EC&M’s Electrical Calculations Handbook - Chapter 16 ppsx

Conversion CalculationsTemperature Conversion Calculations While almost all ambient temperatures in electrical engi-neering are given in degrees Fahrenheit, almost all tem-peratures of e

Conversion Calculations

Temperature Conversion Calculations

While almost all ambient temperatures in electrical engi-neering are given in degrees Fahrenheit, almost all tem-peratures of equipment operation are stated in degrees Celsius Therefore, it is important that a quick and easy conversion method be available Figure 16-1 shows how to make these conversions and provides a quick cross-refer-ence tool from which the conversion can be derived instantly in either direction.

Frequently Used Conversion Calculations

For work in the electrical industry, many other conversion calculations are encountered repeatedly Figure 16-2 pro-vides the conversion formulas for these common conver-sions, and Fig 16-3 is a set of typical sample calculations showing how to use the conversion formulas correctly Each

of these conversion calculations has been changed into a simple one-step calculation for ease of use.

Multiple Conversion Calculations

Sometimes it is necessary to multiply an item in one set of units by another item to determine an answer in a completely

Chapter

16

425 v

Copyright 2001 by The McGraw-Hill Companies, Inc Click here for Terms of Use.

solve for degrees Celsius given degrees Fahrenheit or for degrees Fahrenheit given

different set of units When this is necessary, it is frequently expedient to multiply by “one” in the calculation to make the units come out as desired For example, the rental cost of a diesel generator is quoted at $10 per gallon of diesel fuel used, and it is stated that the diesel engine consumes one 55-gallon (gal) barrel of fuel per 1-hour (h) running period Determine the rental cost of the diesel generator for a 72-h running period.

The cost equals the dollar rate per hour times the quantity

of hours in operation:

Cost  $ rate per hour  quantity of hours in operation

Figure 16-3 Use these calculation methods with the conversion formulas of Fig 16-2

The problem is that the dollar rate per hour is unknown and must be resolved so that the main equation can be solved This is done simply by inserting into the main equation what is known, as follows:

By setting up the equation in this manner, unwanted units cancel as shown:

Thus the final rental cost of the diesel generator is deter-mined to be $550 per hour of operation Therefore, the rental cost of the machine becomes

Cost  $ rate per hour  quantity of hours in operation

 $550/h  72 h of operation  $39,600

The rental cost of the machine is calculated to be $39,600 The key to this type of conversion calculation is to multiply

by “one.” Multiplying anything by “one” does not change its value at all In this calculation, the cost per hour was multi-plied by 55 gal/barrel, which is “one” because there are 55 gal

in one barrel of fuel Just as correctly, it could have been mul-tiplied by 1 barrel/55 gal, but this would not have helped with the unit manipulation in this specific equation Other exam-ples of “one” that can be used in other equations are

■ 24 hours/day

■ 60 minutes/hour

■ 2000 pounds/ton

The entire key is to insert the units that resolve the problem

at hand and then determine the correct coefficients to pre-cede each unit value.

1 barrel

 hour

55 gal

 barrel

$10

 gal

1 barrel

 hour

55 gal

 barrel

$10

 gal

NOTES

Ambient temperature, 165

Ampacity, cable, 154, 172

Ampere, definition, 3

Autotransformers, 275

Batteries, 8

Battery output, 14

Battery voltage, 4

Bill, electrical utility, 90

Box sizing, pull, 357

Bus bars, 132

Capacitance and voltage, 100

Capacitor bank, 103

Capacitor bank detuning, 108

Capacitors, 46, 100

Circuit breakers, 323

Clearance, working, 358

Conductors, equipment-grounding,

214

Conduit cross-sectional area, 315

Conduit fill, 314

Conduit types, 311

Conversion, units, 425

Current:

alternating, 5

definition, 3

direct, 5

through impedance, 75

motor locked-rotor, 298

motor running, 295

motor starting, 291

true rms, 111

Current flow:

harmonic, 111

through resistance, 16

Damage, thermal, to cable, 159

dc offset, 180 Delta systems, 60 Dirt depreciation, 235 Economics, engineering calculations, 411 Efficiency, motor, 290 Electron flow, 3 Enclosure types, 317 Expense, project, 409 Fault, 180

Filters, harmonic, 117 Footcandles, 221 Frequency, 5 Fuse curve, 320 Fuses, 321 Ground rod resistance, 208 Grounding electrode conductors, 213 Grounding functions, 205 Grounding method, 216, 218 Harmonic-related problems, troubleshooting solutions for, 120

Harmonics, 104, 114, 116 Heat flow, from cable, 168 Heat loss, from conductor, 143 Illuminance, 221, 231 Impedance, 31, 52 cable, 133, 139 Inductive reactance as function of conductor spacing, 136

Index

433 v

Copyright 2001 by The McGraw-Hill Companies, Inc Click here for Terms of Use.

Inrush current, motor, 293

Insulation:

cable, 153, 161, 176

motor, 290

Insulators, 3

Interrupting rating, 181

Jacket, cable, 163

Kilowatthour, 80

Knockout spacing, 364

Labor, installation, 374

Lamps, 234

Lighting:

indoor, 229

outdoor, 221

Load calculations:

commercial building, 347

industrial facility, 349

Luminaire quantity, gross

calculation, 222

Motor design code letters, 288

Motor operating characteristics, 292

Motor speed, 286

Motor voltage, 286

Motors, 285, 308

NEMA enclosures, 317

Neutral point, in wye systems, 58

Ohm’s law, 3, 15

Overcurrent devices, 319

Overcurrent protection:

motor circuit, 296

transformer, 265, 276

Personnel-hour tables, installation

labor, 374

Phase angle, 26

Polar-to-rectangular conversion, 70

Polarity, 6

Power, 29, 33, 79

apparent, 30, 84

in three-phase circuit, 33

Power factor, 26, 75 Power factor correction, 87, 90,

93, 104 Power rating, resistor, 18

Q, of a reactor, 117

Raceways, 311 Reactance:

cable, 139 capacitive, 46 inductive, 30, 42 Reactor, 42, 188 Reflectance, 230 Reflections, veiling, 232 Relay protective schemes: for feeder circuit breaker, 328 for large generator, 330 for large induction motor, 333 for large transformer, 331 for small generator, 329 Relays, 326

Resistance:

conductor, 125, 129 definition, 4

of a ground rod, 212 internal, 8 series, 15 Resistance grounding, 217 Resistivity:

conductor, 133 soil, 210 Resistor, power rating, 18 Resonant frequency, 104 rms values, 6, 23 rpm, and number of poles, 60 Service factor, motor, 289 Short-circuit current sources, 182

Short circuits, 179, 190 Skin effect, 131 Spacing, knockout, 361 Speed, motor, 286 Subtransient reactance, generator, 184

Symbols, 5

Temperature, ambient, 165

Temperature effect on conductor

resistance, 135

Terminal, wire, 169

Thermal damage, cable, 159

Three-phase systems, 57

Torque, motor, 288

Total harmonic current distortion

(THCD), 110

Total harmonic voltage distortion

(THVD), 110, 116

Transformer ratings, 256

Transformers, 251

impedance % rating, 187

K-rated, 114

Transient voltage surge

suppressors, 208

Trigonometry for electrical

solutions, 32

Triplen harmonic currents, 116

Vector addition, 70 Vector multiplication, 74 Vectors, 69

Voltage:

definition, 1 effective, rms, 6, 23 system, 56, 271 Voltage distortion, 107 Voltage drop:

cable, 140, 144

in series circuit, 17 Wire characteristics, 125 Wire size, square millimeter, 130 Wires, quantity of in raceway or cable, 165

Wiring methods, 175, 313 Withstand rating, 181 Wye systems, 58, 65

X/R ratio, transformer, 280 X/R value, system, 180

This page intentionally left blank.

ABOUT THE AUTHOR

John M Paschal, Jr., P.E., is a Senior Technical Electrical Engineering Specialist with Bechtel, and has held a master electrician’s license for three decades He has worked in all facets of the electrical industry He has worked “with the tools” in the electrical construction trade, as an electrical construction project manager, as chief electrical estimator for national and international construction firms, as a college assistant professor, and as an electrical engineer

of some of the largest and most demanding commercial, health care, and industrial electrical systems throughout the world Mr Paschal is the author of many books on practical electrical engineering and construction methods,

and books dealing with the National Electrical Code He is

the technical editor of EC&M Books and an author for

Electrical Construction & Maintenance magazine.

Electrical Construction & Maintenance magazine,

estab-lished in 1901, is the electrical industry’s premier magazine for electrical design, construction, and maintenance.