbered. _ 5 circuits For 15 A circuits: 56 A>(15 A _ 80%) _ 4.7 _ 4 circuits For 20 A circuits: 56 A>(20 A _ 80%) _ 3.5 For the residential system example, there are a total of 14 circuits. Total poles required 22 poles 4-240-V circuits (4 _ 2 poles) 8 poles 14-120-V circuits 14 poles Panelboard Selection Select the panelboard based on the number of circuits and the required amperage. Be certain that all the pole space is not taken up so there is room for expansion. Keep in mind that each 120 V circuit takes up one pole (for a one-pole circuit breaker), while each 240 V or 120/240 V circuit takes up two poles. For the residential system example, there are: A 150 A, 24-pole panelboard should be selected (from Tbl. .4, Section 18). Prev: Electrical
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