Conductor Ampacity Calculation – Part Nine


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).



Today, I will explain how to use the NEC, Annex B tables and figures used for Ampacity Calculations by the Neher–McGrath formula. 



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.


                Also, 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. 



                First: Tables Group#1: tables for conductors installed in free air


                This group includes two tables as follows:

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









                1.1 Tables Group#1 Construction


                For example, 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).



                Table B.310.15(B)(2)(1) consists of: 






                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)(1) , these conditions are:

                • Conductors  Rated 0 Through 2000 Volts,
                • Actual conductor temperature ranges from 60°C Through 90°C (140°F Through 194°F),
                • Two or Three Insulated Conductors  Within an Overall Covering (Multiconductor Cable), in Raceway in Free Air,
                • An ambient temperature of 30°C (86°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 Temperature Categories

                Each of The two main sections for conductor material types is divided into three temperature categories:

                • 60°C (140°F),
                • 75°C (167°F) and
                • 90°C (194°F).


                Noting that each temperature category is applicable with certain insulation properties which are listed in the row under each temperature category.







                1.1.D Conductor Sizes Columns

                Two Columns for conductor sizes range from 14 AWG to 1,000 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.






                Another example in this group is 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).


                This table has the same construction of table B.310.15(B)(2)(1) but it differs from it as in the following table:

                 

                TABLE B.310.15(B)(2)(1)
                TABLE B.310.15(B)(2)(3)
                Conductors grouping
                Conductors within an Overall Covering (Multi-conductor Cable)
                none
                Installation method
                in Raceway in Free Air
                in Free Air
                An ambient temperature
                30°C (86°F)
                40°C (104°F)
                Insulation materials
                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

                Types TC, MC, MI, UF, and USE Cables





                1.2 Notes for Tables Group#1





                Note#1

                The asterisk (*) next to the ambient temperature of 30°C (86°F) included in table description at the top of the table, Refer to  table 310.15(B)(2) for application of the ampacity correction factors where the ambient temperature is other than 30°C (86°F).






                Note#2

                The asterisk (**) next to wire sizes 10, 12, and 14 AWG refers to article 240.4(D) and is letting you know that conductor sizes #14, #12, and #10 copper and aluminum are limited to certain size overcurrent protection devices (OCPD) even if the ampacity of the #14, #12, and #10 conductor is higher. The limitations are:

                A- For Copper Conductors:
                • #14 copper limited to 15 amp OCPD
                • #12 copper limited to 20 amp OCPD
                • #10 copper limited to 30 amp OCPD


                B- For Aluminum Conductors:
                • #14 aluminum not allowed
                • #12 aluminum limited to 15 amp OCPD
                • #10 aluminum limited to 25 amp OCPD







                Note#3

                Copper conductors of the same size have three different allowable ampacities. The same is true for aluminum (and copper-clad aluminum) conductors. This is because The maximum allowable ampacities depend on the conductor’s temperature rating.

                For example, a 3 AWG copper conductor with a temperature rating of 60°C has a maximum allowable ampacity of 76 amperes (A). The maximum allowable ampacity of the same 3 AWG copper conductor with a temperature rating of 75°C is 90A. If the temperature rating of the 3 AWG copper conductor is 90°C, the allowable ampacity is 102A .






                Note#4

                All The Allowable Ampacities Tables refer to table 310.104(A) which contains information about conductors rated 600V. Conductor information in this table includes trade name, type letter, maximum operating temperature, application provisions, insulation, thickness of insulation, and outer covering (if any).

                To download a PDF file for table 310.104(A), click on the link.










                Note#5: Conductor Lettering

                You can note in allowable ampacity tables that each temperature category is applicable with certain wiring properties which is listed in the row under each temperature category.

                When looking at these wiring properties, you will notice that the wires contain some type of letter identifier such as THHN or THW.
                These letters serve to identify specific properties of the conductor and/or its insulation. Listed below are some of the letters commonly used in allowable ampacity tables:


                Letter Identifier
                conductor and/or insulation property
                T
                Thermoplastic
                W
                Wet or Damp
                R
                Rubber
                F
                Fixture Wires
                FF
                Fixture wire, flexible stranding
                -2
                Conductor is permitted 90ºC operating temperature
                H
                75ºC insulation rating
                HH
                90ºC insulation rating
                N
                Nylon outer cover


                The combination of letters will tell you most of what you need to know about the conductor.

                NOTE: This is a general rule and there are exceptions such as THW. The one H indicates 75° C rating but it can be used at a 90° C conductor with ballasts in dry locations.






                Examples for Conductor Lettering:

                • TW - Thermoplastic (T) insulation, (W) suitable for wet and dry locations (When suitable for wet generally means it can be used for dry also), and is rated 60° C (no H in group). 
                • RHW - Rubber (R) insulation, (H) rated 75° C and (W) suitable for wet and dry locations. 
                • TF - Thermoplastic (T) insulation, (F) fixture wire.
                • RFH – Rubber (R) insulation, (F) fixture wire, (H) rated 75° C.





                1.3 How to use these Tables group#1



                Step#1: Determine the proper table to use based on the existing wiring method conditions

                For example, if we have a multi-conductor cable with three 6 AWG Type THWN copper installed in a raceway in free air based on an ambient air temperature of 30° C (86° F), which table we will use; TABLE B.310.15(B)(2)(1) or TABLE B.310.15(B)(2)(3)?

                Yes, it is table B.310.15(B)(2)(1), because tables TABLE B.310.15(B)(2)(3) don’t be used with the following:

                • THWN insulation material or 
                • Ambient temperature 30° C (86° F) or 
                • Multi-conductor cable in a raceway. 





                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)(1).



                Step#3: Reading the Ampacity Value

                In the table section selected above, move down to the given conductor size, which is 6 AWG in our example, the amapcity value are existing in the 6 AWG Row.

                Now, move to the right in the 6AWG row until you reach the column for THWN insulation for a copper conductor. This will be the 75° C (167° F) column.



                Step#4: Now read the amapcity of the conductor, you should read 58.





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




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