x Features of the Textbook 117 Copyright Goodheart-Willcox Co., Inc. Chapter Outline 8.1 Overcurrent 8.2 Overcurrent Protection Requirements 8.2.1 Overcurrent Device Sizing Factors 8.2.2 Overcurrent Device Location 8.3 Overcurrent Protective Devices 8.3.1 Voltage Rating 8.3.2 Ampere Rating 8.3.3 Fuses 8.3.4 Circuit Breakers 8.4 Conductor Taps Objectives After completing this chapter, you will be able to: • Recognize overloads and short circuits. • Identify the types, ratings, and characteristics of electrical protective devices. • List types of fuses. • Compare fuses and circuit breakers. CHAPTER 8 Overcurrent Protection Technical Terms circuit breaker current-limiting fuse dual-element fuse fuse ground fault nonrenewable fuse overcurrent device overload renewable fuse short circuit shunted single-element fuse tap rules tapped conductor time-delay fuse Oing vercurrent protection limits the current passing through a conductor, prevent- high temperatures that could damage insulation. Consequently, the equipment con- nected to the circuit is protected as well. This chapter examines the causes of overcurrent and protection methods. 8.1 Overcurrent There are basically three types of overcurrent: • Overload—When a device is connected to a circuit (such as plugging a lamp into a receptacle), it increases the current in the circuit. If enough electrical equipment is connected to a circuit, the current will increase to the level needed to support all of the equipment. If there is too much current, the conductor could melt its insulation, causing a fi re hazard. Overloads generally result from current two to six times the rated current. • Short circuit—The current fl owing through circuit conductors is governed by the resistance of the conductor and the resistance of the load connected to the circuit. If the connected load becomes shunted (paralleled with a very low-resistance path), the normally low resistance of the conductor will allow an extremely high current to fl ow through it. This condition will generate amount of heat, causing temperature The instructional design of the text contains student-focused learning tools to help you succeed. These learning tools are described as follows: Chapter Opening Materials Each chapter opening contains a chapter outline, learning objectives, and technical terms. The Chapter Outline provides a preview of topics to be presented in the content. Objectives clearly identify the knowledge and skills to guide your learning as you progress through the chapter. Technical Terms list the key vocabulary that you will encounter as you read the content. Special Features Special features are used throughout the body of each chapter to extend learning and knowl- edge. Code Alerts point out specifi c items from the National Electrical Code. Notes highlight key points related to the content being discussed. Illustrations have been designed to clearly and simply show concepts, processes, and equip- ment in support of the text material. rtt Consequently, the equipm ent co n e m mee r s d u a ui c melt its insulation, causing a fi re hazard. fi t q y , q p e m r r s d u a c , g 64 Modern Commercial Wiring Copyright Goodheart-Willcox Co., Inc. 5.1.1 Rules for Buried Conductors Conduit, cables, and other raceways that are bur- ied must meet specifi c criteria. The conductors must be protected so that damage does not occur. Table 300.5 of the Code lists the minimum burial depth for conductors 1000 volts nominal or less. The table in Figure 5-3 is a condensed version of Table 300.5. Deeper burial is not uncommon due to soil conditions, interferences with other utilities, and structural footings. Cables under buildings must be installed in a raceway, and the raceway must extend past the exterior walls of the building. Further, where buried conductors and cables emerge from the ground, protection must be provided by race- ways that extend from the minimum cover dis- tance to at least 8′ above grade or to the point of entry into a building. See Figure 5-4. At the point where underground conductors emerge from a raceway, the raceway should be fi tted with a bushing or sealed. Regardless of the method of wiring used, all underground installations must be grounded bonded as required by the Codee. Refer CODE ALERT Protection for Underground Raceway According to Section 300.5(D)(4), where the enclo- sure or raceway is subject to physical damage, the conductors shall be installed in rigid metal conduit, intermediate metal conduit, RTRC-XW, or Schedule 80 PVC, or equivalent. to Chapter 10 of this text and Article 250— Grounding and Bonding of the Code. Splices and taps are permitted in buried con- ductors and cables. Be sure to use materials that are suitable for underground use. All splicing materials must be suitable for the conditions and environment in which they are installed.sin Goodheart-Willcox Publisher Figure 5-3. Condensed version of Table 300.5 from the Codee. Minimum Burial Depths (1000 V or Location of Wiring Method or Circuitt Cables or Conductorss or Rigid Metal Conduitt Rigid Conduit Below streets, and lots 24the ″ 24″ 24″ Below building slab or foundation (in raceway) 0″ 0″ 0″ In trench below 2″ thick concrete or equivalentt 18″ 6″ 12″ Under one- or two-family dwelling driveway 18″ 18″ 18″ Conduit extendslargea 8the ′ above ground or to point of entry into building Conduit extends down to minimum cover requirement (18″ max.) Bushing Goodheart-Willcox Publisher Figure 5-4. Conduit must be installed to protect cables and conductors emerging from underground. C opyright Goodhheaarrt-W W underg round installations must be gr ounded aand nd bonded as required b y the C od . R efe r materials must be suitable for the co a nd environment in which the y are G oodheart- W F igure 5-3. Condensed version o f Table 300.5 f rom the Cod . Minimum Buria l Depths (1000 V or lless)) ess L ocati on of W Wiring Metho d or C ircui Ca bl es or Conductor IIMC MC or Rig Rigid M et al CoIf ondui Rigi d NonNonmetallic Cond Below s treets, aalleys,, alleys and pparkingtemperature arking lots 24 ″ 2 24 ″ 2 4 Below b uildin ing slab o r found ation ( in race way) y) 0 ″ 0 ″ 0 ″ I n tren ch be belo w 2 ″ thick concret e or eq uivale alen 1 8 ″ ″ 6 ″ 1 2 Under o o n ne- or two-fam ily dwe lling d rivew eway 18 ″ 1 8 ″ 1 8 222 Modern Commercial Wiring Copyright Goodheart-Willcox Co., Inc. must be independent of the public utility water supply. The water pump must be supplied by emergency power as well. If the engine is air- cooled, the area where the generator is located must have proper ventilation. Exhausted air must be removed to help keep the generator cool and to prevent air recirculation. The emergency power generator must pro- duce voltage identical to the normal building supply voltage. The generator engine must have a fuel supply suffi cient to sustain at least two hours of operation at full demand. Generators should be sized larger than required. A generator should be sized for the amount of power needed to start all loads, rather than the power needed to run all loads. If the generator is based on the running kVA, then each should be multiplied by 1.25 (125%) to obtain a built-in margin of spare capacity. An emergency power generator should be located near the service equipment, in an area NOTE Code requirements for generator installation are contained in Article 445—Generators. with moderate temperatures. If the temperature is too high, the engine will not cool properly. the is too low, engine may have trouble starting. Refer to the generator specifi ca- tions for specifi c temperature limits. Separate Service Emergency power can be supplied from a sepa- rate service. The emergency service and main service should be supplied from separate source transformers. Ventilation blower motors, elevator motors, lighting circuits, and other critical items should be alternated on both systems to ensure partial service, regardless of which supply fails. Supply Tap Ahead of Main An emergency power supply can be tapped ahead of the main service disconnect, Figure 15-5. However, this method only supplies emergency power if there is a power failure at the main dis- connect or main service distribution equipment. If the power failure originates at the utility, the tapped system is useless. Individual Equipment Power Some items, such as emergency lights, are equipped with an individual battery for use in emergency situations. These units should provide 87 1/2% of the battery rating for at least 1 1/2 hours of operation. The units can be plugged into a regular lighting branch circuit, but cannot be controlled by a wall switch. The units must be permanently attached and regu- larly maintained. See Figure 15-6. 15.1.3 Exit Lights Both the Occupational Safety and Health Administration (OSHA) and the National Fire Protection Agency (NFPA) have requirements NOTE A minimum clearance of 2′ should be kept around the generator to allow for maintenance. Goodheart-Willcox Publisher Figure 15-4. This generator installed near a building’s electrical meter provides emergency power to a two-story office building and attached warehouse.