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.

  1. Equation Application Information. This informative annex provides application information for ampacities calcula‐ ted under engineering supervision.
  2. 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.

  1. 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.
    1. 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.
    2. 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.
  2. 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).
    1. 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.
    2. 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.
  1. 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.
  2. 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 1, the ampacity for Rho = 60, and 50 percent load factor. Each curve is designated for a particular ratio 2/1, where 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, 1 = 583; for Rho = 120 and LF = 100, 2 = 400. The ratio 2/1 = 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 = 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.

image

0.524

12

[B.310.15(B)(7)b]

2

  200.7

 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.

image

0.5N

E

   or , 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]

image

0.524

18

2

  200.7  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
COPPERALUMINUM OR COPPER-CLAD ALUMINUM
14**12**10**8162027361824334321273648— 162128— 182533— 2128371412108
64321486676881025879901051216589102119137385159698045617083955169799310664321
1/02/03/04/0121138158187145166189223163186214253941081241471131291471761271461671971/02/03/04/0
250300350400500205234255274315245281305328378276317345371427160185202218254192221242261303217250273295342250300350400500
60070075080090010003433763873974154484134524664795005424685145295435706172793103213313503823353713843974214603784204354504775216007007508009001000

*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)
COPPERALUMINUM OR COPPER-CLAD ALUMINUM
181118
161616
14**1821242514
12**212830321821242512
10**283641432128303210
839505659303944468
652687579415359616
46989100104547078814
381104116121638191953
29211813213872921031082
1107138154161841081201261
1/0124160178186971251391451/0
2/01431842062151111441601682/0
3/01652132382491291661851943/0
4/01902452742871491922142244/0
250212274305320166214239250250
300237306341357186240268280300
350261337377394205265296309350
400281363406425222287317334400
500321416465487255330368385500
600354459513538284368410429600
700387502562589306405462473700
750404523586615328424473495750
800415539604633339439490513800
900438570639670362469514548900
10004616016747073854995585841000

*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, USETypes RHW, THHW, THW, THWN, XHHW, USETypes RHW, THHW, THW, THWN, XHHW, USETypes RHW, THHW, THW, THWN, XHHW, USETypes RHW, THHW, THW, THWN, XHHW, USETypes RHW, THHW, THW, THWN, XHHW, USE
COPPERALUMINUM OR COPPER-CLAD ALUMINUM
RHO 60 LF 50RHO 90 LF 100RHO 120 LF 100RHO 60 LF 50RHO 90 LF 100RHO 120 LF 100RHO 60 LF 50RHO 90 LF 100RHO 120 LF 100RHO 60 LF 50RHO 90 LF 100RHO 120 LF 100RHO 60 LF 50RHO 90 LF 100RHO 120 LF 100RHO 60 LF 50RHO 90 LF 100RHO 120 LF 100
250350500750410503624794344418511640327396484603386472583736295355431534275330400494369446545674270322387469252299360434320393489626269327401505256310379475302369457581230277337421214258313389288350430538211252305375197235284347250350500750
1000125015001750200093610551160125013327458329079701027700781849907959864970106311421213617686744793836570632685729768776854918975103053358161965168349353657159962874484894110261103593668736796850557627689745794687779863937100549155160465169345350855659863662970376782387743247851755058139944147750753510001250150017502000
Ambient Temp.(°C)
Correction Factors
Ambient Temp. (°F)
6–1011–1516–2021–2526–301.091.041.000.950.901.091.041.000.950.901.091.041.000.950.901.091.041.000.950.901.091.041.000.950.901.091.041.000.950.9043–5052–5961–6870–7779–86

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, USETypes RHW, THHW, THW, THWN, XHHW, USETypes RHW, THHW, THW, THWN, XHHW, USETypes RHW, THHW, THW, THWN, XHHW, USETypes RHW, THHW, THW, THWN, XHHW, USETypes RHW, THHW, THW, THWN, XHHW, USE
COPPERALUMINUM OR COPPER-CLAD ALUMINUM
RHO 60 LF 50RHO 90 LF 100RHO 120 LF 100RHO 60 LF 50RHO 90 LF 100RHO 120 LF 100RHO 60 LF 50RHO 90 LF 100RHO 120 LF 100RHO 60 LF 50RHO 90 LF 100RHO 120 LF 100RHO 60 LF 50RHO 90 LF 100RHO 120 LF 100RHO 60 LF 50RHO 90 LF 100RHO 120 LF 100
86421587710113215454719312114053699111813656749612614648638110512146607710011453709111913742546989102395165839545607810312042557294109415471921064357759811437496382943647607889415471921073242547079303951657486421
1/02/03/04/01772032332681601832102401561782042321681922212531371561782021301471581901571792052341161311481681071211371551381581822091251431641871221391591821311501721981071221391581011151311491221401601839010211613184951071211/02/03/04/0
25035050075010002973634445526282653213894785392563103754595182803404145115792222673203884352092502993624052583123774625221842192613143511692022402883212332853524465212072523083864472012442973724302192673284134801742092543143611631962372933362022452993744331441722072542911321581902332662503505007501000
Ambient Temp.(°C)Correction FactorsAmbient Temp.(°F)
6–1011–1516–2021–2526–301.091.041.000.950.901.091.041.000.950.901.091.041.000.950.901.091.041.000.950.901.091.041.000.950.901.091.041.000.950.9043–5052–5961–6870–7779–86

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, USETypes RHW, THHW, THW, THWN, XHHW, USETypes RHW, THHW, THW, THWN, XHHW, USETypes RHW, THHW, THW, THWN, XHHW, USETypes RHW, THHW, THW, THWN, XHHW, USETypes RHW, THHW, THW, THWN, XHHW, USE
COPPERALUMINUM OR COPPER-CLAD ALUMINUM
RHO 60 LF 50RHO 90 LF 100RHO 120 LF 100RHO 60 LF 50RHO 90 LF 100RHO 120 LF 100RHO 60 LF 50RHO 90 LF 100RHO 120 LF 100RHO 60 LF 50RHO 90 LF 100RHO 120 LF 100RHO 60 LF 50RHO 90 LF 100RHO 120 LF 100RHO 60 LF 50RHO 90 LF 100RHO 120 LF 100
86432163841111291471715877100116132153577598113128148618010512213916151678699112128496381941061215775981131291494456738393106415367778698496686101115133456078911031194458768910011547637983108126405267778710038496373829445597784101116344457657383324152606777864321
1/02/03/04/01972262603011752002282631691932202531852122432801461661892151371561772011701942222551211361541751111261421611531762032351361561782051321511721981441651892191141301471681071211381571331511731999410612113787981111261/02/03/04/0
250300350400334373409442290321351376279308337361310344377394236260283302220242264280281310340368192210228243176192209223261293321349227252276297218242265284242272296321185204222238172190207220220245266288150165179191137151164174250300350400
500600700750503552602632427468509529409447486505460511553574341371402417316343371385412457492509273296319330249270291301397446488508338373408425323356389405364408443461270296321334250274297309326365394409216236255265197215232241500600700750
80090010006546927305445756055205495765976286594284504723954154355275545813383553723083233385305635974394664944184444714815105383443653853183373554274504752732883042472612768009001000
Ambient Temp.(°C)Correction FactorsAmbient Temp.(°F)
6–1011–1516–2021–2526–301.091.041.000.950.901.091.041.000.950.901.091.041.000.950.901.091.041.000.950.901.091.041.000.950.901.091.041.000.950.9043–5052–5961–6870–7779–86

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)
TYPESTYPES

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
COPPERALUMINUM OR COPPER-CLAD ALUMINUM
864216485107137155751001251611826081100128145709511715017051688310712159759712614247607811011355709111713286421
1/02/03/04/01772012292592082362693041651882132411932202502821381571792031621842102381291461661881511711952201/02/03/04/0
2503505007501000—————333401481585657—————308370442535600—————261315381473545—————2412903504334972503505007501000
Ambient Temp. (°C)Correction FactorsAmbient Temp. (°F)
6–1011–1516–2021–2526–301.121.061.000.940.871.091.041.000.950.901.121.061.000.940.871.091.041.000.950.901.121.061.000.940.871.091.041.000.950.901.121.061.000.940.871.091.041.000.950.9043–5052–5961–6870–7779–86

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 7See Fig. B.310.15(B)(2)(2),Detail 8See Fig. B.310.15(B)(2)(2),Detail 7See 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)
TYPESTYPES
UFUSEUFUSEUFUSEUFUSE
COPPERALUMINUM OR COPPER-CLAD ALUMINUM
86421729111915317384107139179203668410914015977991281641865572921191356584108139158516685109124607710012814586421
1/02/03/04/01972232542892312622983391812052322632122402723081541751992261802052332651411591812061651872122411/02/03/04/0
2503505007501000—————370445536654744—————336403483587665—————289349424525608—————2633163824715442503505007501000
Ambient Temp. (°C)Correction FactorsAmbient Temp. (°F)
6–1011–1516–2021–2526–301.121.061.000.940.871.091.041.000.950.901.121.061.000.940.871.091.041.000.950.901.121.061.000.940.871.091.041.000.950.901.121.061.000.940.871.091.041.000.950.9043–5052–5961–6870–7779–86

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 9See Fig. B.310.15(B)(2)(2),Detail 10See Fig. B.310.15(B)(2)(2),Detail 9See 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)
TYPESTYPES
UFUSEUFUSEUFUSEUFUSE
COPPERALUMINUM OR COPPER-CLAD ALUMINUM
8642184107139178201981261632092367810113016518792118152194219668410813915777981271631846178101129146729211815117186421
1/02/03/04/02302612973362703063483942122412743092492833213621792042322622102392723071651882132411942202502831/02/03/04/0
250350500750————429516626767————394474572700————335403490605————308370448552250350500750
10001250150017502000—————887979106311331195—————80889196510271082—————706787862930990—————64271678384389710001250150017502000
Ambient Temp. (°C)Correction FactorsAmbient Temp. (°F)
6–1011–1516–2021–2526–301.121.061.000.940.871.091.041.000.950.901.121.061.000.940.871.091.041.000.950.901.121.061.000.940.871.091.041.000.950.901.121.061.000.940.871.091.041.000.950.9043–5052–5961–6870–7779–86

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

image

Number of Conductors*

Percent of Values in Tables as Adjusted for Ambient Temperature if Necessary

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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.

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image

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. 1 = ampacity for Rho = 60, 50 LF; 2 = ampacity for Rho = 120, 100 LF (load factor); desired ampacity

= F × 1.

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190 mm

(7.5 in.)

Detail 1

290 mm  290 mm

(11.5 in.  11.5 in.)

Electrical duct bank One electrical duct

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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

image
image
image

190 mm (7.5 in.)

Detail 3

image

475 mm  675 mm

image

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

image
image

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.)

image
image

         600 mm          (24 in.)

675 mm  475 mm

(27 in.  19 in.) Electrical duct bank

image
image

         600 mm          (24 in.)

image
image

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

image

(7.5 in.)

190 mm

(7.5 in.)

190 mm

(7.5 in.)

190 mm

(7.5 in.)

image

(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

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Backfill

(earth or concrete) image Electrical duct

image

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

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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.)

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.)

image

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:

  1. 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.
  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
TYPESRHW, THHW, THW, THWN, XHHW, USE, OR MV-90*
Size kcmil
Total per Phase Ampere Rating
RHO EARTH 60LF 50RHO EARTH 90LF 100RHO 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 20C (68F), multiply the ampacities shown above by the appropriate factor shown below.Ambient Temp. (F)
6–101.091.091.091.091.0943–50
11–151.041.041.041.041.0452–59
16–201.001.001.001.001.0061–68
21–250.950.950.950.950.9570–77
26–300.900.900.900.900.9079–86

*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).

  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 60LF 50RHO EARTH 90LF 100RHO EARTH 120LF 100
282018601680
750(705A/Cable)(465A/Cable)(420A/Cable)750
330021401920
1000(825A/Cable)(535A/Cable)(480A/Cable)1000

1250
3700(925A/Cable)2380(595A/Cable)2120(530A/Cable)
1250
406025802300
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 20C (68F), multiply the ampacities shown above by the appropriate factor shown below.Ambient Temp. (F)
6–101.091.091.091.091.0943–50
11–151.041.041.041.041.0452–59
16–201.001.001.001.001.0061–68
21–250.950.950.950.950.9570–77
26–300.900.900.900.900.9079–86

*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).

image

Surface

N and EGC

150 mm (6 in.)

150 mm (6 in.)

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:

  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
TYPESRHW, THHW, THW, THWN, XHHW, USE, OR MV-90*
Size kcmil
Total per Phase Ampere Rating
RHO EARTH 60LF 50RHO EARTH 90LF 100RHO EARTH 120LF 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)
6–101.091.091.091.091.0943–50
11–151.041.041.041.041.0452–59
16–201.001.001.001.001.0061–68
21–250.950.950.950.950.9570–77
26–300.900.900.900.900.9079–86

*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).

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