Conductor Ampacity Calculation – Part Eleven


In Article " Conductor Ampacity Calculation – Part Ten ", I explained how to use the Annex B Tables included under Group#2-A “ tables for conductors installed in underground electrical duct bank” which includes: 

  • TABLE B.310.15(B)(2)(5) in Accordance with Figure B.310.15(B)(2)(2), 
  • TABLE B.310.15(B)(2)(6) in Accordance with Figure B.310.15(B)(2)(2), 
  • TABLE B.310.15(B)(2)(7) in Accordance with Figure B.310.15(B)(2)(2), 


Today, I will explain how to use Other Annex B tables and figures. 




For more information about the Second method for Conductors ampacity calculations by using Annex B Tables under engineering supervision, please review the following articles:




For more information about the first method for Conductors ampacity calculations by using Tables as provided in 310.15(B), please review the following articles:






              Using Annex B tables and figures:







              Rule#1: Annex B Tables And Figures

              Annex B consists of many tables and figures as follows:

              1- Tables:

              • TABLE B.310.15(B)(2)(1),
              • TABLE B.310.15(B)(2)(3),
              • TABLE B.310.15(B)(2)(5) in Accordance with Figure B.310.15(B)(2)(2),
              • TABLE B.310.15(B)(2)(6) in Accordance with Figure B.310.15(B)(2)(2),
              • TABLE B.310.15(B)(2)(7) in Accordance with Figure B.310.15(B)(2)(2),
              • TABLE B.310.15(B)(2)(8)  in Accordance with Figure B.310.15(B)(2)(2),
              • TABLE B.310.15(B)(2)(9) in Accordance with Figure B.310.15(B)(2)(2),
              • TABLE B.310.15(B)(2)(10) in Accordance with Figure B.310.15(B)(2)(2),
              • TABLE B.310.15(B)(2)(11).


              2- Figures:

              The following figures represent Underground electrical duct bank configurations:

              • Figure B.310.15(B)(2)(1)
              • Figure B.310.15(B)(2)(2)
              • Figure B.310.15(B)(2)(3),
              • Figure B.310.15(B)(2)(4),
              • Figure B.310.15(B)(2)(5).





              To download a PDF copy of 
              Annex B tables and figures, click on the link.


              The above tables can be divided to groups as follows: 



              • Tables Group#1: tables for conductors installed in free air. 
              • Tables Group#2: tables for conductors installed in underground electrical duct bank tables. 
              • Tables Group#3: adjustment factor tables 


              The above figures can be divided to groups as follows

              • Figures Group#1: adjustment figure for Thermal Resistivity and Load Factor. 
              • Figures Group#2: special case figures. 



              Second: Tables Group#2: tables for conductors installed in underground electrical duct bank or directly Buried tables.


              This tables group can be divided to two sub-groups as follows: 


              Tables Group#2-A: tables for conductors installed in underground electrical duct bank which includes:

              • TABLE B.310.15(B)(2)(5) in Accordance with Figure B.310.15(B)(2)(2), 
              • TABLE B.310.15(B)(2)(6) in Accordance with Figure B.310.15(B)(2)(2), 
              • TABLE B.310.15(B)(2)(7) in Accordance with Figure B.310.15(B)(2)(2), 



              Tables Group#2-B: tables for conductors directly Buried in earth which includes:

              • TABLE B.310.15(B)(2)(8) in Accordance with Figure B.310.15(B)(2)(2), 
              • TABLE B.310.15(B)(2)(9) in Accordance with Figure B.310.15(B)(2)(2), 
              • TABLE B.310.15(B)(2)(10) in Accordance with Figure B.310.15(B)(2)(2), 



              1.1 Tables Group#2-B Construction:


              For example, 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.



              Fig (1)



              Table B.310.15(B)(2)(8) consists of: (see fig.1)






              1.1.A Conditions For Table Application

              These conditions are existing in the table description at the top of the table, and in table  B.310.15(B)(2)(8) , these conditions are:

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








              1.1.B Conductor Material Sections

              Two main Sections for conductor material type are existing:

              • One for copper,
              • One for aluminum or copper-clad aluminum conductors.







              1.1.C Cables Installation Configurations

              Each of The two main sections for conductor material types is divided into Two Cable Installation Configurations:

              • 1 Cable (Fig. B.310.15(B)(2)(2), Detail 5),
              • 2 Cables (Fig. B.310.15(B)(2)(2), Detail 6).



              Noting that:
              • Each Cable Installation Configuration is applied for Two Conductor Temperatures 60°C (140°F) for Type UF insulation material  and 75°C (167°F) for Types RHW, THHW, THW, THWN, XHHW, USE.







              1.1.D Conductor Sizes Columns

              Two Columns for conductor sizes range from 8 AWG to 1,000 kcmil:

              • The left one beside copper conductors column is for copper conductors sizes,
              • The right one beside aluminum or copper-clad aluminum conductors column is for aluminum or copper-clad aluminum conductors sizes.







              1.1.E Correction Factors for Ambient Temperature

              Under each main Material Section; copper and aluminum/copper-clad aluminum conductors, there is a section of Correction Factors for Ambient Temperature
              Which gives the value of a correction factor for Ambient Temperature based on the Actual Ambient temperature columns in the right and left of this section.






              Other tables included in Group#2-B have the same construction as explained above for table B.310.15(B)(2)(8) but of course they will differ as per the following table: 





              Table B.310.15(B)(2)(8)
              Table B.310.15(B)(2)(9)
              Table B.310.15(B)(2)(10)
              Conditions for table application
              Two or Three Insulated Conductors  Cabled Within an Overall (Two- or Three-Conductor) Covering, Directly Buried in Earth

              Three Triplexed Single Insulated Conductors,
              Directly Buried in Earth


              Three Single Insulated Conductors, Directly Buried in Earth

              Number of Electrical Cables/ Circuits
              1 and 2 Cable
              1 and 2 Circuit
              1 and 2 Circuit
              Allowable Cable Installation Configurations (see fig.2)

              Detail 5 and 6
              Detail 7 and 8
              Detail 9 and 10
              Allowable Conductor Sizes
              from 8 AWG to 1,000 kcmil
              from 8 AWG  to 1,000 kcmil
              from 8 AWG  to 2,000 kcmil
              Insulation Material Types
              Types UF, RHW, THHW, THW, THWN, XHHW, USE
              Types UF, USE
              Types UF, USE














              Fig (2)

              1.2 Notes for Tables Group#2-A:





              Note#1

              All tables included under group#2-B are used in Accordance with Cable installation configurations included in Figure B.310.15(B)(2)(2) (see fig.2)






              Note#2

              Only the conductors/cables with the following insulation materials are used under tables of group#2-A:



              Table B.310.15(B)(2)(8)
              Table B.310.15(B)(2)(9)
              Table B.310.15(B)(2)(10)
              Insulation Material Types
              Types UF, RHW, THHW, THW, THWN, XHHW, USE
              Types UF, USE
              Types UF, USE


              Don’t apply tables group#2-B for conductors/cables with other insulation materials listed above.






              Note#3

              Apply Ambient temperature correction factors if the actual earth ambient temperature not 20°C (68°F) and as per the Correction Factors section included in each table of group#2-B







              Note#4

              Minimum burial depths to top cables shall be in accordance with NEC Section 300.5. Maximum depth to the top of direct buried cables shall be 900 mm (36 in.)






              Note#5

              In Table B.310.15(B)(2)(8), For ampacities of Type UF cable in underground electrical ducts, multiply the ampacities shown in the table by 0.74.






              1.3 How to use these Tables group#2-B


              Step#1: Determine the proper table to use based on the existing wiring method conditions
              For example, if we have Three Triplex Insulated Conductors 2 AWG Copper , UF Directly Buried in Earth based on an earth ambient temperature of 30° C (86° F), which table we will use; Table B.310.15(B)(2)(8) or Table B.310.15(B)(2)(9) or Table B.310.15(B)(2)(10)? (See fig.3).



              Fig (3)


              • Yes, it is table B.310.15(B)(2)(9), 
              • Why it is not Table B.310.15(B)(2)(8) because we have three Triplex Insulated Conductors not Three Insulated Conductors Cabled Within an Overall Covering 
              • Why it is not Table B.310.15(B)(2)(10) because we have Three Triplex Insulated Conductors not Three single Insulated Conductors. 


              Fig (4)


              Step#2: Determine the proper section in the selected table based on the conductor material type (Copper – Aluminum or Copper Clad Aluminum) (see fig.4)
              In same example above, because this is a copper conductor, we will use the section in the left side of table B.310.15(B)(2)(6).



              Step#3: Reading the Ampacity Value 
              (see fig.4)

              In the table section selected above, move down to the given conductor size, which is 2 AWG in our example, the amapcity value are existing in the 2 AWG Row.

              Now, move to the right in the 2AWG row until you reach the column for detail#7 (1 circuit) configuration, move to the Column Type UF, the reading will be in this column.



              Step#4: Now read the amapcity of the conductor, you should read 153.




              In the next Article, I will explain how to use Other Annex B tables and figures groups. Please, keep following.



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