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

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

Figure

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

Switchboard and transfer switches require working clearance in front, but not to the side.

time, lest the motor be damaged or destroyed by excessive I2R

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

Figure

Figure

341

Figure

Figure

343

Figure

Figure

345

Figure

to the National Electrical Code (NEC), which now sets the

standards for the characteristics of the required electrical system Accordingly, this book both points out the require-ments using calculation methodology and provides NEC ref-erence information where the engineer and designer can obtain further information

For every feeder and switchgear bus, panelboard bus, or motor control center bus, a separate calculation must be made; however, these calculations are all very similar, with only the connected loads changing The first of the calcula-tions that must be made is for the service feeder and service equipment

The following six general groups of loads must be consid-ered within commercial buildings:

1 Lighting

2 Receptacle loads

3 Special appliance loads

4 Motor loads other than heating, ventilation, and air-con-ditioning (HVAC) loads

5 The greater of

a HVAC compressor loads and hermetically sealed

motor loads, or

b Heating loads

Lighting loads consist of

1 The greater of 125 percent (for continuous operation) of the quantity of voltamperes per square foot shown in NEC Table 220-3(a) or 125 percent of the actual lighting fixture load, including low-voltage lighting (Article 411), outdoor lighting, and 1200-voltampere (VA) sign circuit [600-4(b)(3)]

2 125 percent of show window lighting [220-12(a)]

3 Track lighting at 125 percent of 150 VA per lineal foot [220-12(b)]

Receptacle loads consist of

1 100 percent of the quantity of 1 VA/ft2shown in Table 220-3(a) or 100 percent of 180 VA per receptacle [220-3(b)(9)]

2 100 percent of specific appliance loads that “plug into” receptacles or 125 percent of specific appliance loads that can operate continuously (for 3 hours or longer) that

“plug into” receptacles

3 100 percent of 180 VA per 5 ft of multioutlet assembly or

100 percent of 180 VA per 1 ft of multioutlet assembly,

Control room of an industrial facility.

Induction motors driving pumps in industrial facility.

Special appliance loads consist of 100 percent of the voltampere rating of each appliance that will not run con-tinuously and 125 percent of each appliance that will run continuously

Motor loads other than HVAC loads, in accordance with NEC Article 430, consist of 125 percent of the largest table ampere size motor plus 100 percent of all other motors, except that any motor that is 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 noncoinci-dental, with heating loads only running when cooling is not required, and vice versa The feeder that serves both nonco-incidental loads only needs to be sized for the larger of the two Note, however, that some 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 220-13 lists specific factors), to commercial cooking units (NEC Table 220-20 lists specific factors), to welders (630-11), to cranes (610-14), and to motors that will not be running simul-taneously (430-26) The demand factor for a group of motor loads must be determined by the engineer or designer

As an example, Fig 13-10 shows a calculation of the mini-mum service feeder ampacity required for a commercial building that consists of 100,000 ft2of 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 three-phase, one 10-hp domestic water pump, and one 30-kilowatt (kW) heating coil that provides reheat dehumidification

Designing an Electrical System for an

Industrial Facility

Engineering and design work in an industrial facility is sim-ilar to that in commercial buildings, except that the loads are

Figure

quite different in type and rating The electrical load of the buildings in a typical industrial plant is normally much smaller than the large-process load, and the design

reliabili-ty considerations are different In commercial buildings, the emphasis is on personnel safety first and equipment func-tionality second In many industrial plants, the process equipment is unmanned, and process interruptions can cause

Motor control center.

Figure

millions of dollars in damage losses 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 tem in an industrial plant, as shown in Fig 13-11 This sys-tem consists of the following loads:

1 A 3000-ft2control building

2 A 2000-ft2switchgear 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, 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

Industrial plants commonly incorporate prefabricated switchgear buildings.

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, each service is capable of oper-ating the entire plant and is sized to support the load of the entire plant