Conductor Ampacity Calculation – Part Ten


In Article " Conductor Ampacity Calculation – Part Eight ", I listed the Rules, Tables and figures that construct the second method for Conductors ampacity calculations by using Neher–McGrath formula under engineering supervision, as provided in 310.15(C).



Also, In Article " Conductor Ampacity Calculation – Part Nine ", I explained how to use the Tables included under Group#1 which were:

  • TABLE B.310.15(B)(2)(1), 
  • TABLE B.310.15(B)(2)(3). 



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



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:

              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).


              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:

              1- 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), 


              2- Tables Sup-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-A Construction















              For example, 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).




              Fig(1)



              Table B.310.15(B)(2)(5) 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)(5) , these conditions are:

              • Single Insulated Conductors  in Nonmagnetic Underground Electrical Ducts (One Conductor per Electrical Duct),
              • 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).









              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 Electrical Duct Configurations

              Each of The two main sections for conductor material types is divided into three electrical Duct Configurations:

              • 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)



              Noting that:

              • Each electrical Duct Configuration is applied for same insulation materials as other configurations which are listed in the row under each electrical Duct Configurations and are (Types RHW, THHW, THW, THWN, XHHW, USE).
              • Each electrical Duct Configuration is applied for certain values of thermal resistivity (RHO) and Load Factor (LF) which are listed in the row under Conductor Material Sections  and are as follows:


              RHO
              RHO
              RHO
              60
              90
              120
              LF
              LF
              LF
              50
              100
              100







              1.1.D Conductor Sizes Columns

              Two Columns for conductor sizes range from 250 to 2,000 kcmil:

              • The right 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-A have the same construction as explained above for table B.310.15(B)(2)(5) but of course they will differ as per the following table: 







              Table B.310.15(B)(2)(5)
              Table B.310.15(B)(2)(6)
              Table B.310.15(B)(2)(7)
              Conditions for table application (see fig.2)
              Single Insulated Conductors  in Nonmagnetic Underground Electrical Ducts (One Conductor per Electrical Duct),

              Three Insulated Conductors Within an Overall Covering (Three-Conductor Cable) in Underground Electrical Ducts (One Cable
              per Electrical Duct)

              Three Single Insulated Conductors in Underground Electrical Ducts (Three Conductors per Electrical Duct)

              Number of Electrical Ducts
              3,6 and 9
              1,3 and 6
              1,3 and 6
              Allowable Electrical Duct Configurations (see fig.3)

              Detail 2 ,3 and 4
              Detail 1 ,2 and 3
              Detail 1 ,2 and 3
              Allowable Conductor Sizes
              from 250 to 2,000 kcmil
              from 8 AWG  to 1,000 kcmil
              from 8 AWG  to 1,000 kcmil





              Fig(2)




              Fig(3)






              1.2 Notes for Tables Group#2-A:





              Note#1

              All tables included under group#2-A are used in Accordance with electrical duct bank configurations included in Figure B.310.15(B)(2)(2) (see fig.3)






              Note#2

              Only the conductors/cables with the following insulation materials are used under tables of group#2-A:
              Types RHW, THHW, THW, THWN, XHHW, USE.

              Don’t apply tables group#2-A 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 if each table of group#2-A







              Note#4

              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#5

              For two and four electrical duct installations with electrical ducts installed in a single row, see B.310.15(B)(5).






              Note#6

              All three tables included in group#2-A have same values of thermal resistivity (RHO) and Load Factor (LF) which are as follows:

              RHO
              RHO
              RHO
              60
              90
              120
              LF
              LF
              LF
              50
              100
              100

              But if in some installations we have different values of thermal resistivity (RHO) and Load Factor (LF), what will be done?

              The answer is using Figure B.310.15(B)(2)(1) in below image, which make Interpolation for the values of current ampacity for these installations based on two known ampacities for standard values of thermal resistivity (RHO) and Load Factor (LF). (this will be explained in detail in next Articles)









              1.3 How to use these Tables group#2-A


              Step#1: Determine the proper table to use based on the existing wiring method conditions
              For example, if we have Three Insulated Conductors 2 AWG Copper , THHW Within an Overall Covering (Three-Conductor Cable) in Underground Electrical Ducts (One Cable per Electrical Duct) based on an earth ambient temperature of 30° C (86° F), which table we will use; Table B.310.15(B)(2)(5) or Table B.310.15(B)(2)(6) or Table B.310.15(B)(2)(7)?




              • Yes, it is table B.310.15(B)(2)(6), 
              • why it is not Table B.310.15(B)(2)(5) because we have three Insulated Conductors not single one, 
              • why it is not Table B.310.15(B)(2)(7) because we have Three Insulated Conductors Within an Overall Covering (Three-Conductor Cable) not Three single Insulated Conductors. 









              Step#2: Determine the proper section in the selected table based on the conductor material type (Copper – Aluminum or Copper Clad Aluminum)
              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 (Given RHO = 90 and LF=100, Detail #2)
              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 6AWG row until you reach the column for detail#2 configuration and for RHO = 90 and LF=100 a the reading will be in this column.



              Step#4: Now read the amapcity of the conductor, you should read 105.



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


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