Conductor Ampacity Calculation – Part Thirteen


In Article " Conductor Ampacity Calculation – Part Twelve ", I explained the following:

  • Third: Tables Group#3: adjustment factor tables, which includes TABLE B.310.15(B)(2)(11). 
  • Fourth: Figures Group#1: adjustment figure for Thermal Resistivity and Load Factor, which includes Figure B.310.15(B)(2)(1). 
  • Fifth: Figures Group#2-A: Detail Figures Which includes B.310.15(B)(2)(2). 


And In Article " Conductor Ampacity Calculation – Part Eleven ", I explained how to use the Annex B Tables included under 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), 


Also,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-A: Detail Figures. 
              • Figures Group#2-B: special case figures. 



              Sixth: Figures Group#2-B: special case figures


              This tables group includes three figures as follows:

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


              1- Figure B.310.15(B)(2)(3):


              1.1 Construction:



              Figure B.310.15(B)(2)(3) shows 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).


              Figure B.310.15(B)(2)(3) consists of:

              1. Detail and Design criteria,
              2. Table,
              3. Notes.





              1- Detail and Design criteria

              Detail are as per fig.1 below which must be used under the following 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






              Fig (1)




              Important!!!

              If the above design criteria is verified for fig.1 above, table of  Figure B.310.15(B)(2)(3) can be applied.







                
              2- Table

              2.1 Conditions For Table Application
              These conditions are existing in the Figure description at the bottom of the figure  Figure B.310.15(B)(2)(3), which are:

              • Single Insulated Conductors in Underground Electrical Ducts (Three Conductors per Electrical Duct),
              • Nine Single-Conductor Cables per Phase,
              • Rated 0 through 5000 Volts,
              • Based on Ambient Earth Temperature of 20°C (68°F),
              • Based on Conductor Temperature 75°C (167°F).


              2.2 Allowable Insulation materials
              • TYPES RHW, THHW, THW, THWN, XHHW, USE, OR MV-90 are only the permitted to be used under Figure B.310.15(B)(2)(3).
              • MV-90 is Limited to 75°C conductor temperature.


              2.3 Two Conductor Sizes Columns (Right and Left)
              In these two right and left Two Conductor Sizes Columns, Three Conductor sizes are permitted to be used under Figure B.310.15(B)(2)(3), which are 250, 350 and 500 Kcmil.

              2.4 three Columns for Thermal resistivity (RHO) and Load Factor (LF) Values
              certain values of Thermal resistivity (RHO) and Load Factor (LF) permitted to be used under Figure B.310.15(B)(2)(3) as follows:

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


              2.5 Correction Factors for Ambient Temperature

              This section of Correction Factors for Ambient Temperature gives the value of a correction factor for Ambient Temperature (other than 20°C (68°F)) based on the Actual Ambient temperature columns in the right and left of this section.







              3- Notes

              1. Neutral configurationper 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.
              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.







              2- Figure B.310.15(B)(2)(4):


              2.1 Construction:






              Figure B.310.15(B)(2)(4) shows 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).


              Figure B.310.15(B)(2)(4) consists of:

              1. Detail and Design criteria,
              2. Table,
              3. Notes.




              1- Detail and Design criteria

              Detail are as per fig.2 below which must be used under the following design criteria:

              • Neutral 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
              • Number of Cables per Phase = 4
              • Rho concrete = Rho Earth – 5
              • Rho PVC Duct = 650
              • Rho Cable Insulation = 500
              • Rho Cable Jacket = 650







              Fig (2)





              Important!!!

              If the above design criteria is verified for fig.2 above, table of  Figure B.310.15(B)(2)(4) can be applied.








              2- Table

              2.1 Conditions For Table Application
              These conditions are existing in the Figure description at the bottom of the figure  Figure B.310.15(B)(2)(4), which are:

              • Single Insulated Conductors in  Nonmagnetic Underground Electrical Ducts (One Conductor per Electrical Duct),
              • Four Single-Conductor Cables per Phase,
              • Rated 0 through 5000 Volts,
              • Based on Ambient Earth Temperature of 20°C (68°F),
              • Based on Conductor Temperature 75°C (167°F).


              2.2 Allowable Insulation materials
              • TYPES RHW, THHW, THW, THWN, XHHW, USE, OR MV-90 are only the permitted to be used under Figure B.310.15(B)(2)(3).
              • MV-90 is Limited to 75°C conductor temperature.


              2.3 Two Conductor Sizes Columns (Right and Left)
              In these two right and left Two Conductor Sizes Columns, five Conductor sizes are permitted to be used under Figure B.310.15(B)(2)(3), which are as below table:


              750 Kcmil
              1000  Kcmil
              1250  Kcmil
              1500  Kcmil
              1750  Kcmil


              2.4 three Columns for Thermal resistivity (RHO) and Load Factor (LF) Values
              certain values of Thermal resistivity (RHO) and Load Factor (LF) permitted to be used under Figure B.310.15(B)(2)(3) as follows:

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


              2.5 Correction Factors for Ambient Temperature

              This section of Correction Factors for Ambient Temperature gives the value of a correction factor for Ambient Temperature (other than 20°C (68°F)) based on the Actual Ambient temperature columns in the right and left of this section.
                








              3- 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.
              3. Metallic shields of Type MV-90 cable shall be grounded at one point only.






              3- Figure B.310.15(B)(2)(5):


              3.1 Construction:






              Figure B.310.15(B)(2)(5) shows 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).


              Figure B.310.15(B)(2)(5) consists of:

              1. Detail and Design criteria,
              2. Table,
              3. Notes.




              1- Detail and Design criteria

              Detail are as per fig.3 below which must be used under the following design criteria:

              • Neutral 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
              • Number of Cables per Phase = 5
              • Rho concrete = Rho Earth – 5
              • Rho PVC Duct = 650
              • Rho Cable Insulation = 500
              • Rho Cable Jacket = 650







              Fig (3)




              Important!!!

              If the above design criteria is verified for fig.3 above, table of  Figure B.310.15(B)(2)(5) can be applied.
                








              2- Table

              2.1 Conditions For Table Application
              These conditions are existing in the Figure description at the bottom of the figure  Figure B.310.15(B)(2)(4), which are:

              • Single Insulated Conductors in  Nonmagnetic Underground Electrical Ducts (One Conductor per Electrical Duct),
              • Five Single-Conductor Cables per Phase,
              • Rated 0 through 5000 Volts,
              • Based on Ambient Earth Temperature of 20°C (68°F),
              • Based on Conductor Temperature 75°C (167°F).


              2.2 Allowable Insulation materials
              • TYPES RHW, THHW, THW, THWN, XHHW, USE, OR MV-90 are only the permitted to be used under Figure B.310.15(B)(2)(3).
              • MV-90 is Limited to 75°C conductor temperature.


              2.3 Two Conductor Sizes Columns (Right and Left)
              In these two right and left Two Conductor Sizes Columns, one Conductor size is permitted to be used under Figure B.310.15(B)(2)(3), which is 2000 Kcmil.



              2.4 three Columns for Thermal resistivity (RHO) and Load Factor (LF) Values
              certain values of Thermal resistivity (RHO) and Load Factor (LF) permitted to be used under Figure B.310.15(B)(2)(3) as follows:

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


              2.5 Correction Factors for Ambient Temperature

              This section of Correction Factors for Ambient Temperature gives the value of a correction factor for Ambient Temperature (other than 20°C (68°F)) based on the Actual Ambient temperature columns in the right and left of this section.








              3- 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.
              3. Metallic shields of Type MV-90 cable shall be grounded at one point only.







              4- Differences between (3) special case figures:



                

              Figure B.310.15(B)(2)(3)
              Figure B.310.15(B)(2)(4)
              Figure B.310.15(B)(2)(5)
              Duct Type
              Underground magnetic  or Nonmagnetic Electrical Ducts

              Nonmagnetic Underground Electrical Ducts
              Nonmagnetic Underground Electrical
              Number of Cables per Duct
              3
              1
              1
              Number of Cables per Phase
              9
              4
              5
              Conductor Sizes (Kcmil)
              250 - 350 - 500
              750 – 1000 – 1250 – 1500 - 1750
              2000
              Phase Ducts Size

              75 to 125 mm (3 to 5 in.)
              75 mm (3 in.)

              75 mm (3 in.)

              Phasing Configurations
              In case of Nonmagnetic Ducts, Phasing is A, B, C in rows or columns

              Phasing is A, B, C in rows or columns

              Phasing is A, B, C in rows or columns

              In case of Nonmagnetic Ducts, 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.








              In the next Article, I will explain the Voltage Drop Calculations. Please, keep following.




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