## NFPA 70, 2017

#### Informative Annex J ADA Standards for Accessible Design

Heads up: There are no amended sections in this chapter.

# Informative Annex B Application Information for Ampacity Calculation

This informative annex is not a part of the requirements of this NFPA document but is included for informational purposes only.

### B.1 Equation Application Information

This informative annex provides application information for ampacities calculated under engineering supervision.

### B.2 Typical Applications Covered by Tables

Typical ampacities for conductors rated 0 through 2000 volts are shown in Table B.310.15(B)(2)(1) through Table B.310.15(B)(2)(10). Table B.310.15(B)(2)(11) provides the adjustment factors for more than three current-carrying conductors in a raceway or cable with load diversity. Underground electrical duct bank configurations, as detailed in Figure B.310.15(B)(2)(3), Figure B.310.15(B)(2)(4), and Figure B.310.15(B)(2)(5), are utilized for conductors rated 0 through 5000 volts. In Figure B.310.15(B)(2)(2) through Figure B.310.15(B)(2)(5), where adjacent duct banks are used, a separation of 1.5 m (5 ft) between the centerlines of the closest ducts in each bank or 1.2 m (4 ft) between the extremities of the concrete envelopes is sufficient to prevent derating of the conductors due to mutual heating. These ampacities were calculated as detailed in the basic ampacity paper, AIEE Paper 57-660, The Calculation of the Temperature Rise and Load Capability of Cable Systems, by J. H. Neher and M. H. McGrath. For additional information concerning the application of these ampacities, see IEEE STD 835-1994, Standard Power Cable Ampacity Tables.

Typical values of thermal resistivity (Rho) are as follows:

Average soil (90 percent of USA) = 90

Concrete = 55

Damp soil (coastal areas, high water table) = 60

Paper insulation = 550

Polyethylene (PE) = 450

Polyvinyl chloride (PVC) = 650

Rubber and rubber-like = 500

Very dry soil (rocky or sandy) = 120

Thermal resistivity, as used in this informative annex, refers to the heat transfer capability through a substance by conduction. It is the reciprocal of thermal conductivity and is normally expressed in the units°C-cm/watt. For additional information on determining soil thermal resistivity (Rho), see ANSI/IEEE STD 442-1996, Guide for Soil Thermal Resistivity Measurements.

### B.3 Criteria Modifications

Where values of load factor and Rho are known for a particular electrical duct bank installation and they are different from those shown in a specific table or figure, the ampacities shown in the table or figure can be modified by the application of factors derived from the use of Figure B.310.15(B)(2)(1).

Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity can be used beyond the point of transition, a distance equal to 3 m (10 ft) or 10 percent of the circuit length calculated at the higher ampacity, whichever is less.

Where the burial depth of direct burial or electrical duct bank circuits are modified from the values shown in a figure or table, ampacities can be modified as shown in (a) and (b) as follows.

(a) Where burial depths are increased in part(s) of an electrical duct run to avoid underground obstructions, no decrease in ampacity of the conductors is needed, provided the total length of parts of the duct run increased in depth to avoid obstructions is less than 25 percent of the total run length.

(b) Where burial depths are deeper than shown in a specific underground ampacity table or figure, an ampacity derating factor of 6 percent per increased 300 mm (foot) of depth for all values of Rho can be utilized. No rating change is needed where the burial depth is decreased.

Table B.310.15(B)(2)(1) Ampacities of Two or Three Insulated Conductors, Rated 0 Through 2000 Volts, Within an Overall Covering (Multiconductor Cable), in Raceway in Free Air Based on Ambient Air Temperature of 30°C (86°F)*

Size (AWG or kcmil) Temperature Rating of Conductor. [See Table 310.104(A).] Size (AWG or kcmil)
60°C
(140°F)
75°C
(167°F)
90°C
(194°F)
60°C
(140°F)
75°C
(167°F)
90°C
(194°F)
Types TW, UF Types RHW, THHW, THW, THWN, XHHW, ZW Types THHN, THHW, THW-2, THWN-2, RHH, RWH-2, USE-2, XHHW, XHHW-2, ZW-2 Type TW Types RHW, THHW, THW, THWN, XHHW Types THHN, THHW, THW-2, THWN-2, RHH, RWH-2,USE-2, XHHW, XHHW-2, ZW-2
14** 16 18 21 14
12** 20 24 27 16 18 21 12
10** 27 33 36 21 25 28 10
8 36 43 48 28 33 37 8
6 48 58 65 38 45 51 6
4 66 79 89 51 61 69 4
3 76 90 102 59 70 79 3
2 88 105 119 69 83 93 2
1 102 121 137 80 95 106 1
1/0 121 145 163 94 113 127 1/0
2/0 138 166 186 108 129 146 2/0
3/0 158 189 214 124 147 167 3/0
4/0 187 223 253 147 176 197 4/0
250 205 245 276 160 192 217 250
300 234 281 317 185 221 250 300
350 255 305 345 202 242 273 350
400 274 328 371 218 261 295 400
500 315 378 427 254 303 342 500
600 343 413 468 279 335 378 600
700 376 452 514 310 371 420 700
750 387 466 529 321 384 435 750
800 397 479 543 331 397 450 800
900 415 500 570 350 421 477 900
1000 448 542 617 382 460 521 1000
*Refer to 310.15(B)(2) for the ampacity correction factors where the ambient temperature is other than 30°C (86°F).
**Refer to 240.4(D) for conductor overcurrent protection limitations.

Table B.310.15(B)(2)(3) Ampacities of Multiconductor Cables with Not More Than Three Insulated Conductors, Rated 0 Through 2000 Volts, in Free Air Based on Ambient Air Temperature of 40°C (104°F) (for Types TC, MC, MI, UF, and USE Cables)*

Size (AWG or kcmil) Temperature Rating of Conductor. [See Table 310.104(A).] Size (AWG or kcmil)
60°C
(140°F)
75°C
(167°F)
85°C
(185°F)
90°C
(194°F)
60°C
(140°F)
75°C
(167°F)
85°C
(185°F)
90°C
(194°F)
18 11 18
16 16 16
14** 18 21 24 25 14
12** 21 28 30 32 18 21 24 25 12
10** 28 36 41 43 21 28 30 32 10
8 39 50 56 59 30 39 44 46 8
6 52 68 75 79 41 53 59 61 6
4 69 89 100 104 54 70 78 81 4
3 81 104 116 121 63 81 91 95 3
2 92 118 132 138 72 92 103 108 2
1 107 138 154 161 84 108 120 126 1
1/0 124 160 178 186 97 125 139 145 1/0
2/0 143 184 206 215 111 144 160 168 2/0
3/0 165 213 238 249 129 166 185 194 3/0
4/0 190 245 274 287 149 192 214 224 4/0
250 212 274 305 320 166 214 239 250 250
300 237 306 341 357 186 240 268 280 300
350 261 337 377 394 205 265 296 309 350
400 281 363 406 425 222 287 317 334 400
500 321 416 465 487 255 330 368 385 500
600 354 459 513 538 284 368 410 429 600
700 387 502 562 589 306 405 462 473 700
750 404 523 586 615 328 424 473 495 750
800 415 539 604 633 339 439 490 513 800
900 438 570 639 670 362 469 514 548 900
1000 461 601 674 707 385 499 558 584 1000
*Refer to 310.15(B)(2) for the ampacity correction factors where the ambient temperature is other than 40°C (104°F).
**Refer to 240.4(D) for conductor overcurrent protection limitations.

Table B.310.15(B)(2)(5) Ampacities of Single Insulated Conductors, Rated 0 Through 2000 Volts, in Nonmagnetic Underground Electrical Ducts (One Conductor per Electrical Duct), Based on Ambient Earth Temperature of 20°C (68°F), Electrical Duct Arrangement in Accordance with Figure B.310.15(B)(2)(2), Conductor Temperature 75°C (167°F)

Size (kcmil) 3 Electrical Ducts [Fig. B.310.15(B)(2)(2), Detail 2] 6 Electrical Ducts [Fig. B.310.15(B)(2)(2), Detail 3] 9 Electrical Ducts [Fig. B.310.15(B)(2)(2), Detail 4] 3 Electrical Ducts [Fig. B.310.15(B)(2)(2), Detail 2] 6 Electrical Ducts [Fig. B.310.15(B)(2)(2), Detail 3] 9 Electrical Ducts [Fig. B.310.15(B)(2)(2), Detail 4] Size (kcmil)
Types RHW, THHW, THW, THWN, XHHW, USE Types RHW, THHW, THW, THWN, XHHW, USE Types RHW, THHW, THW, THWN, XHHW, USE Types RHW, THHW, THW, THWN, XHHW, USE Types RHW, THHW, THW, THWN, XHHW, USE Types RHW, THHW, THW, THWN, XHHW, USE
RHO RHO RHO RHO RHO RHO RHO RHO RHO RHO RHO RHO RHO RHO RHO RHO RHO RHO
60 90 120 60 90 120 60 90 120 60 90 120 60 90 120 60 90 120
LF LF LF LF LF LF LF LF LF LF LF LF LF LF LF LF LF LF
50 100 100 50 100 100 50 100 100 50 100 100 50 100 100 50 100 100
250 410 344 327 386 295 275 369 270 252 320 269 256 302 230 214 288 211 197 250
350 503 418 396 472 355 330 446 322 299 393 327 310 369 277 258 350 252 235 350
500 624 511 484 583 431 400 545 387 360 489 401 379 457 337 313 430 305 284 500
750 794 640 603 736 534 494 674 469 434 626 505 475 581 421 389 538 375 347 750
1000 936 745 700 864 617 570 776 533 493 744 593 557 687 491 453 629 432 399 1000
1250 1055 832 781 970 686 632 854 581 536 848 668 627 779 551 508 703 478 441 1250
1500 1160 907 849 1063 744 685 918 619 571 941 736 689 863 604 556 767 517 477 1500
1750 1250 970 907 1142 793 729 975 651 599 1026 796 745 937 651 598 823 550 507 1750
2000 1332 1027 959 1213 836 768 1030 683 628 1103 850 794 1005 693 636 877 581 535 2000
Ambient Temp. (°C) Correction Factors Ambient Temp. (°F)
6—10 1.09 1.09 1.09 1.09 1.09 1.09 43—50
11—15 1.04 1.04 1.04 1.04 1.04 1.04 52—59
16—20 1.00 1.00 1.00 1.00 1.00 1.00 61—68
21—25 0.95 0.95 0.95 0.95 0.95 0.95 70—77
26—30 0.90 0.90 0.90 0.90 0.90 0.90 79—86

Table B.310.15(B)(2)(6) Ampacities of Three Insulated Conductors, Rated 0 Through 2000 Volts, Within an Overall Covering (Three-Conductor Cable) in Underground Electrical Ducts (One Cable per Electrical Duct) Based on Ambient Earth Temperature of 20°C (68°F), Electrical Duct Arrangement in Accordance with Figure B.310.15(B)(2)(2), Conductor Temperature 75°C (167°F)

Size (AWG or kcmil) 1 Electrical Duct [Fig. B.310.15(B)(2)(2), Detail 1] 3 Electrical Ducts [Fig. B.310.15(B)(2)(2), Detail 2] 6 Electrical Ducts [Fig. B.310.15(B)(2)(2), Detail 3] 1 Electrical Duct [Fig. B.310.15(B)(2)(2), Detail 1] 3 Electrical Ducts [Fig. B.310.15(B)(2)(2), Detail 2] 6 Electrical Ducts [Fig. B.310.15(B)(2)(2), Detail 3] Size (AWG or kcmil)
Types RHW, THHW, THW, THWN, XHHW, USE Types RHW, THHW, THW, THWN, XHHW, USE Types RHW, THHW, THW, THWN, XHHW, USE Types RHW, THHW, THW, THWN, XHHW, USE Types RHW, THHW, THW, THWN, XHHW, USE Types RHW, THHW, THW, THWN, XHHW, USE
RHO RHO RHO RHO RHO RHO RHO RHO RHO RHO RHO RHO RHO RHO RHO RHO RHO RHO
60 90 120 60 90 120 60 90 120 60 90 120 60 90 120 60 90 120
LF LF LF LF LF LF LF LF LF LF LF LF LF LF LF LF LF LF
50 100 100 50 100 100 50 100 100 50 100 100 50 100 100 50 100 100
8 58 54 53 56 48 46 53 42 39 45 42 41 43 37 36 41 32 30 8
6 77 71 69 74 63 60 70 54 51 60 55 54 57 49 47 54 42 39 6
4 101 93 91 96 81 77 91 69 65 78 72 71 75 63 60 71 54 51 4
2 132 121 118 126 105 100 119 89 83 103 94 92 98 82 78 92 70 65 2
1 154 140 136 146 121 114 137 102 95 120 109 106 114 94 89 107 79 74 1
1/0 177 160 156 168 137 130 157 116 107 138 125 122 131 107 101 122 90 84 1/0
2/0 203 183 178 192 156 147 179 131 121 158 143 139 150 122 115 140 102 95 2/0
3/0 233 210 204 221 178 158 205 148 137 182 164 159 172 139 131 160 116 107 3/0
4/0 268 240 232 253 202 190 234 168 155 209 187 182 198 158 149 183 131 121 4/0
250 297 265 256 280 222 209 258 184 169 233 207 201 219 174 163 202 144 132 250
350 363 321 310 340 267 250 312 219 202 285 252 244 267 209 196 245 172 158 350
500 444 389 375 414 320 299 377 261 240 352 308 297 328 254 237 299 207 190 500
750 552 478 459 511 388 362 462 314 288 446 386 372 413 314 293 374 254 233 750
1000 628 539 518 579 435 405 522 351 321 521 447 430 480 361 336 433 291 266 1000
Ambient Temp. (°C) Correction Factors Ambient Temp. (°F)
6—10 1.09 1.09 1.09 1.09 1.09 1.09 43—50
11—15 1.04 1.04 1.04 1.04 1.04 1.04 52—59
16—20 1.00 1.00 1.00 1.00 1.00 1.00 61—68
21—25 0.95 0.95 0.95 0.95 0.95 0.95 70—77
26—30 0.90 0.90 0.90 0.90 0.90 0.90 79—86

Table B.310.15(B)(2)(7) Ampacities of Three Single Insulated Conductors, Rated 0 Through 2000 Volts, in Underground Electrical Ducts (Three Conductors per Electrical Duct) Based on Ambient Earth Temperature of 20°C (68°F), Electrical Duct Arrangement in Accordance with Figure B.310.15(B)(2)(2), Conductor Temperature 75°C (167°F)

Size (AWG or kcmil) 1 Electrical Duct [Fig. B.310.15(B)(2)(2), Detail 1] 3 Electrical Ducts [Fig. B.310.15(B)(2)(2), Detail 2] 6 Electrical Ducts [Fig. B.310.15(B)(2)(2), Detail 3] 1 Electrical Duct [Fig. B.310.15(B)(2)(2), Detail 1] 3 Electrical Ducts [Fig. B.310.15(B)(2)(2), Detail 2] 6 Electrical Ducts [Fig. B.310.15(B)(2)(2), Detail 3] Size (AWG or kcmil)
Types RHW, THHW, THW, THWN, XHHW, USE Types RHW, THHW, THW, THWN, XHHW, USE Types RHW, THHW, THW, THWN, XHHW, USE Types RHW, THHW, THW, THWN, XHHW, USE Types RHW, THHW, THW, THWN, XHHW, USE Types RHW, THHW, THW, THWN, XHHW, USE
RHO RHO RHO RHO RHO RHO RHO RHO RHO RHO RHO RHO RHO RHO RHO RHO RHO RHO
60 90 120 60 90 120 60 90 120 60 90 120 60 90 120 60 90 120
LF LF LF LF LF LF LF LF LF LF LF LF LF LF LF LF LF LF
50 100 100 50 100 100 50 100 100 50 100 100 50 100 100 50 100 100
8 63 58 57 61 51 49 57 44 41 49 45 44 47 40 38 45 34 32 8
6 84 77 75 80 67 63 75 56 53 66 60 58 63 52 49 59 44 41 6
4 111 100 98 105 86 81 98 73 67 86 78 76 79 67 63 77 57 52 4
3 129 116 113 122 99 94 113 83 77 101 91 89 83 77 73 84 65 60 3
2 147 132 128 139 112 106 129 93 86 115 103 100 108 87 82 101 73 67 2
1 171 153 148 161 128 121 149 106 98 133 119 115 126 100 94 116 83 77 1
1/0 197 175 169 185 146 137 170 121 111 153 136 132 144 114 107 133 94 87 1/0
2/0 226 200 193 212 166 156 194 136 126 176 156 151 165 130 121 151 106 98 2/0
3/0 260 228 220 243 189 177 222 154 142 203 178 172 189 147 138 173 121 111 3/0
4/0 301 263 253 280 215 201 255 175 161 235 205 198 219 168 157 199 137 126 4/0
250 334 290 279 310 236 220 281 192 176 261 227 218 242 185 172 220 150 137 250
300 373 321 308 344 260 242 310 210 192 293 252 242 272 204 190 245 165 151 300
350 409 351 337 377 283 264 340 228 209 321 276 265 296 222 207 266 179 164 350
400 442 376 361 394 302 280 368 243 223 349 297 284 321 238 220 288 191 174 400
500 503 427 409 460 341 316 412 273 249 397 338 323 364 270 250 326 216 197 500
600 552 468 447 511 371 343 457 296 270 446 373 356 408 296 274 365 236 215 600
700 602 509 486 553 402 371 492 319 291 488 408 389 443 321 297 394 255 232 700
750 632 529 505 574 417 385 509 330 301 508 425 405 461 334 309 409 265 241 750
800 654 544 520 597 428 395 527 338 308 530 439 418 481 344 318 427 273 247 800
900 692 575 549 628 450 415 554 355 323 563 466 444 510 365 337 450 288 261 900
1000 730 605 576 659 472 435 581 372 338 597 494 471 538 385 355 475 304 276 1000
Ambient Temp. (°C) Correction Factors Ambient Temp. (°F)
6—10 1.09 1.09 1.09 1.09 1.09 1.09 43—50
11—15 1.04 1.04 1.04 1.04 1.04 1.04 52—59
16—20 1.00 1.00 1.00 1.00 1.00 1.00 61—68
21—25 0.95 0.95 0.95 0.95 0.95 0.95 70—77
26—30 0.90 0.90 0.90 0.90 0.90 0.90 79—86

Table B.310.15(B)(2)(8) Ampacities of Two or Three Insulated Conductors, Rated 0 Through 2000 Volts, Cabled Within an Overall (Two- or Three-Conductor) Covering, Directly Buried in Earth, Based on Ambient Earth Temperature of 20°C (68°F), Electrical Duct Arrangement in Accordance with Figure B.310.15(B)(2)(2), 100 Percent Load Factor, Thermal Resistance (Rho) of 90

Size (AWG or kcmil) 1 Cable [Fig. B.310.15(B)(2)(2), Detail 5] 2 Cables [Fig. B.310.15(B)(2)(2), Detail 6] 1 Cable [Fig. B.310.15(B)(2)(2), Detail 5] 2 Cables [Fig. B.310.15(B)(2)(2), Detail 6] Size (AWG or kcmil)
60°C
(140°F)
75°C
(167°F)
60°C
(140°F)
75°C
(167°F)
60°C
(140°F)
75°C
(167°F)
60°C
(140°F)
75°C
(167°F)
TYPES TYPES
UF RHW, THHW, THW, THWN, XHHW, USE UF RHW, THHW, THW, THWN, XHHW, USE UF RHW, THHW, THW, THWN, XHHW, USE UF RHW, THHW, THW, THWN, XHHW, USE
8 64 75 60 70 51 59 47 55 8
6 85 100 81 95 68 75 60 70 6
4 107 125 100 117 83 97 78 91 4
2 137 161 128 150 107 126 110 117 2
1 155 182 145 170 121 142 113 132 1
1/0 177 208 165 193 138 162 129 151 1/0
2/0 201 236 188 220 157 184 146 171 2/0
3/0 229 269 213 250 179 210 166 195 3/0
4/0 259 304 241 282 203 238 188 220 4/0
250 333 308 261 241 250
350 401 370 315 290 350
500 481 442 381 350 500
750 585 535 473 433 750
1000 657 600 545 497 1000
Ambient Temp. (°C) Correction Factors Ambient Temp. (°F)
6—10 1.12 1.09 1.12 1.09 1.12 1.09 1.12 1.09 43—50
11—15 1.06 1.04 1.06 1.04 1.06 1.04 1.06 1.04 52—59
16—20 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 61—68
21—25 0.94 0.95 0.94 0.95 0.94 0.95 0.94 0.95 70—77
26—30 0.87 0.90 0.87 0.90 0.87 0.90 0.87 0.90 79—86
Note: For ampacities of Type UF cable in underground electrical ducts, multiply the ampacities shown in the table by 0.74.

Table B.310.15(B)(2)(9) Ampacities of Three Triplexed Single Insulated Conductors, Rated 0 Through 2000 Volts, Directly Buried in Earth Based on Ambient Earth Temperature of 20°C (68°F), Electrical Duct Arrangement in Accordance with Figure B.310.15(B)(2) (2), 100 Percent Load Factor, Thermal Resistance (Rho) of 90

Size (AWG or kcmil) See Fig. B.310.15(B)(2)(2), Detail 7 See Fig. B.310.15(B)(2)(2), Detail 8 See Fig. B.310.15(B)(2)(2), Detail 7 See Fig. B.310.15(B)(2)(2), Detail 8 Size (AWG or kcmil)
60°C (140°F) 75°C (167°F) 60°C (140°F) 75°C (167°F) 60°C (140°F) 75°C (167°F) 60°C (140°F) 75°C (167°F)
TYPES TYPES
UF USE UF USE UF USE UF USE
8 72 84 66 77 55 65 51 60 8
6 91 107 84 99 72 84 66 77 6
4 119 139 109 128 92 108 85 100 4
2 153 179 140 164 119 139 109 128 2
1 173 203 159 186 135 158 124 145 1
1/0 197 231 181 212 154 180 141 165 1/0
2/0 223 262 205 240 175 205 159 187 2/0
3/0 254 298 232 272 199 233 181 212 3/0
4/0 289 339 263 308 226 265 206 241 4/0
250 370 336 289 263 250
350 445 403 349 316 350
500 536 483 424 382 500
750 654 587 525 471 750
1000 744 665 608 544 1000
Ambient Temp. (°C) Correction Factors Ambient Temp. (°F)
6—10 1.12 1.09 1.12 1.09 1.12 1.09 1.12 1.09 43—50
11—15 1.06 1.04 1.06 1.04 1.06 1.04 1.06 1.04 52—59
16—20 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 61—68
21—25 0.94 0.95 0.94 0.95 0.94 0.95 0.94 0.95 70—77
26—30 0.87 0.90 0.87 0.90 0.87 0.90 0.87 0.90 79—86

Table B.310.15(B)(2)(10) Ampacities of Three Single Insulated Conductors, Rated 0 Through 2000 Volts, Directly Buried in Earth Based on Ambient Earth Temperature of 20°C (68°F), Electrical Duct Arrangement in Accordance with Figure B.310.15(B)(2)(2), 100 Percent Load Factor, Thermal Resistance (Rho) of 90

Size (AWG or kcmil) See Fig. B.310.15(B)(2)(2), Detail 9 See Fig. B.310.15(B)(2)(2), Detail 10 See Fig. B.310.15(B)(2)(2), Detail 9 See Fig. B.310.15(B)(2)(2), Detail 10 Size (AWG or kcmil)
60°C (140°F) 75°C (167°F) 60°C (140°F) 75°C (167°F) 60°C (140°F) 75°C (167°F) 60°C (140°F) 75°C (167°F)
TYPES TYPES
UF USE UF USE UF USE UF USE
8 84 98 78 92 66 77 61 72 8
6 107 126 101 118 84 98 78 92 6
4 139 163 130 152 108 127 101 118 4
2 178 209 165 194 139 163 129 151 2
1 201 236 187 219 157 184 146 171 1
1/0 230 270 212 249 179 210 165 194 1/0
2/0 261 306 241 283 204 239 188 220 2/0
3/0 297 348 274 321 232 272 213 250 3/0
4/0 336 394 309 362 262 307 241 283 4/0
250 429 394 335 308 250
350 516 474 403 370 350
500 626 572 490 448 500
750 767 700 605 552 750
1000 887 808 706 642 1000
1250 979 891 787 716 1250
1500 1063 965 862 783 1500
1750 1133 1027 930 843 1750
2000 1195 1082 990 897 2000
Ambient Temp. (°C) Correction Factors Ambient Temp. (°F)
6—10 1.12 1.09 1.12 1.09 1.12 1.09 1.12 1.09 43—50
11—15 1.06 1.04 1.06 1.04 1.06 1.04 1.06 1.04 52—59
16—20 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 61—68
21—25 0.94 0.95 0.94 0.95 0.94 0.95 0.94 0.95 70—77
26—30 0.87 0.90 0.87 0.90 0.87 0.90 0.87 0.90 79—86

Table B.310.15(B)(2)(11) Adjustment Factors for More Than Three Current-Carrying Conductors in a Raceway or Cable with Load Diversity

Number of Conductors* Percent of Values in Tables as Adjusted for Ambient Temperature if Necessary
4—6 80
7—9 70
10—24 70**
25—42 60**
43—85 50**
*Number of conductors is the total number of conductors in the raceway or cable adjusted in accordance with 310.15(B)(4) and (5).
**These factors include the effects of a load diversity of 50 percent.

FIGURE B.310.15(B)(2)(1) Interpolation Chart for Cables in a Duct Bank. I1 = ampacity for Rho = 60, 50 LF; I2 = ampacity for Rho = 120, 100 LF (load factor); desired ampacity = F × I1.

FIGURE B.310.15(B)(2)(2) Cable Installation Dimensions for Use with Table B.310.15(B)(2)(5) Through Table B.310.15(B)(2)(10).

Design Criteria

Neutral and Equipment Grounding conductor (EGC) Duct = 150 mm (6 in.)

Phase Ducts = 75 to 125 mm (3 to 5 in.)

Conductor Material = Copper

Number of Cables per Duct = 3

Number of Cables per Phase = 9

Rho Concrete = Rho Earth — 5

Rho PVC Duct = 650

Rho Cable Insulation = 500

Rho Cable Jacket = 650

Notes:

1. Neutral configuration per 300.5(I), Exception No. 2, for isolated phase installations in nonmagnetic ducts.
2. Phasing is A, B, C in rows or columns. Where magnetic electrical ducts are used, conductors are installed A, B, C per electrical duct with the neutral and all equipment grounding conductors in the same electrical duct. In this case, the 6-in. trade size neutral duct is eliminated.
3. Maximum harmonic loading on the neutral conductor cannot exceed 50 percent of the phase current for the ampacities shown in the table below.
4. Metallic shields of Type MV-90 cable shall be grounded at one point only where using A, B, C phasing in rows or columns.
Size kcmil TYPES
RHW, THHW, THW, THWN, XHHW, USE, OR MV-90*
Size kcmil
Total per Phase Ampere Rating
RHO EARTH 60 LF 50 RHO EARTH 90 LF 100 RHO EARTH 120 LF 100
250 2340
(260A/Cable)
1530
(170A/Cable)
1395
(155A/Cable)
250
350 2790
(310A/Cable)
1800
(200A/Cable)
1665
(185A/Cable)
350
500 3375
(375A/Cable)
2160
(240A/Cable)
1980
(220A/Cable)
500

Ambient Temp. (°C) For ambient temperatures other than 20°C (68°F), multiply the ampacities shown above by the appropriate factor shown below. Ambient Temp. (°F)
6—10 1.09 1.09 1.09 1.09 1.09 43—50
11—15 1.04 1.04 1.04 1.04 1.04 52—59
16—20 1.00 1.00 1.00 1.00 1.00 61—68
21—25 0.95 0.95 0.95 0.95 0.95 70—77
26—30 0.90 0.90 0.90 0.90 0.90 79—86
*Limited to 75°C conductor temperature.

Informational Note Figure B.310.15(B)(2)(3) Ampacities of Single Insulated Conductors Rated 0 Through 5000 Volts in Underground Electrical Ducts (Three Conductors per Electrical Duct), Nine Single-Conductor Cables per Phase Based on Ambient Earth Temperature of 20°C (68°F), Conductor Temperature 75°C (167°F).

Design Criteria

Neutral and Equipment Grounding conductor (EGC) Duct = 150 mm (6 in.)

Phase Ducts = 75 (3 in.)

Conductor Material = Copper

Number of Cables per Duct = 1

Number of Cables per Phase = 4

Rho Concrete = Rho Earth — 5

Rho PVC Duct = 650

Rho Cable Insulation = 500

Rho Cable Jacket = 650

Notes:

1. Neutral configuration per 300.5(I), Exception No 2.
2. Maximum harmonic loading on the neutral conductor cannot exceed 50 percent of the phase current for the ampacities shown in the table below.
3. Metallic shields of Type MV-90 cable shall be grounded at one point only.
Size kcmil TYPES
RHW, THHW, THW, THWN, XHHW, USE, OR MV-90*
Size kcmil
Total per Phase Ampere Rating
RHO EARTH 60 LF 50 RHO EARTH 90 LF 100 RHO EARTH 120 LF 100
750 2820
(705A/Cable)
1860
(465A/Cable)
1680
(420A/Cable)
750
1000 3300
(825A/Cable)
2140
(535A/Cable)
1920
(480A/Cable)
1000
1250 3700
(925A/Cable)
2380
(595A/Cable)
2120
(530A/Cable)
1250
1500 4060
(1015A/Cable)
2580
(645A/Cable)
2300
(575A/Cable)
1500
1750 4360
(1090A/Cable)
2740
(685A/Cable)
2460
(615A/Cable)
1750

Ambient Temp. (°C) For ambient temperatures other than 20°C (68°F), multiply the ampacities shown above by the appropriate factor shown below. Ambient Temp. (°F)
6—10 1.09 1.09 1.09 1.09 1.09 43—50
11—15 1.04 1.04 1.04 1.04 1.04 52—59
16—20 1.00 1.00 1.00 1.00 1.00 61—68
21—25 0.95 0.95 0.95 0.95 0.95 70—77
26—30 0.90 0.90 0.90 0.90 0.90 79—86
*Limited to 75°C conductor temperature.

Informational Note Figure B.310.15(B)(2)(4) Ampacities of Single Insulated Conductors Rated 0 Through 5000 Volts in Nonmagnetic Underground Electrical Ducts (One Conductor per Electrical Duct), Four Single-Conductor Cables per Phase Based on Ambient Earth Temperature of 20°C (68°F), Conductor Temperature 75°C (167°F).

Design Criteria

Neutral and Equipment Grounding conductor (EGC) Duct = 150 mm (6 in.)

Phase Ducts = 75 (3 in.)

Conductor Material = Copper

Number of Cables per Duct = 1

Number of Cables per Phase = 5

Rho Concrete = Rho Earth — 5

Rho PVC Duct = 650

Rho Cable Insulation = 500

Rho Cable Jacket = 650

Notes:

1. Neutral configuration per 300.5(I), Exception No. 2.
2. Maximum harmonic loading on the neutral conductor cannot exceed 50 percent of the phase current for the ampacities shown in the table below.
3. Metallic shields of Type MV-90 cable shall be grounded at one point only.
Size kcmil TYPES
RHW, THHW, THW, THWN, XHHW, USE, OR MV-90*
Size kcmil
Total per Phase Ampere Rating
RHO EARTH 60 LF 50 RHO EARTH 90 LF 100 RHO EARTH 120 LF 100
2000 5575
(1115A/Cable)
3375
(675A/Cable)
3000
(600A/Cable)
2000

Ambient Temp. (°C) For ambient temperatures other than 20°C (68°F), multiply the ampacities shown above by the appropriate factor shown below. Ambient Temp. (°F)
6—10 1.09 1.09 1.09 1.09 1.09 43—50
11—15 1.04 1.04 1.04 1.04 1.04 52—59
16—20 1.00 1.00 1.00 1.00 1.00 61—68
21—25 0.95 0.95 0.95 0.95 0.95 70—77
26—30 0.90 0.90 0.90 0.90 0.90 79—86
*Limited to 75°C conductor temperature.

Informational Note Figure B.310.15(B)(2)(5) Ampacities of Single Insulated Conductors Rated 0 Through 5000 Volts in Nonmagnetic Underground Electrical Ducts (One Conductor per Electrical Duct), Five Single-Conductor Cables per Phase Based on Ambient Earth Temperature of 20°C (68°F), Conductor Temperature 75°C (167°F).

### B.4 Electrical Ducts

The term electrical duct(s) is defined in 310.60.

### B.5 Table B.310.15(B)(2)(6) and Table B.310.15(B)(2)(7)

(a) To obtain the ampacity of cables installed in two electrical ducts in one horizontal row with 190-mm (7.5-in.) center-to-center spacing between electrical ducts, similar to Figure B.310.15(B)(2)(2), Detail 1, multiply the ampacity shown for one duct in Table B.310.15(B)(2)(6) and Table B.310.15(B)(2)(7) by 0.88.

(b) To obtain the ampacity of cables installed in four electrical ducts in one horizontal row with 190-mm (7.5-in.) center-to-center spacing between electrical ducts, similar to Figure B.310.15(B)(2)(2), Detail 2, multiply the ampacity shown for three electrical ducts in Table B.310.15(B)(2)(6) and Table B.310.15(B)(2)(7) by 0.94.

### B.6 Electrical Ducts Used in Figure B.310.15(B)(2)(2)

If spacing between electrical ducts, as shown in Figure B.310.15(B)(2)(2), is less than as specified where electrical ducts enter equipment enclosures from underground, the ampacity of conductors contained within such electrical ducts need not be reduced.

### B.7 Examples Showing Use of Figure B.310.15(B)(2)(1) for Electrical Duct Bank Ampacity Modifications

Figure B.310.15(B)(2)(1) is used for interpolation or extrapolation for values of Rho and load factor for cables installed in electrical ducts. The upper family of curves shows the variation in ampacity and Rho at unity load factor in terms of I 1, the ampacity for Rho = 60, and 50 percent load factor. Each curve is designated for a particular ratio I 2/I 1, where I 2 is the ampacity at Rho = 120 and 100 percent load factor.

The lower family of curves shows the relationship between Rho and load factor that will give substantially the same ampacity as the indicated value of Rho at 100 percent load factor.

As an example, to find the ampacity of a 500-kcmil copper cable circuit for six electrical ducts as shown in Table B.310.15(B)(2)(5): At the Rho = 60, LF = 50, I 1 = 583; for Rho = 120 and LF = 100, I 2 = 400. The ratio I 2/I 1 = 0.686. Locate Rho = 90 at the bottom of the chart and follow the 90 Rho line to the intersection with 100 percent load factor where the equivalent Rho = 90. Then follow the 90 Rho line to I 2/I 1 ratio of 0.686 where F = 0.74. The desired ampacity = 0.74 × 583 = 431, which agrees with the table for Rho = 90, LF = 100.

To determine the ampacity for the same circuit where Rho = 80 and LF = 75, using Figure B.310.15(B)(2)(1), the equivalent Rho = 43, F = 0.855, and the desired ampacity = 0.855 × 583 = 498 amperes. Values for using Figure B.310.15(B)(2)(1) are found in the electrical duct bank ampacity tables of this informative annex.

Where the load factor is less than 100 percent and can be verified by measurement or calculation, the ampacity of electrical duct bank installations can be modified as shown. Different values of Rho can be accommodated in the same manner.

Informational Note: The ampacity limit for 10 through 85 current-carrying conductors is based on the following equation. For more than 85 conductors, special calculations are required that are beyond the scope of this table.

 where: A1 = ampacity from Table 310.15(B)(16), Table 310.15(B)(18), Table B.310.15(B)(2)(1), Table B.310.15(B)(2)(6), or Table B.310.15(B)(2)(7) multiplied by the appropriate adjustment factor from Table B.310.15(B)(2)(11). N = total number of conductors used to select adjustment factor from Table B.310.15(B)(2)(11) E = number of conductors carrying current simultaneously in the raceway or cable A2 = ampacity limit for the current-carrying conductors in the raceway or cable

Example 1

Calculate the ampacity limit for twelve 14 AWG THWN current-carrying conductors (75°C) in a raceway that contains 24 conductors that may, at different times, be current-carrying.

Example 2

Calculate the ampacity limit for eighteen 14 AWG THWN current-carrying conductors (75°C) in a raceway that contains 24 conductors that may, at different times, be current-carrying.