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:
2- Figures:
The following
figures represent Underground electrical duct bank configurations:
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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.
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| 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:
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1.1.B Conductor Material Sections
Two main Sections for
conductor material type are existing:
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1.1.C Cables Installation Configurations
Each of The two main sections
for conductor material types is divided into Two Cable Installation
Configurations:
Noting that:
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1.1.D Conductor Sizes Columns
Two Columns for conductor
sizes range from 8 AWG to 1,000 kcmil:
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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.
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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)
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Table B.310.15(B)(2)(9)
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Table B.310.15(B)(2)(10)
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Conditions for table
application
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Two or Three Insulated Conductors Cabled Within an Overall (Two- or Three-Conductor)
Covering, Directly Buried in Earth
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Three Triplexed Single Insulated Conductors,
Directly Buried in Earth
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Three Single Insulated Conductors, Directly
Buried in Earth
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Number of Electrical Cables/ Circuits
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1 and
2 Cable
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1 and
2 Circuit
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1 and
2 Circuit
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Allowable Cable Installation Configurations (see fig.2)
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Detail 5 and 6
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Detail 7 and 8
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Detail 9 and 10
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Allowable Conductor Sizes
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from 8
AWG to 1,000 kcmil
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from
8 AWG to 1,000 kcmil
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from
8 AWG to 2,000 kcmil
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Insulation Material
Types
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Types UF, RHW, THHW, THW, THWN, XHHW, USE
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Types UF, USE
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Types UF, USE
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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)
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Note#2
Only the conductors/cables
with the following insulation materials are used under tables of group#2-A:
Don’t apply tables group#2-B
for conductors/cables with other insulation materials listed above.
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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
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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.)
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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.
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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).
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| 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.
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| 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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