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Contents
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi
1 National Electrical Safety Code Overview
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Rule 230. Clearances, General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Rule 231. Clearances of Supporting Structures From Other Objects. . . . . . . . . . . . . . . . . . . . . 5
Figure 1-3. Clearances from railroad tracks. Rule 231C. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Rule 232. Vertical Clearances of Wires, Conductors, and Equipment Above Ground,
Roadway, Rail, or Water Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Rule 233. Clearances Between Wires, Conductors, and Cables Carried on
Different Supporting Structures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Rule 234. Clearance of Wires, Conductors, Cables, and Equipment from Buildings,
Bridges, Rail Cars, Swimming Pools, and Other Installations . . . . . . . . . . . . . . . . . . . . . . . . . 23
Rule 235. Clearance for Wires, Conductors, or Cables Carried on the
Same Supporting Structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Rule 236. Climbing Space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
Rule 237. Working Space. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
Rule 238. Vertical Clearance Between Certain Communications and
Supply Facilities Located on the Same Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
Rule 239. Clearance of Vertical and Lateral Facilities From Other Facilities and
Surfaces On the Same Supporting Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54Application of the NESC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
2 Pole Line Design
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
Route Selection and Control Points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
NESC Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
Conductor Loading Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
Conductor Tensions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
Ruling Spans. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
Conductor Sag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
Weight Span (Load Span) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
Negative Load Span . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
Wind Span . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
Horizontal Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
Total Bending Moment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
Vertical Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
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Extreme Wind Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
Anchors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
Slack Spans (Reduced Tension Spans) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
Pole Foundations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
Selection of Pole Top Assemblies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
Pole Line Design Parting Thoughts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
3 Wood, Steel, and Concrete Poles
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
Wood Poles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
Wood Pole Specifications, Codes, Standards. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
Steel Poles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
Concrete Poles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
4 Fiber Optic Cable
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
Basics of Optical Fiber Communications Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
Fiber Optic Transmission Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
Fiber Optic Cable Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
Overall Design and Construction Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
5 Street Lighting
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
Factors Contributing to Roadway Design Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
Roadway Lighting Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
Roadway Lighting Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164
State Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
Example of Roadway Lighting System Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
Power Supply and Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
Street Lighting Voltage Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183
Voltage Drop Example. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183
Lighting System Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186Closing Thoughts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
6 Line Protection
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190
Fuses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190
Distribution Transformer Protection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
Fuse-Fuse Coordination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203
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Reclosers and Breakers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209
Recloser Recloser Coordination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211
Sectionalizers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217
Recloser Sectionalizer Fuse Coordination. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220
Switches. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221
Surge Arresters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222
7 System Grounding
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232
Types of Grounding Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232
Earth as a Grounding Medium. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237
The Grounding Electrode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241
Pole Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245
Grounding of System Neutral. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246
Pole Grounding for Line Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246
Improving System Grounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
Substation Grounding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248
Electric Shock and the Human Being . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249
8 Capacitors
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256
Power Factor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256
Capacitor Installations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260Capacitor Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262
Distribution Line Capacitor Banks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277
Design of Line Capacitor Installations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279
Substation Capacitor Banks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293
9 Protective Relaying
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298
Fault Current Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298
Transformer Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301Relay Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313
Instrument Transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319
Overcurrent Relaying. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324
Zone/Distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343
Differential Relaying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349
Miscellaneous Relaying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359
Other Transformer Protection Schemes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 360
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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 368
Basics of Distribution Automation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 368
SCADA Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369
Fault Detection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373
Power Quality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375
Distribution Sensing, Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378
Distribution Automation Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 381
Communication Systems for Distribution Automation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 387
Automation Software. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391
Benefits of Distribution Automation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393
Environmental Conditions to Consider . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 394
11 Underground DistributionIntroduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402
Overhead vs. Underground. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402
Engineering URD Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405
Code Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430
Design of URD Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430
Installation of URD Facilities Some Considerations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442
Operating the URD System Some Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 445
12 Transformer ConnectionsIntroduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 452
Transformer Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 452
Paralleling Single-Phase Distribution Transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 455
Phase Rotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 469
Ferroresonance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 469
Single-Phase Transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 470
Considerations for Polyphase Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477
Common Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 478
Banks With Two Transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485
13 Metering
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 490
Basic Construction and Operation of an Induction Watthour Meter . . . . . . . . . . . . . . . . . . 490
Demand Metering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499
Reactive Power Metering. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 502
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Standard Metering Installations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 510
Special Metering Installations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513
Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 514
Automated Meter Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 516
Safety Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 519
14 Dispersed Generation
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 524
Distribution Generation Policy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 524
Commonly Applied Distributed Resources. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 525
New and Emerging Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 533
Switching and Protection Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 534
Overview of the DG Utility Interface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 536
Requirements of IEEE 1547 Standard for Interconnecting Distributed Resourceswith Electric Power Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 539
Glossary of Interconnection Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 549
DG Interconnection Standards. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 551
15 Engineering Economics
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 554
Cost Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 554
Cost Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 556
Introduction to Engineering Economics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 557Time Diagrams. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 561
Inflation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 568
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18. For uncontrolled water flow areas, the surface area shall be that enclosed by its annual high-water mark.
Clearances shall be based on the normal flood level; if available, the 10-year flood level may be assumed as the
normal flood level.
19. The clearance over rivers, streams, and canals shall be based upon the largest surface area of any 1-mile-long
segment that includes the crossing. The clearance over a canal, river, or stream normally used to provide access for
sailboats to a larger body of water shall be the same as that required for the larger body of water.20. Where an overwater obstruction restricts vessel height to less than the applicable reference height given in
Table 232-3 in the NESC, the required clearance may be reduced by the difference between the reference height
and the overwater obstruction height, except that the reduced clearance shall not be less than that required for the
surface area on the line-crossing side of the obstruction.
21. Where the US Army Corps of Engineers, or the state, or surrogate thereof has issued a crossing permit, clearances
of that permit shall govern.
22. See Rule 234I for the required horizontal and diagonal clearances to rail cars.
23. For the purpose of this Rule, trucks are defined as any vehicle exceeding 8 feet in height. Areas not subject to truck
traffic are areas where truck traffic is not normally encountered nor reasonably anticipated.
24. Communication cables and conductors may have a clearance of 15 feet where poles are back of curbs or other
deterrents to vehicular traffic.
25. The clearance values shown in this table are computed by adding the applicable Mechanical and Electrical (M&E)
value of Table A-1 to the applicable Reference Component of Table A-2a of Appendix A in the NESC.
26. When designing a line to accommodate oversized vehicles, these clearance values shall be increased by the
difference between the known height of the oversized vehicle and 14 feet.
Vertical Clearances of Service Conductors
Shown below is a summary diagram of the most commonly required vertical clearances for
service conductors, limited to 300 V to ground.
Vertical clearances shall not be less than those shown in Figure 1-4, and shall be applied asdescribed earlier in Rule 232A. Application. The clearances shown here are as designed and are
not clearances under ambient conditions.
Figure 2.4. Vertical clearances of service conductors.
Street
Residentialdriveway
Not less than16.5 ft (open wire)16.0 ft (multiplex)
See Table 1-2.Not less than
16.5 ft (open wire)16.0 ft (multiplex)
See Table 1-2.Not less than
12.5 ft (open wire)12.0 ft (multiplex)
Accessible areapedestrians only
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Figure 2-3. Point of maximum sag and low point can be different.
Horizontal distance:
and
Vertical distance:
Where:
X = horizontal distance to low point in sag from the lower support, ft
Y = vertical distance to low point in sag from the lower support, ft
L = span length, ft
H = difference in attachment elevations, ft (if higher, the value is positive
and if lower, it is negative)S = conductor sag in question (any sag), ft
Note: If the value of (1 H/4S) is negative, the low point in sag (theoretical) occurs beyond the
lower support and thus not in the given span. In this case, the low point in sag is at the
attachment point of the conductor at the lower support.
Anatomy of a Span
Figure 2-4 depicts the important terminology associated with components of conductor sag.
These components are used constantly when designing overhead utility lines.
Y = S 1 H
4S
2
X = 1 L2
H4S
Low point on sag
S
H
Y
L
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Wind Loading on the Conductors
The horizontal loading due to the wind force on a conductor is defined by this equation:
FWC = WH LW OFT
Where:FWC = horizontal load due to wind force on each conductor, lbs
WH = horizontal wind factor, lbs/ft
LW = wind span, ft
OFT = overload factor for transverse wind as required by NESC Table 253-2.
Wind Loading on the Structure (Pole)
For manual calculations, the wind loading on the structure (pole) is to be applied to that
portion of the pole above the support level, see Figure 2-8. For an unguyed pole the support
level is the ground line and for a guyed pole the support level is defined as the top guy level.
The wind loading on that defined pole section is found by using the following formula:
FWS = WPH DA L OFT
Where:
FWS = wind load on structure (pole), lbs
WPH = horizontal wind pressure for given loading conditions, lbs/ft2
DA = average diameter of pole (above pole support level) ft
L = length of pole section above pole support level, ft
OFT = overload factor for transverse wind as required by NESC Table 253-2.
Figure 2-8. Wind loading on a structure.
Guyed pole support level
Unguyed pole support leve
L (guyed) (guyed)
L (unguyed
(unguyed
WS (unguyed
FW (guyed)
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Figure 3-8. Prestressed spun concrete pole.
Here are brief descriptions of the major components that make up a prestressed concrete pole:
Prestressed Strands
Prestressed strands are the main strength members of a concrete pole. The number of
prestressed strands, their size and location depends on the manufacturer and the class and/or
strength requirement of the pole. For ease of drilling, the location of these prestressed strands
can be specified. The prestressed strands are loaded to a specific tension to insure the concrete
stays in compression. Strands are generally located symmetrically, which makes it fairly easy to
avoid them when drilling. Prestressed strands are made of high strength carbon steel and can
only be cut with a torch. Due to their importance in the overall strength of the pole, they
should never be drilled, cut, or left exposed. Poles with this type of damage should be removed
from service.
Spiral Wrap
Spiral wrap wires are smaller than the prestressed strands and are made of mild steel. They arewrapped helically around the prestressed strands for the full length of the concrete pole.
Although they provide some strength to the concrete pole, cutting the spiral wrap wires for
boltholes does not generally have a significant effect on the strength of the pole.
Non-Tensioned Reinforcement
Non-tensioned reinforcement is nothing more than mild steel rebar. It is often used at the
ground line to provide extra strength in this area. It is placed beside the prestressed strand and
normally will not be located where it might be cut.
Concrete cover
Concrete cover
Spiral wire isc oser toget er
at each en
Longitudinal
Longitudinal wire(if required)
Prestressed str
Prestressed s
Spiral wire
Spiral wire
Non-tensionedreinforcement(if required
on-tens one ren orcem(if required) Seam line
s requ re
5 8 in. minimum
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Figure 4-8. Typical hardware for optical ground wire (OPGW). Courtesy: Preformed Line Products.
Figure 4-9. Typical hardware for ADSS cable. Courtesy: Preformed Line Products.
Air flow spoiler Dielectricdamper
ADSScoronaTM
coil
Downleadcushion &mountingaccessories
Splice casewithin steelballistic shield
Dielectricdeadends
or litetension
deadend
Cable abrasionprotector
In spanstorage system
Verticalcablestorage
Closure orsplice case
Dielectricsupport,aluminum
support, orlite support
Dielectricsuspension or
aluminumsuspension
Air flow spoiler
Spiral vibration damper
Support
Downleadcushion & lattice
tower clamp
Formed wire deadend
OR
Suspension
Cushion clamp
OR
U-Bolt deadend
Defender& vertical
cable storage
Splicecase/
closure
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Figure 5-6. Street lighting example.
From Table 5-5, we determine the roadway surface classification as R1. The residential area in
question is classified as a low pedestrian conflict area as listed in Table 5-3. From Table 5-6 we
see that the average maintained illuminance value (footcandles) for a major roadway in a low
pedestrian conflict residential area is 0.6 footcandles.
To calculate the average maintained footcandles, we assume the Lamp Depreciation Factor to
be 90% and the Luminaire Dirt Depreciation to be 80%. Using Figure 5-3 (American Electric
Roadway Luminaire) photometric data, the Coefficient of Utilization graph shows the CU to
be 43%, based on this calculation:
Street sideHouse side
CL of luminaire
Sidewalk
30'
Sidewalk
45'
39'6'
P2
P2
Points P1 and P2avg. maint. 0.6 FC
P1
P1
160.52'
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Figure 6-7. Inrush and damage curves.
Fuse-Fuse CoordinationFuse link coordination can be achieved by the use of TCC curves, coordination tables, or
industry established rules of thumb. When fuses are installed in series on a power system, the
down-line fuse is referred to as the protecting link and the up-line fuse is referred to as the
protected link. To achieve coordination, the protecting link should operate for faults in its zone
of protection without causing damage to the protected link.
0.01 0.6
0.02 1.2
0.03 1.8
0.04 2.4
0.05 3
0.06 3.60.07 4.20.08 4.80.09 5.40.1 6
0.2 12
0.3 18
0.4 24
0.5 30
0.6 360.7 420.8 480.9 54
1 60
2 120
3 180
4 240
5 300
6 3607 4208 4809 540
10 600
20 1200
30 1800
40 2400
50 3000
60 360070 420080 480090 5400
100 6000
0.5
0.6
0.7
0.80
.9 1 2 3 4 5 6 7 8 910
20
30
40
50
60
70
80
90
100
200
300
400
500
Current in Amperes: x 1 at 12.5 kV.
TimeinSeconds
TimeInCycles(60-Hz
Basis)
Damage CurveInrush Curve
Transformer Fuse
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Deep Driven Ground Rods
Using longer rods or sectional ground rods to drive the rod deeper into the earth usually
decreases the electrode resistance. In addition, with the ground electrode down further into the
earth it is more likely to exhibit lower levels of resistance due to water tables and constant soil
temperatures. It may also encounter soil with a lower resistivity. A rule of thumb is thatdoubling the length of the ground rod will lower the resistance by 40%.
Multiple Ground Rods
Paralleling multiple ground rods can achieve lower resistances by providing multiple paths for
the current to divide. However, paralleling two rods will not reduce the resistance by one-half
as is usually the case with parallel conductors. The reason for this is that the rods are not
widely separated from one another and the rods will mutually interact on each other through
their common resistances to remote earth. Separation of the rod grounds will greatly influence
the reduction in resistance. It is recommended that the rods be spaced from one another at a
distance of two times the length of the rods and tied together with a #6 or #4 copperconductor. If they are spaced in this manner, the reduction in resistance using two ground rods
is about 60%, three reduces it to 40%, and four reduces it to 33%.
Install Ground Away From the Pole
Ground rods installed adjacent to poles are typically in soil which was disturbed when the pole
was installed. This can result in a relatively high ground resistance. Installing the rod 2 to 3 feet
from the pole may reduce the resistance considerably. This is especially true when installing
ground rods for padmounted equipment.
Chemical TreatmentsIn some extreme cases, the soil surrounding the ground electrode must be chemically treated to
reduce the ground resistance. This method also helps to reduce the change in soil resistivity
during wet and dry seasons. The major disadvantages of this method are that it is expensive
and is not generally permanent (chemicals are washed away by rain). However, it may be the
only solution for areas with underlying rock layers preventing the use of deeper or even
multiple rods. The materials used for this purpose are generally referred to as Grounding
Enhancement Material or GEM, and this abbreviation is used in some trade names. Buried
electrodes are sometimes placed in Bentonite clay which has a favorable resistivity.
Substation GroundingThe function of a substation ground is to provide proper operation of electric equipment and
to provide personnel safety. These functions can be met by using the lowest practical resistance
between the circuit neutrals and the earth. Typically, a ground grid that is designed to meet the
safety standards for personnel will also be satisfactory for equipment operations.
The substation ground grid generally consists of driven ground rods tied together with buried
cables and equipment ground mats, which are all tied to the system neutral. Current flows into
the ground from lightning surges, ground faults. Switching surges can cause potential
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Figure 9-17. Line breaker with instantaneous relay.
As can be seen, the two are properly coordinated. However, instantaneous relaying can be
applied at breaker A to increase its sensitivity without mis-coordinating with breaker B. This is
accomplished by setting the instantaneous relay to see 80-85% of the distance to B. This can
be easily calculated as a ratio of fault current decrease to line length since this can be approxi-
mated as a linear relationship.
IInst = 8000 - [(8000 6000) 0.8] = 6,400 amps
The instantaneous relay can, for all practical purposes, be drawn as a vertical line at the 6,400
amps location. This time-overcurrent curve can be indicated with a dashed line for all currents
greater than the instantaneous setting.
Figure 9-17 has been redrawn in Figure 9-18 with a tap at point C that is protected by a fuse.
Source20 miles
80-85
Current
ime
6000 A 6400 A
6,400=
IF @ B IF @ A
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Figure 9-18. Radial line with fused tap.
As can be seen, for a fault with a magnitude of 7,200 amps on the tap at C just beyond the fuse,
the instantaneous trip setting on breaker A will operate faster than the fuse. The only way to
alleviate this problem is to set the instantaneous relay above 7,200 amps or to remove the
instantaneous relay. However, this greatly reduces the sensitivity of the breaker relay scheme to
high-magnitude, close-in faults.
Application of instantaneous relaying can be accomplished by utilizing a reclosing relay with
an instantaneous trip lockout feature. After a preset number of reclosers (usually one or two),
this device locks out the instantaneous relay from operation. Thus, breaker A can be set tooperate twice on instantaneous and then revert to time delay for two additional operations. As
a result, an intermittent (transient) fault on the tap at C can be cleared without the fuse
blowing. It is estimated that two-thirds of all faults are transient in nature, this results in a
considerable reduction in outage time for the consumer. If the fault is persistent, the instanta-
neous relay will be locked out and, under time delay at breaker A, the fuse will clear the fault
providing correct coordination.
For a persistent fault on the main feeder, the time delay of the final two operations can be
undesirable. However, due to the small percentage of persistent faults and, by liberal
application of fuses on all taps off the main feeder, this problem can be reduced to an
acceptable level.
Ground Overcurrent Relaying
Though all of the principles above are applicable to relaying for ground faults, ground fault
protection requires special consideration. The magnitude and detection of ground faults
depends upon transformer connection (delta or wye) and the presence of a ground impedance
in the case of a wye system. The following discussion will concentrate on the solidly grounded
wye system prevalent in most distribution systems.
C
6400 A
Tim
e
A
7200 A
ource
8000 A
IF @ B IF @ AIF @ C