Overhead Conductors Explained

First some History

The "American Wire Gauge" or AWG
This method of defining or "Gauging" wire sizes is puzzling to many… Engineers Included.
Why do the wires get smaller when the gauge gets larger?
Well… It originated in Europe in the 13th century with the method of "Drawing" wire.
In this process the metal is pulled, or "Drawn" through a number of holes, each hole smaller than the previous,
until it passes through the hole having the desired diameter.
The gauge number originated from how many progressively smaller holes the wire is drawn through.
Therefore... The higher the gauge number .... The smaller the diameter.

AWG

AWG 0 AWG 1 AWG 2

This Wire is 112.29% of the Diameter of this Wire -- OR -- This Wire is 89.05% of the Diameter of this Wire

Like all new technology that comes along, Manufacturers had their own “Proprietary” method of defining their “Drawn” sizes.
The ASTM (American Society for Testing and Materials) stepped in and defined Standard (B258-02).
This standard defined the ratio of successive sizes to be the 39th root of 92 (1.122932).
Odd as are many of the early “English” non-metric units.
Also states that the diameter of 36AWG is 0.4600” and that 0000AWG is 0.005”.
0000AWG or 4/0 is three sizes larger than 0AWG.
 Overhead Conductors Explained (cont.)

The American Wire Gauge does not specify the cross-sectional area in square inches
Instead it uses the circular mil or cmil …
More specifically it uses thousands of circular mils or kcmil
Commonly denoted as KCM
Now a short explanation of the unit of area known as the circular mil
The length of one mil is 1/1000 of an inch or 0.001”
The circular mil is the cross-sectional area of a round conductor that has a diameter of 1.0 mil

0.0012
0.001" Area = 1cmil 1𝑐𝑚𝑖𝑙 = 𝜋 = 7.854 ∙ 10−7 𝑖𝑛2
4

If you want to know how many thousands of circular mils a of a round conductor….
divide the square inch area by one thousand circular mils area

𝐷2
𝜋
D" ? KCM 𝑘𝑐𝑚𝑖𝑙𝑠 = 𝐾𝐶𝑀 = 4 = 1000𝐷2
0.0012
1000 ∙ 4 𝜋
Overhead Conductors Explained (cont.)

Thanks to Wikipedia for the following equation…

It relates the AWG number to wire diameter
Let n = AWG gauge number
36−𝑛
𝐷𝑖𝑎𝑚𝑒𝑡𝑒𝑟 𝑖𝑛𝑐ℎ𝑒𝑠 = 0.005 ∙ 92 39
 Overhead Conductors Explained (cont.)
How many circular mils for a solid round conductor with diameter = 1, ¾, ½ inch ?

1.0" 𝐾𝐶𝑀 = 1000 1.0 2 = 1000 𝐾𝐶𝑀

0.75" ? KCM 𝐾𝐶𝑀 = 1000 0.75 2 = 563 𝐾𝐶𝑀
0.5" 𝐾𝐶𝑀 = 1000 0.5 2 = 250 𝐾𝐶𝑀

Now for Bare Overhead Conductors

Used for High Voltage Power Transmission
Although overhead conductors could be solid such as 3/0 or 4/0 AWG …
larger capacity conductors are “stranded” aluminum and generally have steel stranding for strength.
an example would be ACSR 26/7 …
this is Aluminum Conductor Steel Reinforced with 26 strands of aluminum and 7 strands of steel

Also unique to bare overhead conductors is the use of “code” names

that specify the conductor size and stranding.
ACSR uses birds. 30 strands of aluminum
For example:
“Oriole” is understood to be 336.4 KCM ACSR 30/7 7 strands of steel

It is important to realize now that not all of the cross-sectional

area of the round conductor is actual “conductor”…
Some of it is steel, some of it is air. So the KCM rating is
the “net” conductor cross-sectional area. 30 times the
cross-sectional area of one strand of aluminum for this example.
 ACSR Conductor Data
SIZE STRANDING CROSS SECTION O.D. RESISTANCE AMPS 75°C Conductor - 32°C Ambient
Nomenclature CODE AWG OR NO. X DIA. INCHES SQ. INCHES Ω/Mile MVA @ kV
kcmil Al/Steel ALUM STEEL TOTAL ALUM in AC@40C AC@75C 69 115 138 161 230 345
TURKEY 6 6/1 TURKEY 6 6/1 6x.0661 1x.0661 0.024 0.0206 0.198 3.685 4.335 100 12 20 24 28 40 60
SWAN 4 6/1 SWAN 4 6/1 6x.0834 1x.0834 0.0383 0.0328 0.25 2.323 2.777 130 16 26 31 36 52 78
SPARROW 2 6/1 SPARROW 2 6/1 6x.1052 1x.1052 0.0608 0.0521 0.316 1.468 1.785 170 20 34 41 47 68 102
ROBIN 1 6/1 ROBIN 1 6/1 6x.1181 1x.1181 0.0767 0.0657 0.354 1.167 1.436 195 23 39 47 54 78 117
RAVEN 1/0 6/1 RAVEN 1/0 6/1 6x.1327 1x.1327 0.0967 0.029 0.398 0.929 1.156 225 27 45 54 63 90 134
QUAIL 2/0 6/1 QUAIL 2/0 6/1 6x.1489 1x.1489 0.1219 0.1045 0.447 0.744 0.935 260 31 52 62 73 104 155
PIGEON 3/0 6/1 PIGEON 3/0 6/1 6x.1672 1x.1672 0.1537 0.1317 0.502 0.591 0.755 295 35 59 71 82 118 176
PENGUIN 4/0 6/1 PENGUIN 4/0 6/1 6x.1878 1x.1878 0.1939 0.1662 0.563 0.470 0.612 340 41 68 81 95 135 203
OWL 266.8 6/7 OWL 266.8 6/7 6x.2109 7x.0703 0.2367 0.2095 0.633 0.364 0.475 400 48 80 96 112 159 239
WAXWING 266.8 18/1 WAXWING 266.8 18/1 18x.1217 1x.1217 0.221 0.2095 0.609 0.370 0.417 420 50 84 100 117 167 251
PARTRIDGE 266.8 26/7 PARTRIDGE 266.8 26/7 26x.1013 7x.0788 0.2436 0.2095 0.642 0.364 0.412 430 51 86 103 120 171 257
OSTRICH 300 26/7 OSTRICH 300 26/7 26x.1074 7x.0835 0.2738 0.2355 0.68 0.324 0.366 460 55 92 110 128 183 275
PIPER 300 30/7 PIPER 300 30/7 30x.1000 7x.1000 0.2906 0.2356 0.7 0.322 0.364 465 56 93 111 130 185 278
MERLIN 336.4 18/1 MERLIN 336.4 18/1 18x.1367 1x.1367 0.2789 0.2642 0.684 0.294 0.332 485 58 97 116 135 193 290
LINNET 336.4 26/7 LINNET 336.4 26/7 26x.1137 7x.0884 0.307 0.264 0.72 0.289 0.327 495 59 99 118 138 197 296
ORIOLE 336.4 30/7 ORIOLE 336.4 30/7 30x.1059 7x.1059 0.3259 0.2642 0.741 0.287 0.324 500 60 100 120 139 199 299
CHICKADEE 397.5 18/1 CHICKADEE 397.5 18/1 18x.1486 1x.1486 0.3295 0.3122 0.743 0.249 0.281 540 65 108 129 151 215 323
IBIS 397.5 26/7 IBIS 397.5 26/7 26x.1236 7x.0961 0.363 0.3122 0.783 0.245 0.277 550 66 110 131 153 219 329
LARK 397.5 30/7 LARK 397.5 30/7 30x.1151 7x.1151 0.385 0.3122 0.806 0.243 2.746 560 67 112 134 156 223 335
PELICAN 477 18/1 PELICAN 477 18/1 18x.1628 1x.1628 0.3954 0.3746 0.814 0.208 0.234 605 72 121 145 169 241 362
TOUCAN 477 22/7 TOUCAN 477 22/7 22x.1472 7x.0818 0.4114 0.3746 0.834 0.206 0.233 610 73 122 146 170 243 365
HAWK 477 26/7 HAWK 477 26/7 26x.1354 7x.1053 0.4357 0.3746 0.858 0.204 0.231 620 74 123 148 173 247 370
HEN 477 30/7 HEN 477 30/7 30x.1261 7x.1261 0.4621 0.3746 0.883 0.203 0.229 625 75 124 149 174 249 373
HERON 500 30/7 HERON 500 30/7 30x.1291 7x.1291 0.4843 0.3927 0.904 0.193 0.219 645 77 128 154 180 257 385
SAPSUCKER 556.5 22/7 SAPSUCKER 556.5 22/7 22x.1590 7x.0883 0.48 0.4368 0.901 0.187 0.200 670 80 133 160 187 267 400
DOVE 556.5 26/7 DOVE 556.5 26/7 26x.1463 7x.1138 0.5083 0.4371 0.927 0.175 0.198 680 81 135 163 190 271 406
EAGLE 556.5 30/7 EAGLE 556.5 30/7 30x.1362 7x.1362 0.5391 0.4371 0.953 0.174 0.196 690 82 137 165 192 275 412
DUCK 605 54/7 DUCK 605 54/7 54x.1058 7x.1058 0.5369 0.4752 0.952 0.165 0.188 705 84 140 169 197 281 421
GOLDFINCH 636 22/7 GOLDFINCH 636 22/7 22x.1700 7x.0944 0.5485 0.4995 0.963 0.155 0.175 730 87 145 174 204 291 436
GROSBEAK 636 26/7 GROSBEAK 636 26/7 26x.1564 7x.1216 0.5808 0.4995 0.99 0.154 0.174 740 88 147 177 206 295 442
EGRET 636 30/19 EGRET 636 30/19 30x.1456 19x.0874 0.6133 0.4995 1.019 0.153 0.172 750 90 149 179 209 299 448
GOOSE 636 54/7 GOOSE 636 54/7 54x.1085 7x.1058 0.5642 0.4995 0.977 0.157 0.180 725 87 144 173 202 289 433
GULL 666.6 54/7 GULL 666.6 54/7 54x.1111 7x.1111 0.5914 0.5235 1 0.150 0.171 750 90 149 179 209 299 448
STARLING 715.5 26/7 STARLING 715.5 26/7 26x.1659 7x.1290 0.6535 0.562 1.051 0.137 0.154 800 96 159 191 223 319 478
REDWING 715.5 30/19 REDWING 715.5 30/19 30x.1544 19x.0926 0.69 0.562 1.081 0.136 0.153 805 96 160 192 224 321 481
CROW 715.5 54/7 CROW 715.5 54/7 54x.1151 7x.1151 0.6348 0.562 1.036 0.140 0.159 780 93 155 186 218 311 466
DRAKE 795 26/7 DRAKE 795 26/7 26x.1749 7x.1360 0.7264 0.6244 1.108 0.124 0.139 850 102 169 203 237 339 508
MALLARD 795 30/19 MALLARD 795 30/19 30x.1628 19x.0977 0.7668 0.6244 1.14 0.122 0.138 860 103 171 206 240 343 514
MACAW 795 42/7 MACAW 795 42/7 42x.1376 7x.0764 0.6565 0.6244 1.055 0.126 0.142 835 100 166 200 233 333 499
CONDOR 795 54/7 CONDOR 795 54/7 54x.1213 7x.1213 0.7053 0.6244 1.092 0.126 0.144 835 100 166 200 233 333 499
CRANE 874.5 54/7 CRANE 874.5 54/7 54x.1273 7x.1273 0.7759 0.6868 1.146 0.126 0.144 850 102 169 203 237 339 508
CANARY 900 54/7 CANARY 900 54/7 54x.1291 7x.1291 0.7985 0.7069 1.162 0.111 0.127 900 108 179 215 251 359 538
CARDINAL 954 54/7 CARDINAL 954 54/7 54x.1329 7x.1329 0.8464 0.7493 1.196 0.106 0.120 940 112 187 225 262 374 562
PHOENIX 954 42/7 PHOENIX 954 42/7 42x.1507 7x.0387 0.7878 0.7493 1.155 0.105 0.118 935 112 186 223 261 372 559
SNOWBIRD 1033.5 42/7 SNOWBIRD 1033.5 42/7 42x.1569 7x.0872 0.8535 0.8117 1.203 0.098 0.110 980 117 195 234 273 390 586
CURLEW 1033.5 54/7 CURLEW 1033.5 54/7 54x.1383 7x.1383 0.9169 0.8117 1.245 0.098 0.111 985 118 196 235 275 392 589
BEUAMONT 1113 42/7 BEUAMONT 1113 42/7 42x.1628 7x.0904 0.919 0.8741 1.248 0.091 0.102 1025 122 204 245 286 408 612
FINCH 1113 54/19 FINCH 1113 54/19 54x.1436 19x.0862 0.985 0.8741 1.293 0.091 0.103 1030 123 205 246 287 410 615
GRACKLE 1192.5 54/19 GRACKLE 1192.5 54/19 54x.1486 19x.0892 1.0553 0.9366 1.338 0.086 0.097 1075 128 214 257 300 428 642
SCISSORTAIL 1272 42/7 SCISSORTAIL 1272 42/7 42x.1740 7x.0967 1.0501 0.999 1.334 0.080 0.090 1115 133 222 267 311 444 666
PHEASANT 1272 54/19 PHEASANT 1272 54/19 54x.1535 19x.0921 1.1256 0.999 1.382 0.080 0.091 1120 134 223 268 312 446 669
MARTIN 1351.5 54/19 MARTIN 1351.5 54/19 54x.1582 19x.0949 1.1959 1.0615 1.424 0.076 0.086 1160 139 231 277 323 462 693
PLOVER 1431 54/19 PLOVER 1431 54/19 54x.1628 19x.0977 1.2663 1.1239 1.465 0.072 0.082 1200 143 239 287 335 478 717
PARROT 1510.5 54/19 PARROT 1510.5 54/19 54x.1672 19x.1003 1.3364 1.1863 1.505 0.069 0.078 1240 148 247 296 346 494 741
FALCON 1590 54/19 FALCON 1590 54/19 54x.1716 19x.1030 1.4071 1.2488 1.545 0.065 0.074 1280 153 255 306 357 510 765
 Overhead Conductors Explained (cont.)

Notes regarding the current carrying capacity or “Ampacity” of conductors

Because of the Inherent resistance of conductors… The I2R losses generate heat.
As current Increases… heat Increases.
Aluminum has a relatively high coefficient of thermal expansion.
When aluminum heats up... It expands and gets "Longer“.
This causes "Sag" between supporting structures.
High voltage conductors must not sag low enough to cause safety concerns for "Grounded" Objects.
So the "Cooler" a conductor is... The less it will sag.
It is all about "Dissipating" the heat generated by the current flowing in the conductor.

There are many factors that determine how much heat is dissipated and what the steady-state conductor temperature will be.
Among these factors are:
Ambient Temperature or outside air temperature.
Wind Speed and Direction (relative to conductor orientation)
Location of Conductor (Latitude) for sun contribution to conductor temperature.
Emissivity - the "Reflective" properties of the line to sunlight.
Absorptivity - the ability to take on the ambient temperature conditions.

Conductors will be rated with a Maximum Operating Temperature or MOT to limit sagging and possible safety concerns.
The rating methodology used will determine the ampacity of the line, and will be higher in the Winter compared to Summer.
A "Derated” MOT is commonly used for normal operating conditions....
Allowing the MOT to be used for Emergency operating conditions.

Since American engineers generally relate to F …

0C Freezing point of water (32F)
25C Considered normal indoor "Laboratory" temperature (77F)
30C Considered "Borderline" high outside temperature (86F)
100C Boiling point of water (212F)
 Overhead Conductors Explained (cont.)
What would be the “ideal” overhead conductor?
Infinite Ampacity -- Zero Sag -- Zero weight -- Zero Diameter -- Zero Cost
None of these are possible but they can be maximized
Below are some of the conductors manufactured today
ACSR Aluminum Conductor Steel Reinforced Birds as Code Names
ACSR/TW Trapezoidal Conductors Rivers or Birds as Code Names
ACSS Aluminum Conductor Steel Supported Birds as Code Names
ACCC Aluminum Conductor Composite Core Cities or Birds as Code Names
ACCR Aluminum Conductor Composite Reinforced Birds as Code Names
AAC All Aluminum Conductor Flowers as Code Names.
AAC/TW Trapezoidal Conductors Flowers or Mountains as Code Names
AAAC All Aluminum Alloy Conductor Cities as Code Names.
ACAR Aluminum Conductor Alloy Reinforced Birds as Code Names
AACSR Aluminum Alloy Conductor Steel Reinforced Uses Code Numbers
ACSR TP Twisted Pair controls wind galloping Birds as Code Names
ACSR/AW AlumiWeld Aluminum Clad Steel reinforced Birds as Code Names
ACSR/SD Self Dampening with gaps between inner layers Birds as Code Names

Lets take a closer look at conductor construction and properties

ACSR ACSR/TW
ACSS ACSS/TW ACCC
 Overhead Conductors Explained (cont.)

ACSR - ACSS

ACSR and ACSS have no significant difference in appearance or construction

• both have round aluminum conductor strands
• both have round steel reinforcement strands
• ACSR use hard drawn aluminum
• ACSS uses annealed or “soft” aluminum.
Now an attempt to demonstrate how using annealed aluminum can allow for
A much higher MOT (Maximum Operating Temperature)

Consider using a standard ACSR conductor and you want an MOT of 120C
with a minimum safe clearance of 20 feet

ACSR
 Overhead Conductors Explained (cont.)
For the above example…
when the increased current flowing in the conductor heats the conductor to 70C …
the ground clearance reaches the minimum of 20 ft.
No problem, just increase the structure heights.

ACSR

Ok, now when the current flowing in the conductor heats the conductor to 120C …
the ground clearance reaches the minimum of 20 ft.
Good to go? … No!
The problem is that hard drawn aluminum begins to anneal at about 95C.
this causes the aluminum to soften and loose it’s strength.
The total strength of an ACSR conductor relies on both the aluminum and the steel.
So this loss of strength could cause the conductor to break under it’s own weight or
during high wind or ice loading conditions.
The tensile strength of hard drawn aluminum is about 3 times that of annealed or soft aluminum.
 Overhead Conductors Explained (cont.)
When designing with ACSS…
The annealed ACSS aluminum has higher conductivity (lower resistance) than hard drawn ACSR aluminum.
The annealed ACSS aluminum has lower coefficient of expansion than hard drawn ACSR aluminum.
The “pre-annealed” decrease in aluminum strength is taken into account.
The total tensile strength of the conductor is effectively the steel stranding alone.
Using Extra-High-Strength (EHS) steel and more steel stranding allows ACSS to be operated up to 250C

ACSS
 Overhead Conductors Explained (cont.)

Trapezoidal or Trap Wire

Trap Wire replaces the round aluminum conductor strands with trapezoidal shaped strands
• Effectively eliminates the empty space between the aluminum strands
• The aluminum portion of the conductor imitates a solid aluminum conductor
• Reduces the overall diameter of the conductor by about 10%
• The reduced diameter decreases wind and ice loading
• Allows a reduction in the strength requirements for towers and poles (less expensive structures)
Two ways to consider advantages:
• Smaller diameter and same ampacity as equivalent ACSR
• Higher ampacity compared to ACSR with same diameter

Smaller Diameter Same Diameter

Same cross-sectional area More cross-sectional area
Same ampacity More ampacity
 Overhead Conductors Explained (cont.)

ACCC

ACCC is a registered trademark of CTC Global

• Considered a high-temperature low-sag conductor
• Uses trapezoidal stranding
• Replaces steel stranding with carbon composite fiber
• Has a much lower coefficient of thermal expansion, allowing higher operating temperatures
• Lighter weight, therefore can allow more aluminum conductor
• Uses fully annealed aluminum for maximizing high temperature operation
• Ampacity is about twice as much conventional ASCR of same size and weight
• Popular for reconductoring existing transmission lines without replacing structures
• Large bend radius due to the stiff composite core
• Maximum operating Temperature of about 250C
 Overhead Conductors Explained (cont.)
Overhead conductor specification do not specify inductance or capacitance
This is because the inductance and capacitance depends on conductor size
and the geometry of the conductor spacing
For more details --> transmission line constants and conductor ampacity explained

r I
+++++

Magnetic Field

D Electric Field

--------

l
Electric Field creates “Changing” Magnetic Field creates
Capacitance Inductance
Capacitance is constant and is a function of Inductance is constant and is a function of
conductor geometry. conductor geometry.
Capacitance increases with conductor radius. Inductance increases with conductor spacing.
Capacitance decreases with conductor spacing. Inductance decreases with conductor radius.

2 0 D
C L ln
ln D r 2 r

Capacitors supply Reactive Power (VARs) Inductors absorb Reactive Power (VARs)
Reactive Power supplied by capacitors is Reactive Power absorbed by inductors is not
fairly constant and equal to: constant, increases with current and equal to:

Qc  2fV 2 C QL  2fI 2 L
There is a point with increasing load current that the reactive power supplied by the capacitance equals the reactive power demand of the inductance.
This “Sweet Spot” is called the Surge Impedance Loading.

C B
PSIL  V 2 or in per unit quantities:
PSIL 
L X PU

If the Reactive Power Supplied by the Capacitance is greater than the Reactive Power Demand of the Inductance…… Voltages will Increase.
If the Reactive Power Supplied by the Capacitance is less than the Reactive Power Demand of the Inductance…… Voltages will Decrease.
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