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.
- Equation Application Information. This informative annex provides application information for ampacities calcula‐ ted under engineering supervision.
- Typical Applications Covered by Tables. Typical ampaci‐ ties 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.
- 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 modi‐ fied 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.
- 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.
- 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.
- 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).
- To obtain the ampacity of cables installed in two elec‐ trical 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.
- 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.
- Electrical Ducts Used in Figure B.310.15(B)(2)(2). If spac‐ ing 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.
- 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 electri‐ cal 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 ampac‐ ity 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 ampac‐ ity 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 I2/I1 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 infor‐ mative annex.
Where the load factor is less than 100 percent and can be verified by measurement or calculation, the ampacity of electri‐ cal 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.
[B.310.15(B)(7)a]
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.
0.524
12
[B.310.15(B)(7)b]
2
A 200.7
14 amperes i.e., 50 percent diversity
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.
0.5N
E
A A or A , whichever is less
2
A | = | 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 | ||
A 2 | = | ampacity limit for the current-carrying |
conductors in the raceway or cable |
where:
1
1 1
[B.310.15(B)(7)c]
0.524
18
2
A 200.7 11.5 amperes
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 | ||
COPPER | ALUMINUM OR COPPER-CLAD ALUMINUM | ||||||
14**12**10**8 | 16202736 | 18243343 | 21273648 | — 162128 | — 182533 | — 212837 | 1412108 |
64321 | 48667688102 | 587990105121 | 6589102119137 | 3851596980 | 4561708395 | 51697993106 | 64321 |
1/02/03/04/0 | 121138158187 | 145166189223 | 163186214253 | 94108124147 | 113129147176 | 127146167197 | 1/02/03/04/0 |
250300350400500 | 205234255274315 | 245281305328378 | 276317345371427 | 160185202218254 | 192221242261303 | 217250273295342 | 250300350400500 |
6007007508009001000 | 343376387397415448 | 413452466479500542 | 468514529543570617 | 279310321331350382 | 335371384397421460 | 378420435450477521 | 6007007508009001000 |
*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) | ||
COPPER | ALUMINUM OR COPPER-CLAD ALUMINUM | ||||||||
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 | ||||||||||||||
COPPER | ALUMINUM OR COPPER-CLAD ALUMINUM | ||||||||||||||||||
RHO 60 LF 50 | RHO 90 LF 100 | RHO 120 LF 100 | RHO 60 LF 50 | RHO 90 LF 100 | RHO 120 LF 100 | RHO 60 LF 50 | RHO 90 LF 100 | RHO 120 LF 100 | RHO 60 LF 50 | RHO 90 LF 100 | RHO 120 LF 100 | RHO 60 LF 50 | RHO 90 LF 100 | RHO 120 LF 100 | RHO 60 LF 50 | RHO 90 LF 100 | RHO 120 LF 100 | ||
250350500750 | 410503624794 | 344418511640 | 327396484603 | 386472583736 | 295355431534 | 275330400494 | 369446545674 | 270322387469 | 252299360434 | 320393489626 | 269327401505 | 256310379475 | 302369457581 | 230277337421 | 214258313389 | 288350430538 | 211252305375 | 197235284347 | 250350500750 |
10001250150017502000 | 9361055116012501332 | 7458329079701027 | 700781849907959 | 864970106311421213 | 617686744793836 | 570632685729768 | 7768549189751030 | 533581619651683 | 493536571599628 | 74484894110261103 | 593668736796850 | 557627689745794 | 6877798639371005 | 491551604651693 | 453508556598636 | 629703767823877 | 432478517550581 | 399441477507535 | 10001250150017502000 |
Ambient Temp.(°C) | Correction Factors | Ambient Temp. (°F) | |||||||||||||||||
6–1011–1516–2021–2526–30 | 1.091.041.000.950.90 | 1.091.041.000.950.90 | 1.091.041.000.950.90 | 1.091.041.000.950.90 | 1.091.041.000.950.90 | 1.091.041.000.950.90 | 43–5052–5961–6870–7779–86 |
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 | ||||||||||||||
COPPER | ALUMINUM OR COPPER-CLAD ALUMINUM | ||||||||||||||||||
RHO 60 LF 50 | RHO 90 LF 100 | RHO 120 LF 100 | RHO 60 LF 50 | RHO 90 LF 100 | RHO 120 LF 100 | RHO 60 LF 50 | RHO 90 LF 100 | RHO 120 LF 100 | RHO 60 LF 50 | RHO 90 LF 100 | RHO 120 LF 100 | RHO 60 LF 50 | RHO 90 LF 100 | RHO 120 LF 100 | RHO 60 LF 50 | RHO 90 LF 100 | RHO 120 LF 100 | ||
86421 | 5877101132154 | 547193121140 | 536991118136 | 567496126146 | 486381105121 | 466077100114 | 537091119137 | 42546989102 | 3951658395 | 456078103120 | 42557294109 | 41547192106 | 43577598114 | 3749638294 | 3647607889 | 41547192107 | 3242547079 | 3039516574 | 86421 |
1/02/03/04/0 | 177203233268 | 160183210240 | 156178204232 | 168192221253 | 137156178202 | 130147158190 | 157179205234 | 116131148168 | 107121137155 | 138158182209 | 125143164187 | 122139159182 | 131150172198 | 107122139158 | 101115131149 | 122140160183 | 90102116131 | 8495107121 | 1/02/03/04/0 |
2503505007501000 | 297363444552628 | 265321389478539 | 256310375459518 | 280340414511579 | 222267320388435 | 209250299362405 | 258312377462522 | 184219261314351 | 169202240288321 | 233285352446521 | 207252308386447 | 201244297372430 | 219267328413480 | 174209254314361 | 163196237293336 | 202245299374433 | 144172207254291 | 132158190233266 | 2503505007501000 |
Ambient Temp.(°C) | Correction Factors | Ambient Temp.(°F) | |||||||||||||||||
6–1011–1516–2021–2526–30 | 1.091.041.000.950.90 | 1.091.041.000.950.90 | 1.091.041.000.950.90 | 1.091.041.000.950.90 | 1.091.041.000.950.90 | 1.091.041.000.950.90 | 43–5052–5961–6870–7779–86 |
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 | ||||||||||||||
COPPER | ALUMINUM OR COPPER-CLAD ALUMINUM | ||||||||||||||||||
RHO 60 LF 50 | RHO 90 LF 100 | RHO 120 LF 100 | RHO 60 LF 50 | RHO 90 LF 100 | RHO 120 LF 100 | RHO 60 LF 50 | RHO 90 LF 100 | RHO 120 LF 100 | RHO 60 LF 50 | RHO 90 LF 100 | RHO 120 LF 100 | RHO 60 LF 50 | RHO 90 LF 100 | RHO 120 LF 100 | RHO 60 LF 50 | RHO 90 LF 100 | RHO 120 LF 100 | ||
864321 | 6384111129147171 | 5877100116132153 | 577598113128148 | 6180105122139161 | 51678699112128 | 49638194106121 | 577598113129149 | 4456738393106 | 415367778698 | 496686101115133 | 45607891103119 | 44587689100115 | 47637983108126 | 4052677787100 | 384963738294 | 45597784101116 | 344457657383 | 324152606777 | 864321 |
1/02/03/04/0 | 197226260301 | 175200228263 | 169193220253 | 185212243280 | 146166189215 | 137156177201 | 170194222255 | 121136154175 | 111126142161 | 153176203235 | 136156178205 | 132151172198 | 144165189219 | 114130147168 | 107121138157 | 133151173199 | 94106121137 | 8798111126 | 1/02/03/04/0 |
250300350400 | 334373409442 | 290321351376 | 279308337361 | 310344377394 | 236260283302 | 220242264280 | 281310340368 | 192210228243 | 176192209223 | 261293321349 | 227252276297 | 218242265284 | 242272296321 | 185204222238 | 172190207220 | 220245266288 | 150165179191 | 137151164174 | 250300350400 |
500600700750 | 503552602632 | 427468509529 | 409447486505 | 460511553574 | 341371402417 | 316343371385 | 412457492509 | 273296319330 | 249270291301 | 397446488508 | 338373408425 | 323356389405 | 364408443461 | 270296321334 | 250274297309 | 326365394409 | 216236255265 | 197215232241 | 500600700750 |
8009001000 | 654692730 | 544575605 | 520549576 | 597628659 | 428450472 | 395415435 | 527554581 | 338355372 | 308323338 | 530563597 | 439466494 | 418444471 | 481510538 | 344365385 | 318337355 | 427450475 | 273288304 | 247261276 | 8009001000 |
Ambient Temp.(°C) | Correction Factors | Ambient Temp.(°F) | |||||||||||||||||
6–1011–1516–2021–2526–30 | 1.091.041.000.950.90 | 1.091.041.000.950.90 | 1.091.041.000.950.90 | 1.091.041.000.950.90 | 1.091.041.000.950.90 | 1.091.041.000.950.90 | 43–5052–5961–6870–7779–86 |
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 | ||
COPPER | ALUMINUM OR COPPER-CLAD ALUMINUM | ||||||||
86421 | 6485107137155 | 75100125161182 | 6081100128145 | 7095117150170 | 516883107121 | 597597126142 | 476078110113 | 557091117132 | 86421 |
1/02/03/04/0 | 177201229259 | 208236269304 | 165188213241 | 193220250282 | 138157179203 | 162184210238 | 129146166188 | 151171195220 | 1/02/03/04/0 |
2503505007501000 | ————— | 333401481585657 | ————— | 308370442535600 | ————— | 261315381473545 | ————— | 241290350433497 | 2503505007501000 |
Ambient Temp. (°C) | Correction Factors | Ambient Temp. (°F) | |||||||
6–1011–1516–2021–2526–30 | 1.121.061.000.940.87 | 1.091.041.000.950.90 | 1.121.061.000.940.87 | 1.091.041.000.950.90 | 1.121.061.000.940.87 | 1.091.041.000.950.90 | 1.121.061.000.940.87 | 1.091.041.000.950.90 | 43–5052–5961–6870–7779–86 |
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 | ||
COPPER | ALUMINUM OR COPPER-CLAD ALUMINUM | ||||||||
86421 | 7291119153173 | 84107139179203 | 6684109140159 | 7799128164186 | 557292119135 | 6584108139158 | 516685109124 | 6077100128145 | 86421 |
1/02/03/04/0 | 197223254289 | 231262298339 | 181205232263 | 212240272308 | 154175199226 | 180205233265 | 141159181206 | 165187212241 | 1/02/03/04/0 |
2503505007501000 | ————— | 370445536654744 | ————— | 336403483587665 | ————— | 289349424525608 | ————— | 263316382471544 | 2503505007501000 |
Ambient Temp. (°C) | Correction Factors | Ambient Temp. (°F) | |||||||
6–1011–1516–2021–2526–30 | 1.121.061.000.940.87 | 1.091.041.000.950.90 | 1.121.061.000.940.87 | 1.091.041.000.950.90 | 1.121.061.000.940.87 | 1.091.041.000.950.90 | 1.121.061.000.940.87 | 1.091.041.000.950.90 | 43–5052–5961–6870–7779–86 |
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 | ||
COPPER | ALUMINUM OR COPPER-CLAD ALUMINUM | ||||||||
86421 | 84107139178201 | 98126163209236 | 78101130165187 | 92118152194219 | 6684108139157 | 7798127163184 | 6178101129146 | 7292118151171 | 86421 |
1/02/03/04/0 | 230261297336 | 270306348394 | 212241274309 | 249283321362 | 179204232262 | 210239272307 | 165188213241 | 194220250283 | 1/02/03/04/0 |
250350500750 | ———— | 429516626767 | ———— | 394474572700 | ———— | 335403490605 | ———— | 308370448552 | 250350500750 |
10001250150017502000 | ————— | 887979106311331195 | ————— | 80889196510271082 | ————— | 706787862930990 | ————— | 642716783843897 | 10001250150017502000 |
Ambient Temp. (°C) | Correction Factors | Ambient Temp. (°F) | |||||||
6–1011–1516–2021–2526–30 | 1.121.061.000.940.87 | 1.091.041.000.950.90 | 1.121.061.000.940.87 | 1.091.041.000.950.90 | 1.121.061.000.940.87 | 1.091.041.000.950.90 | 1.121.061.000.940.87 | 1.091.041.000.950.90 | 43–5052–5961–6870–7779–86 |
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.
10 20 30 40 50 60 70 80 90 100 110 120 130140 150 160 170 180 190 200
1.15
1.10
1.05
1.00
0.95
0.95
0.90
0.90
0.85
I 2
Ratio — I
1
0.85
0.80
F
0.80
0.75
0.75
0.70
0.70
0.65
0.65
0.60
0.60
0.55
0.55
0.50
0.50
0.45
0.40
100
95
90
85
80
75
Load Factor
70
65
60
55
50
45
40
35
30
0 10 20 30 40 50 60 70 80 90 100 110 120 130140 150 160 170 180 190 200
RHO
FIGURE B.310.15(B)(2)(1) Interpolation Chart for Cables in a Duct Bank. I 1 = ampacity for Rho = 60, 50 LF; I 2 = ampacity for Rho = 120, 100 LF (load factor); desired ampacity
= F × I 1.
190 mm
(7.5 in.)
Detail 1
290 mm 290 mm
(11.5 in. 11.5 in.)
Electrical duct bank One electrical duct
190 mm (7.5in.)
190 mm (7.5 in.)
Detail 2
475 mm 475 mm
(19 in. 19 in.) Electrical duct bank Three electrical ducts
or
190 mm (7.5 in.)
Detail 3
475 mm 675 mm
190 mm
(7.5 in.)
190 mm 190 mm
(7.5 in.) (7.5 in.)
190 mm
(7.5 in.)
Detail 4
190 mm
(7.5 in.)
190 mm
(7.5 in.)
190 mm
(7.5 in.)
(19 in. 27 in.) Electrical duct bank Six electrical ducts
or
675 mm 675 mm
190 mm
(7.5 in.)
(27 in. 27 in.) Electrical duct bank Nine electrical ducts
675 mm 290 mm
(27 in. 11.5 in.) Electrical duct bank Three electrical ducts
190 mm
(7.5 in.)
190 mm
(7.5 in.)
600 mm (24 in.)
675 mm 475 mm
(27 in. 19 in.) Electrical duct bank
600 mm (24 in.)
Detail 5
Buried 3 conductor cable
Detail 6
Buried 3 conductor cables
Detail 7
Buried triplexed cables (1 circuit)
Detail 8
Buried triplexed cables (2 circuits)
190 mm
(7.5 in.)
190 mm
(7.5 in.)
190 mm
(7.5 in.)
190 mm
(7.5 in.)
(24 in.)
600 mm
190 mm
(7.5 in.)
190 mm
(7.5 in.)
Detail 9
Buried single-conductor cables (1 circuit)
Detail 10
Buried single-conductor cables (2 circuits)
Note 1: Minimum burial depths to top electrical ducts or cables shall be in accordance with 300.5. Maximum depth to the top of electrical duct banks shall be 750 mm (30 in.) and maximum depth to the top of direct-buried cables shall be 900 mm (36 in.)
Note 2: For two and four electrical duct installations with electrical ducts installed in a single row, see B.310.15(B)(5).
Legend
Backfill
(earth or concrete) Electrical duct
Cable or cables
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).
Surface
150 mm (6 in.)
N and EGC
150 mm (6 in.)
600 mm (24 in.)
190 mm (7.5 in.)
750 mm (30 in.)
190 mm (7.5 in.)
190 mm 190 mm
(7.5 in.) (7.5 in.)
675 mm (27 in.)
(Typical for all)
N and EGC
Surface 150 mm (6 in.)
150 mm (6 in.)
AØ
250 mm (10 in.) BØ
250 mm (10 in.) CØ
600 mm (24 in.)
950 mm (38 in.)
250 mm
(10 in.)
250 mm
(10 in.)
250 mm
(10 in.)
Design Criteria
Neutral and Equipment Number of Cables per Phase = 9
Grounding conductor (EGC) Rho Concrete = Rho Earth – 5
Duct = 150 mm (6 in.) Rho PVC Duct = 650 Phase Ducts = 75 to 125 mm (3 to 5 in.) Rho Cable Insulation = 500 Conductor Material = Copper Rho Cable Jacket = 650 Number of Cables per Duct = 3
Notes:
- Neutral configuration per 300.5(I), Exception No. 2, for isolated phase installations in
Design CriteriaNeutral and Equipment Grounding conductor (EGC) Duct = 150 mm (6 in.)Phase Ducts = 75 mm (3 in.) Conductor Material = Copper Number of Cables per Duct = 1
Notes:1.1 m (44 in.)
Number of Cables per Phase = 4 Rho Concrete = Rho Earth – 5 Rho PVC Duct = 650Rho Cable Insulation = 500Rho Cable Jacket = 650nonmagnetic ducts. - 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.
- Maximum harmonic loading on the neutral conductor cannot exceed 50 percent of the phase current for the ampacities shown in the table below.
- 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 | TYPESRHW, THHW, THW, THWN, XHHW, USE, OR MV-90* | Size kcmil | ||
Total per Phase Ampere Rating | ||||
RHO EARTH 60LF 50 | RHO EARTH 90LF 100 | RHO EARTH 120LF 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 20C (68F), 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 75C 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).
- Neutral configuration per 300.5(I), Exception No 2.
- Maximum harmonic loading on the neutral conductor cannot exceed 50 percent of the phase current for the ampacities shown in the table below.
- 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 60LF 50 | RHO EARTH 90LF 100 | RHO EARTH 120LF 100 | ||
2820 | 1860 | 1680 | ||
750 | (705A/Cable) | (465A/Cable) | (420A/Cable) | 750 |
3300 | 2140 | 1920 | ||
1000 | (825A/Cable) | (535A/Cable) | (480A/Cable) | 1000 |
1250 | 3700(925A/Cable) | 2380(595A/Cable) | 2120(530A/Cable) | 1250 |
4060 | 2580 | 2300 | ||
1500 | (1015A/Cable) | (645A/Cable) | (575A/Cable) | 1500 |
1750 | 4360(1090A/Cable) | 2740(685A/Cable) | 2460(615A/Cable) | 1750 |
Ambient Temp. (C) | For ambient temperatures other than 20C (68F), 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 75C 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).
Surface
N and EGC
150 mm (6 in.)
150 mm (6 in.)
AØ
250 mm (10 in.) BØ
250 mm (10 in.) CØ
600 mm (24 in.)
950 mm (38 in.)
250 mm 250 mm 250 mm 250 mm
(10 in.)
(10 in.)
(10 in.)
(10 in.)
1.1 m (44 in.)
Design Criteria
Neutral and Equipment Number of Cables per Phase = 5 Grounding conductor (EGC) Rho Concrete = Rho Earth – 5 Duct = 150 mm (6 in.) Rho PVC Duct = 650
Phase Ducts = 75 mm (3 in.) Rho Cable Insulation = 500 Conductor Material = Copper Rho Cable Jacket = 650 Number of Cables per Duct = 1
Notes:
- Neutral configuration per 300.5(I), Exception No. 2.
- Maximum harmonic loading on the neutral conductor cannot exceed 50 percent of the phase current for the ampacities shown in the table below.
- Metallic shields of Type MV-90 cable shall be grounded at one point only.
Size kcmil | TYPESRHW, THHW, THW, THWN, XHHW, USE, OR MV-90* | Size kcmil | ||
Total per Phase Ampere Rating | ||||
RHO EARTH 60LF 50 | RHO EARTH 90LF 100 | RHO EARTH 120LF 100 | ||
2000 | 5575(1115A/Cable) | 3375(675A/Cable) | 3000(600A/Cable) | 2000 |
Ambient Temp. (C) | For ambient temperatures other than 20C (68F), 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 75C 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).