EC&M’s Electrical Calculations Handbook - Chapter 13 doc

Circuits for Special Loads Designing Circuits for Various Electrical Loads The normal procedure used to determine circuit sizes and characteristics for typical loads is to determine the wiring method and conductor and insulation types to be used from the installation-site environmental data, solve for the cur- rent that will be drawn by the load, select the conductor size, and then determine the raceway size. After all this is completed, a calculation of the voltage drop in the system is normally done to determine that sufficient operating volt- age is available at the terminals of the load for it to operate satisfactorily. A solution using this methodology is shown in Fig. 13-1. When the load can operate for 3 hours or longer, it is considered to be a continuous-duty load, and the circuit must be designed for continuous operation. Figure 13-2 is the solution method used for designing the electrical circuit to a continuous load. There are many types of electrical loads that exhibit special operational characteristics, such as large inrush currents on initial energization. The electrical system must be designed to Chapter 13 335 v Copyright 2001 by The McGraw-Hill Companies, Inc. Click here for Terms of Use. Figure 13-1 Solve for the conductor size and overcurrent device rating for a general load. 336 Figure 13-2 Solve for the conductor size and overcurrent device rating for a general continuous load. 337 permit these unique loads to start and operate successfully while still providing protection from abnormal current flow. For example, when starting a motor, a large inrush current flows until the motor can establish a “back emf” to limit line current. The electrical system must permit this inrush cur- rent to flow to start the motor, but it also must be able to inter- rupt it if the motor does not accelerate to speed in normal 338 Chapter Thirteen Switchboard and transfer switches require working clearance in front, but not to the side. time, lest the motor be damaged or destroyed by excessive I 2 R heat. Then the same electrical system must continuously monitor motor running current to also prevent long-time over- load from damaging the motor. The following figures illustrate calculations required by specific types of appliances in the design of electric circuits to permit unique loads to start and to continue to protect them during operation: Figure 13-3: Air-conditioning equipment Figure 13-4: Household appliances Figure 13-5: Heat tracing with self-regulating cable (see Note below) Figure 13-6: Heat tracing with constant-wattage cable Figure 13-7: Lighting fixtures Figure 13-8: Electrical power receptacles with unidenti- fied loads Figure 13-9: Electrical power receptacle with specific load Note: Some of these examples use actual field experience rather than code requirements for sizing. For example, field experience with self-regulating heat-tracing cable shows that initial energization inrush current of up to 4.9 times full-load rating current occurs for 3 to 5 minutes in dry locations and indefinitely in wet locations. Therefore, the circuit breaker and conductors must be sized to deliver this current, lest the electrical power system fail or trip “off” to this most important load. For this reason, the electrical engineer and designer should make every effort to obtain the appliance manufactur- er’s actual load characteristics for each electrical load. Designing an Electrical System for a Commercial Building The electrical system for a commercial building must be large enough to safely supply the facility electrical loads. While this can be done based on physics, over the last century, the best minds in the electrical industry have contributed information Circuits for Special Loads 339 Figure 13-3 Solve for the conductor size, overcurrent device rating, and dis- connect rating for an HVAC load. 340 Figure 13-4 Solve for the conductor size and overcurrent device rating for a general household appliance. 341 Figure 13-5 Solve for the conductor size and overcurrent device rating for a self-regulated heat tracing cable. 342 Figure 13-6 Solve for the conductor size and overcurrent device rating for a constant-wattage heat tracing cable. 343 Figure 13-7 Solve for the conductor size and overcurrent device rating for a commercial lighting load. 344 [...]... motors, 460 V d One 20-kW cathodic protection system e 50 kW of lighting in the process facility 5 One 10-ton air-conditioner package unit with a 50-hp motor 354 Chapter Thirteen 6 One 6000-kW compressor motor that has an operating power factor of 0.90 at 13. 8 kV The electrical power system to the industrial plant is redundant from the electrical utility, serving the industrial plant at 13. 8 kV; therefore,... shown in Fig 1 3-1 1 This system consists of the following loads: 1 A 3000-ft2 control building 2 A 2000-ft2 switchgear building 3 A 10-kilovoltampere (kVA) redundant uninterruptible power system (UPS) to energize the distributed control system (DCS) 4 The following series of pumps and fans: a P-1101A and P-1101B, 10 horsepower (hp), 460 volts (V) b P-1601A, P1601B, 50 hp, 460 V c Ten 30-hp finfan motors,... loads, such as air-handling loads, operate with either heating or cooling and must be added to the load listing for either case Demand factors can be applied to receptacle loads (NEC Table 22 0-1 3 lists specific factors), to commercial cooking units (NEC Table 22 0-2 0 lists specific factors), to welders (63 0-1 1), to cranes (61 0-1 4), and to motors that will not be running simultaneously (43 0-2 6) The demand... engineer or designer As an example, Fig 1 3-1 0 shows a calculation of the minimum service feeder ampacity required for a commercial building that consists of 100,000 ft2 of office space with one 50-horsepower (hp) air-handling unit, one 250-hp chiller with a branch circuit selection current of 250 A at 480 V threephase, one 10-hp domestic water pump, and one 30-kilowatt (kW) heating coil that provides... quantity of voltamperes per square foot shown in NEC Table 22 0-3 (a) or 125 percent of the actual lighting fixture load, including low-voltage lighting (Article 411), outdoor lighting, and 1200-voltampere (VA) sign circuit [60 0-4 (b)(3)] 2 125 percent of show window lighting [22 0-1 2(a)] 3 Track lighting at 125 percent of 150 VA per lineal foot [22 0-1 2(b)] Receptacle loads consist of 1 100 percent of the quantity... noncoincidental with another need not be considered For example, if a building contains two chilled water pumps, P-1A and P-1B, where P-1B serves as a backup to P-1A, then only one of the two pumps can be expected to be running In this case, the load flow analysis should include P-1A but not P-1B HVAC loads consist of heating and cooling loads Frequently, these heating and cooling loads are noncoincidental,...345 Figure 1 3-8 Solve for the conductor size and overcurrent device rating for general receptacle loads 346 Figure 1 3-9 Solve for the conductor size and overcurrent device rating for specific receptacle loads Circuits for Special Loads 347 to the National Electrical Code (NEC), which now sets the standards for the characteristics of the required electrical system Accordingly,... Therefore, the continuity of electrical power to the process equipment is frequently treated with as much design care as the design for continuity of electrical power to the life safety electrical system branch of a hospital electrical system These two changes, the size of the loads and system redundancy requirements, are shown clearly in the following example of sizing of an electrical system in an industrial... provides reheat dehumidification Designing an Electrical System for an Industrial Facility Engineering and design work in an industrial facility is similar to that in commercial buildings, except that the loads are 350 Figure 1 3-1 0 Solve for service for a commercial building Circuits for Special Loads 351 Motor control center quite different in type and rating The electrical load of the buildings in a typical... typical industrial plant is normally much smaller than the large-process load, and the design reliability considerations are different In commercial buildings, the emphasis is on personnel safety first and equipment functionality second In many industrial plants, the process equipment is unmanned, and process interruptions can cause 352 Figure 1 3-1 1 Solve for service for an industrial plant Circuits for . operation: Figure 1 3-3 : Air-conditioning equipment Figure 1 3-4 : Household appliances Figure 1 3-5 : Heat tracing with self-regulating cable (see Note below) Figure 1 3-6 : Heat tracing with constant-wattage. tracing with constant-wattage cable Figure 1 3-7 : Lighting fixtures Figure 1 3-8 : Electrical power receptacles with unidenti- fied loads Figure 1 3-9 : Electrical power receptacle with specific load Note:. an electrical sys- tem in an industrial plant, as shown in Fig. 1 3-1 1. This sys- tem consists of the following loads: 1. A 3000-ft 2 control building 2. A 2000-ft 2 switchgear building 3. A 10-kilovoltampere