Electrical Service and Utilization PDF

Service and Utilization direct burial installation with no concrete encasement. NEMA (National Electrical...

Service and Utilization direct burial installation with no concrete encasement. NEMA (National Electrical Manufacturers Association) Type I - is manufactured with a thinner wall and is intended for ELECTRIC SERVICE encasement in a minimum of 2 in. (50 mm) of concrete. The codes and standards that apply to electric service include: Plastic conduit (PVC) - plastic 1. National Electrical Code 2. National Electrical Safety Code Nonmetallic conduit is most frequently used 3. Standards of the utility supplying electrical without concrete encasement for low-voltage and services. signal wiring and with encasement for high-voltage wiring. It offers several advantages over steel *Electrical construction work on private property is conduit for underground work, such as lower cost usually at the owner’s expense. and freedom from corrosion. Beautification - to decrease the objectionable Handholes are used for low-voltage power and appearance of much of their equipment. signal cables and for runs with a small number of cables. Factors to consider when building overhead or underground: Manholes are used for high-voltage cables and Length of the service run where large duct banks must be accommodated. Type of terrain Service voltage Type SE - basic service entrance cable, Weather conditions constructed with a moisture- and flame resistant covering. OVERHEAD SERVICE SE type U or simply USE - When it is provided low cost with moisture proofing for underground use. Cost saving ranged from 10% to 50% type UF (underground feeder) - Underground UNDERGROUND SERVICE cable for other than service runs. Preferable in areas with severe weather conditions, called heavy loading areas. attractiveness, service reliability, and long SERVICE EQUIPMENT life. high cost Transformer - interposed between the high-voltage UNDERGROUND WIRING incoming utility lines and the secondary service conductors The methods available for underground wiring are: - required whenever the building utilization Direct burial - low cost and ease of voltage is different from the service installation. voltage. Installation in Type I, concrete-encased duct - high strength and permanence, but at the highest price. TRANSFORMERS - device that changes or transforms Installation in Type II, direct burial duct alternating current (ac) of one voltage to - median cost but little strength. alternating current of another voltage. Nonmetallic duct (conduit) - intended for Step-down transformer - has a larger number of underground electric use is commercially available turns in its primary winding than in its secondary in two wall thicknesses. winding. NEMA (National Electrical Step-up transformer - has a larger number of turns Manufacturers Association) Type II - with a heavy in its secondary winding than in its primary winding. wall provides the physical protection required for Low or secondary voltages - 120, 208, 240, 277, greater weight, and larger size than liquid-filled and 480 V. units. High or primary voltages - 2400, 4160, 7200, TRANSFORMER VAULTS 12,470, and 13,200 V. - a fire-rated enclosure provided because of the possibility of fire caused by rupture of Load center transformers are installed in unit an oil-filled transformer case. substations, both indoor and outdoor. Distribution transformers are mounted on a pole SERVICE EQUIPMENT or on a concrete pad outdoors. ARRANGEMENTS AND METERING Substation transformers are large and are always Metering must be provided ahead (electrically) of concrete-pad-mounted. the building’s service entrance switch(es) *a transformer is specified by type, phase, voltages, If high-voltage service is purchased, then kVA rating, sound power level, and insulation class transformers and all other equipment beyond the service connection will be *A service transformer bank is necessary when furnished by the owner. the facility utilization voltage is different from the If low-voltage service is purchased, all the utility voltage. equipment necessary to provide such low voltage is furnished by the utility. Single meter - single-occupant building TRANSFORMERS OUTDOORS Submetering - apartment houses The advantages of an outdoor transformer up installation are: SERVICE SWITCHES No enclosed building space required Reduced probability of a noise problem The purpose of electric service switches is to within the building disconnect the normal service to the building. Lower first cost Ease of maintenance and replacement SWITCHES No interior heat production Opportunity to use low-cost, long-life, Mechanical switches close and open an electric oil-filled units circuit by physically moving two electrical conductors into contact with each other to close the * The most popular type of exterior transformer circuit and physically separating them to open the installation is the pad mount. circuit. (a) Oil-Insulated Transformers - most oil-filled Solid-state switches perform the same electrical transformers must be installed in a fire-resistant switching function but by a completely different vault. process, without moving parts. (b) “Less-Flammable” Liquid-Insulated TYPES OF SWITCHES Transformers - Transformers rated 35 kV or less that are insulated with a liquid whose fire point is 1. Single-pole single-throw switch not less than 300ºC may be installed indoors - one output and one input without a vault. - on-off switching - Basic function: Controls a single circuit (c) Non Flammable Fluid-Filled Transformers - from one location. do not require a vault unless the voltage is very - Example: A simple light switch in your high, require a sump or catch basin of sufficient home. capacity for all of the contained liquid. 2. Two-pole single-throw switch (d) Dry-Type Transformers - transformers of - Basic function: Controls two separate choice in the majority of indoor installations, despite circuits from one location. When the switch having shorter life, higher losses, higher noise level,  is in the "on" position, it connects both Summarizing, the following adequately describes a circuits. When it's in the "off" position, it switch: switch, HD, 3P & SN, 200A/150AF, 600 V, in disconnects both circuits. NEMA 12 enclosure. This “translates” as a - Example: Used in electrical panels to heavy-duty switch, 3 poles and solid neutral, 200- control two different branches of a circuit amp rating with 150-amp fuses, 600-V rating, in an industrial use enclosure. 3. Three-pole and solid-neutral (3P and SN) Switch. CONTACTORS - Basic function: Connects or disconnects - A contactor is a switch, a contactor uses three circuits independently, with a shared contact blocks of silver-coated copper, neutral. A 3P and SN switch is like a which are forced together to make (close) three-way switch that controls three or are separated to break (open) a circuit. separate circuits instead of just one. - a contactor whose contacts are open when - Example: Commonly used in motor control the coil is not energized is referred to as applications. normally open (NO), whereas a contactor whose contacts are closed when de 4. Single-pole double-throw switch also energized is referred to as normally called, in small sizes, a 3-way switch). closed (NC) - Basic function: Controls a single circuit from two locations. It has three terminals: SPECIAL SWITCHES one common terminal and two traveler terminals. - Example: Used to control a light from both (a) Remote-Control (RC) Switches - contactor that ends of a hallway. latches mechanically after being operated. 5. Single-pole double-throw switch with (b) Automatic Transfer Switch - on failure of center “off” position (in control work normal service it automatically transfers to the called a hand-off automatic switch) emergency service. - Basic function: Controls a single circuit from two locations, with a neutral "off" c) Time-Controlled Switches - switches whose position. operation is time-based. - Example: Used in control systems to provide a "manual-automatic" switch. SOLID-STATE SWITCHES, 6. Use of two single-pole, double-throw (3- PROGRAMMABLE SWITCHES, way) switches for switching a lighting MICROPROCESSORS, AND circuit from two locations. PROGRAMMABLE CONTROLLERS - Two 3-way switches that can provide more flexibility in lightning control from two locations. Solid-state switch - electronic device with a - Operation: Flipping either switch will conducting state and a nonconducting state, change the path of electricity, allowing the corresponding to a conventional switch in its closed light to be turned on or off from either and open positions. location. If a small, programmable memory circuit, in the form of an erasable programmable read-only memory current rating is the amount of current that the (EPROM) chip is added, the device becomes a switch can carry continuously and interrupt safely. programmable time switch. Switches for normal (occasional) use in lighting and A programmable controller is an electronic device power circuits are called general-duty safety that uses a programmable memory chip for internal switches. storage of instructions that in turn implement specific functions such as logic, timing, counting, Switches for frequent interrupting, high-fault and so on. currents, and ease of maintenance are rated HD for The internal control/memory is frequently referred to heavy duty. as a microprocessor; with respect to the overall programmable controller, it acts as a central processing unit (CPU).  Air-circuit breakers Programmable controllers have all but re-placed (1) molded-case breaker the ubiquitous hard-wired relay panel in - consist of a complete mechanism applications. encased in a molded phenolic case. EQUIPMENT ENCLOSURES (2) large air-circuit breaker - Is a small structure, cabinet, or vault that - a more complicated and highly adjustable used to house and protect the equipment device used in larger applications that can necessary for processing wireless be used in applications that preclude the communication signals. use of molded-case breakers. (c) Characteristics of Fuses and Circuit NEMA Designation: Description Type Breakers 1 General-purpose Although both fuses and circuit breakers are circuit-protective devices, their characteristics differ 2 Dripproof markedly, as seen in the following comparisons. 3 Dust-tight, rain-tight, and sleet-resistant Fuses—Switch-and-Fuse Combination 3R Rainproof and sleet-resistant ADVANTAGES 3S Dust-tight, rain-tight, and sleet-proof Simple and foolproof Constant characteristics (no aging) 4 Watertight and dust-tight Initial economy Very high interrupting capacity (IC) 4X Watertight, dust-tight, corrosion-resistant No maintenance 7-9 Hazardous Instantaneous; energy-limiting 12 Industrial use, dust-tight, and drip-tight DISADVANTAGES Fuses are single-pole only Necessity for storage of replacement fuses CIRCUIT-PROTECTIVE DEVICES Nonrenewable (one-time operation) Nonadjustable Nonindicatinga To protect insulation, wiring, switches, and other No electric or remote controla apparatus from the destructive effects of overload Not trip-free and short-circuit currents, an automatic means for opening the circuit is required: Circuit Breakers ADVANTAGES (a) Fuses - The fuse is a simple device consisting Usable as switches of a fusible link or wire of low melting temperature Multipole that, when enclosed in an insulating fiber tube, is No storage of replacements required called a cartridge fuse (available up to 6000 A and Resettable 600 V) and, when enclosed in a porcelain cup, is Indicates trip known as a plug fuse (rated 5 to 30 A, 150 V to Trip-free ground, maximum). Remote-control potential Adjustable b) Circuit Breakers - electromechanical device that performs the same protective function as a fuse DISADVANTAGES and, in addition, acts as a switch, it can be used in Low to medium IC, except for special units lieu of a switch-and-fuse combination to both Periodic maintenance required protect and disconnect a circuit. High initial cost - The thermal trip acts on overload, Complex construction changes with age whereas the magnetic trip acts on a short circuit. SWITCHBOARDS AND SWITCHGEAR switchboard is a large, free-standing assembly of switches and fuses (and/or circuit breakers), which normally provides switching and overcurrent protection to a number of circuits connected to a PRINCIPLES OF ELECTRIC LOAD single electric source. CONTROL - The key to energy conservation and Modern switchboards are all dead front; that is, electric demand limitation is load control. they have all circuit breakers, switches, fuses, and live parts completely enclosed in a metal structure. INTELLIGENT PANELBOARDS Switchgear - low voltage switchboards with large circuit breakers and all high-voltage equipment - This central panel-mounted controller can (above 600 V). also accept signal data from individual remote or network sources and can a building-type switchboard - when molded-case provide status reports, alarm signals, circuit breakers are utilized in a switchboard. operational logs, and local bypass and override functions. When switchgear is to be installed outdoors, one of three methods is employed: 1. build a small structure to enclose normal indoor gear, ELECTRIC MOTORS 2. utilize weatherproof outdoor gear, 3. or utilize switchgear that is built into its (a) Direct-Current Motors - These motors are not own exterior enclosing structure normally used in building work. (b) Alternating-Current Motors - These motors fall UNIT SUBSTATIONS (TRANSFORMER into three general classifications: poly-phase LOAD CENTERS) induction motors, poly-phase synchronous motors, and single-phase motors. Within these categories unit substation or a load-center substation there are further subdivisions. - An assembly, comprising a primary voltage switch and-fuse or circuit breaker, (c) Squirrel-Cage Induction Motors - invented by a step-down transformer, meters, controls, Nikola Tesla. Squirrel-cage motors are buswork, and secondary (low-voltage) manufactured in four different NEMA designs to switchgear. meet different application requirements. -. Its function is to accept an incoming high-voltage power supply, transform the (d) Electric Motor Energy Considerations high voltage to a voltage that can be premium efficiency or PE motors - single-speed, utilized in the facility, and distribute the poly-phase, squirrel-cage induction motors, designs low-voltage power through associated A and B, continuous rating, and operating at low-voltage (secondary) switchgear. 230/460 V, 60 Hz. PANELBOARDS - same function as a switchboard except on a smaller scale. It accepts a relatively large block of power at some downstream point in a system and distributes it in smaller MOTOR CONTROL EQUIPMENT blocks. - The panelboard components are mounted combination starter - this disconnect switch may inside an open metal cabinet called a be combined with the starter into a single unit. backbox. Backbox WIRING DEVICES: GENERAL DESCRIPTION - is prefabricated with knockouts at the top, bottom, and sides to permit connection of wiring devices - includes all devices that are conduits carrying circuit conductors. normally installed in wall outlet boxes. WIRING DEVICES: RECEPTACLES SURGE SUPPRESSION Receptacle The full term for this aspect of power conditioning is - a contact device installed at the outlet for transient voltage surge suppression (TVSS). the connection of a single attachment plug. Because transient voltages, also called surges or - properly called a duplex convenience spikes, can cause major physical damage to receptacle or duplex convenience outlet computer systems and other types of electronic - identified by the number of poles, the equipment. number of wires, and whether the device is designed for connection of a separate (a) Terminology equipment ground. Avalanche diode - A solid-state device placed in - The common wall outlet is properly called parallel with a protected load. a convenience receptacle outlet, a receptacle outlet, or a convenience outlet. Clamping voltage - The voltage at which a shunting device begins to conduct an incoming voltage surge. The lower a device’s clamping * The basic components of the entire system are voltage, the better its suppression action. the control relay and the switches. EMI/RFI rejection - A measure of the attenuation, * power line carrier (PLC) - power line carries the by a TVSS device, of electromagnetic interference control signal (EMI) and RFI. Gas tube surge protector - A high-energy shunt device, typically found in high-capacity, good-quality POWER CONDITIONING TVSS units that are subject to large surges. - relatively new electrical term that describes the process of converting utility-supplied Isolation transformer - A transformer with two electrical power, which is frequently separate windings and no conductive electrical characterized by transient surges, spikes, connection between them. radio frequency noise, and voltage fluctuation, to a pure (noiseless), Joule rating - A device’s joule rating is a measure accurately voltage-regulated sinusoidal of its capacity to absorb and dissipate heat waveform, normally referred to as generated by its action. SI unit of energy. computer-grade power. - due to their extreme sensitivity to Maximum surge current - A measure of a device’s fluctuations in the power supply voltage ability to divert and dissipate surge current without and particularly to random high-frequency failing. voltages superimposed on the power supply voltage, known as radio frequency Metal-oxide variable resistor (MOV) - A shunt interference (RFI) or simply radio noise device somewhat slower than an avalanche diode but with much higher energy absorption capability SOURCE OF DISTURBANCE and higher clamping voltage. - Utility power systems are designed to maintain voltage and frequency within Response time - measure of the rapidity with which certain limits on an average, long-term a TVSS device begins to clamp a voltage surge. basis. Sinewave tracking - term of uncertain meaning 1. Voltage variations - caused by short-time found frequently in manufacturers’ literature. current drain, such as during motor starts, deliberate voltage reductions (brownouts) 2. Electrical noise - low-amplitude, higher-frequency voltage that, when superimposed UNINTERRUPTIBLE POWER SUPPLY on the power line, results in a distorted waveform. 3. Transients - large, short-duration voltage power-conditioning equipment can supply clean variations, also known as surges or spikes utility power; it cannot, however, supply any power during a utility outage. An uninterruptible power supply (UPS) is an arrangement of normal and backup power supplies that transfer a facility’s critical load from the normal to the backup supply in so short a time that no computer malfunction results. ELECTRICAL EQUIPMENT RATINGS programmable microprocessors and effective (a) Voltage - The voltage rating of an item of sensors and transducers have been added to basic electrical equipment is the maximum voltage that UPS systems to produce what is known as can safely be applied to the unit continuously. intelligent UPS equipment. (b) Current - The current rating of a device is Emergency systems are intended to supply determined by the maximum operating temperature electric power to equipment essential for human at which its components can operate at full load. safety upon interruption of the normal power supply. Each electric wiring system is inspected at least twice by the local inspection authorities: once after INTERIOR WIRING SYSTEMS raceways (roughing) have been installed and before the wiring and closing-in of walls, and once after the Electrical power system - primary purpose of a entire job is complete. system is to distribute electrical energy Electrical signal or a communication system - purpose is to transmit information Wiring and Raceways * how to construct a distribution system that safely provides the energy required at the desired THE MAJOR COMPONENTS OF A BUILDING locations? electrical system can be grouped into three major - isolate all electrically conducting elements categories: from the building structure, except at those specific points, such as wall receptacles, wiring and raceways - conductors and where contact is desired. raceways of all types (a) Exposed Insulated Cables - cable power-handling equipment - construction itself provides the necessary electrical transformers, switchboards, panelboards, insulation and mechanical protection large switches, and circuit breakers (b) Insulated Cables in Open Raceways (Trays) - utilization equipment - utilization intended for industrial applications, and it relies on equipment such as lighting, motors, both the cable and the tray for safety. controls, and wiring devices. (c) Insulated Conductors in Closed Raceways - most general type and is applicable to all types of facilities. In general, the raceway is installed first SYSTEM COMPONENTS and the wiring is pulled in or laid in later. * the distribution equipment is largest at the (d) Combined Conductor and Enclosure - cover supply point, and decreases in size on its way to the all types of factory-prepared and farthest utilization points. factory-constructed integral assemblies of conductor and enclosure. * in collection systems, piping is smallest at the initial collection points, growing larger (in steps) as CONDUCTORS the quantities of fluid increase. Electrical conductors (“wires”) are the means by * National Electrical Code (NEC) of the National which current is conducted through the electrical Fire Protection Association (NFPA) defines the system, corresponding to the piping of a hydraulic fundamental safety measures that must be followed system. The standard of the U.S. wire and cable in the selection, construction, and installation of industry for round cross-section conductors is the electrical equipment and systems. This code is American Wire Gauge (AWG). used by all inspectors, electrical designers, engineers, contractors, and operating personnel. a single insulated conductor No. 6 AWG or larger, LIGHT-DUTY BUSWAY, FLAT CABLE or several conductors of any size assembled into a ASSEMBLIES, AND LIGHTING TRACK single unit, are referred to as cable. Single conductors No. 8 AWG and smaller are (a) Light-Duty Plug-In Busway -. Lightduty called wire. busways are rated from 20 to 60 A at 300 V, in 2- and 3-wire construction. Their application is The AWG numbers run in reverse order to the size principally for direct connection (with overcurrent of the wire—that is, the smaller the AWG number, protection) of light machinery and industrial lighting. the larger the size. (b) Flat-Cable Assemblies - consisting of two, conductor current–carrying capacity, or three, or four No. 10 AWG conductors is ampacity, is determined by the maximum safe field-installed in a rigidly mounted standard 15 8-in. operating temperature of the insulation used on the (41 mm) square structural channel. conductor. (c) Lighting Track - a factory-assembled channel * Most conductors are covered with some type of with conductors for one to four circuits permanently insulation that provides both electrical isolation and installed in the track (NEC Article 410-R). Power is a degree of physical protection. Additional physical taken from the track by special tap-off devices that shielding, where necessary, is provided by a jacket contact the track’s electrified conductors and carry placed over the insulation. the power to the attached lighting fixture. * jurisdictions in the United States have banned the use of aluminum wire in branch circuitry. CABLE TRAY - a continuous open support for approved cables. The advantages of this system are free-air-rated cable ampacities, easy installation and maintenance, and relatively low cost. The CONDUCTORS FOR GENERAL WIRING disadvantages are bulkiness and accessibility - The NEC lists the wire types that are requirements. generally installed in raceways and are referred to by the term building wire. These wires consist of a copper conductor covered with insulation and, in some DESIGN CONSIDERATIONS FOR instances, with a jacket RACEWAY SYSTEMS The function of a raceway in such systems is BUSWAY/BUSDUCT/CABLEBUS largely the same as it is for power wiring: protection and isolation of the wiring. - A busway (busduct) is an assembly of copper or aluminum bars in a rigid metallic design considerations for power systems housing. Its use is almost always raceways for data and communication systems preferable, from an economic viewpoint. include: - In the case of a heavy current requirement, I. Number, type, and location of data-processing the alternatives to using busways are to terminals use paralleled sets of round conductors II. Networking requirements: or a single large conductor. Paralleled III. Number, location, and characteristics of major sets of conductors are almost always more peripheral devices, such as mass storage, printing, expensive than a busway of similar current and plotting capacity because of the high installation IV. Location and type of major subsystems, such as cost of multiple conduits. computer-aided design/manufacturing space V. Location of presentation spaces that require - Cablebus is similar to ventilated busduct, interconnection to computer networks except that it uses insulated cables instead of busbars. It carries the ampacity rating of its cables in free air. STEEL CONDUIT - It does not require painting. - drawback is its deleterious effect on many The purpose of conduit is to: types of concrete, causing spalling and 1. Protect the enclosed wiring from cracking when embedded mechanical injury and damage from the - rdifficulties frequently encountered are surrounding atmosphere mechanical freezing of threaded joints 2. Provide a grounded metal enclosure for (because of thread deformation) and the wiring in order to avoid a shock hazard difficulty in obtaining electrical contact with 3. Provide a system ground path grounding straps 4. Protect surroundings against a fire hazard as a result of overheating or arcing FLEXIBLE METAL CONDUIT of the enclosed conductors - consists of an empty, spirally wound, 5. Support the conductors interlocked armor steel or aluminum raceway. It is known to the trade as Greenfield and is covered in NEC Article * the NEC generally requires that all power wiring 350. be enclosed in a rigid metallic corrosion-resistant - used principally for motor connections and conduit. other locations where vibration is present, where movement is encountered, or where physical obstructions make its use three types of steel conduit necessary. - for wiring inside metal partitions. When 1. Heavy-wall steel conduit, also referred to covered with a liquid-tight plastic jacket, it simply as rigid steel conduit; covered by is suitable for use in wet locations. In this NEC Article 346. use the threaded fittings configuration, it is most often known by the trade name Sealtite. 2. Intermediate metal conduit, usually referred to as IMC; covered by NEC Article 345. use the threaded fittings NONMETALLIC CONDUIT - classified under NEC Article 347, is made 3. Metallic tubing, normally known as EMT from materials such as fiber, or thin-wall conduit; covered by NEC asbestos-cement, soapstone, PVC, and Article 348. uses set-screw and pressure high-density polyethylene. it may be used fittings. without restriction in nonhazardous areas within the physical limitations of the the following rules should be observed and material involved. included in all specifications for conduit work in concrete slabs: 1. Conduits shall have an outside diameter (OD) no greater than one-third the slab thickness, as SURFACE METAL RACEWAYS measured at its thinnest point. (METALLIC AND NONMETALLIC) 2. Conduits running parallel to each other shall be These raceways are covered in NEC Article 352. spaced not less than three times the OD of the Surface metal raceways and multioutlet assemblies largest conduit center-to-center. may be utilized only in dry, nonhazardous, non 3. Conduit crossings shall be as close to a right corrosive locations and may generally contain only angle as possible. wiring operating below 300 V. Such raceways are 4. Minimum cover over conduits shall be ¾ in. (19 normally installed exposed, in places not subject to mm). physical injury. ALUMINUM CONDUIT OUTLET AND DEVICE BOXES - being even lighter than EMT - galvanized stamped sheet metal. - aluminum has better corrosion resistance - used in buildings to protect electrical wiring in most atmospheres; and hold devices like switches and outlets. - it is nonmagnetic, - giving a lower voltage drop; - it is non sparking, FLOOR RACEWAYS are supplied by a second set of rectangular raceways called feeder ducts, To bring the various electrical and communication usually laid at right angles to the services to the user, in the absence of any sort of distribution ducts. overall floor raceway system, the installing contractor has one of four choices: - single-level underfloor duct system, the distribution and feeder ducts are on the 1. Channel the floor and install a conduit in the same level, and the interwiring between channel, connecting it to the nearest wall outlet. them is accomplished in junction boxes. Patch the chased portion of the floor. - labor intensive - Because the initial cost of a full underfloor system is high, an alternative arrangement 2. Install a surface floor raceway. The usefulness of utilizes only feeder ducts this technique is very limited because it presents a tripping hazard and problems with routine floor - A two-level underfloor duct system is cleaning. essentially the same as a single-level - unsightly and presents a safety hazard system except that the distribution ducts and feeder ducts are on different levels. 3. Drill through the floor twice and connect the new outlet to a nearby existing floor or wall outlet via a conduit on the underside of the floor slab. Floor Underfloor ducts may be cast into the structural slab penetrations must be fireproof. in lieu of being installed in fill or topping, but the - disturb the occupants below, slab must be designed to accommodate them. The use of a fill or topping on the structural slab for an 4. Drill through the floor and run a conduit in or on underfloor duct installation has these advantages: the ceiling below. When using this technique, special poke-through fittings are available that 1. Ducts can be run in any direction, without conflict restore the fire rating of the slab with structural elements. - disturb the occupants below. 2. Finishing is simplified. 3. Coordination is simplified. 4. Formwork and construction sequence are The NEC recognizes three types of in-floor simplified. raceways: The disadvantages are: Underfloor raceways—Article 354 1. Additional concrete increases costs directly by Cellular metal floor raceways—Article 356 increasing the weight of the structure. This is Cellular concrete floor raceways—Article 358 particularly expensive in seismic designs. 2. The building height may be increased. All three types are applicable to all types of structures, and none may be used in corrosive or hazardous areas. The fundamental difference between them is that underfloor raceways are added on to the structure, whereas cellular floor CELLULAR METAL FLOOR RACEWAY raceways are part of the structure itself—and therefore have a pronounced effect on architectural The underfloor duct system described previously coordination. is best applied to rectilinear arrangements. More free-flowing arrangements, such as those found in UNDERFLOOR DUCT office landscaping layouts, require a fully accessible - simply an arrangement of parallel floor—if the floor is to be used for electrification. rectangular metal or heavy plastic This may be provided by a cellular (metal) floor that raceways laid on the structural slab and is an integrated structural/electrical system. The covered with concrete fill. floor can be partially or completely electrified. - Access to the wiring in these distribution Three types of wiring systems generally run in ducts is via inserts that connect to separate floor cells and header ducts. openings in the ducts and terminate in 1. electric power, floor fittings for both power and signal/data 2. data-transmission wiring, wiring. Cable feeds to the distribution ducts 3. telephone and signal systems PRECAST CELLULAR CONCRETE FLOOR RACEWAYS - large capacity, versatility in that each cell is a potential raceway, and flexibility in outlet placement and movement. - A cell is defined in NEC Article 358 as a “single, enclosed, tubular space in a floor made of precast cellular concrete slabs, the direction of the cell being parallel to the direction of the floor member.” FULL-ACCESS FLOOR - applicable to spaces with very heavy cabling requirements, particularly if frequent recabling and reconnection are required. - developed for data-processing areas that require large, fully accessible cable spaces and large quantities of conditioned air. UNDER-CARPET WIRING SYSTEM - this system, which is covered in NEC Article 328, was originally developed as both an inexpensive alternative to an underfloor or cellular floor system and as a means for providing a flexible floor-level branch circuit wiring system. - Essentially, the system consists of a factory-assembled flat cable (NEC type FCC), approved for floor installation only under carpet squares, plus the accessories necessary for connection to 120-V power outlets CEILING RACEWAYS AND MANUFACTURED WIRING SYSTEMS - These systems are actually more flexible than their underfloor counterparts because they energize lighting, provide power and telephone facilities, and even supply outlets for the floor above, - in addition to permitting very rapid layout changes at low cost. - The hung ceiling must consist of lift-out panels because this type of wiring system is not permitted in spaces rendered inaccessible by the building structure.