In Article " Conductor Ampacity Calculation – Part Eleven ", I explained how to use the Annex B Tables included under Group#2B “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#2A “ 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#2A: Detail Figures.
 Figures Group#2B: special case figures.
Third: Tables Group#3: adjustment factor tables
This tables group includes One Table only which is TABLE B.310.15(B)(2)(11).
1.1 Tables Group#3 Construction:
Table B.310.15(B)(2)(11) consists of: (see above image)
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)(11) , these conditions are:

1.1.B
Conductor Material Sections
Two main Columns for conductor material type
are existing:

1.2 Notes for Tables Group#3:
Note#1
The Application Conditions for table B.310.15(B)(2)(11)
limit the use of it to the following tables:

Note#2
The asterisk (*) next to Number of conductors means that:
The Number of conductors is the total number
of conductors in the raceway or cable adjusted in accordance with
310.15(B)(4) and (5).
For more
information about using Rules of 310.15(B)(4) and (5), please review our Article " Conductor Ampacity Calculation – Part Six ".

310.15(B)(4): Bare or Covered Conductors
Where
bare or covered conductors are installed with insulated conductors, the
temperature rating of the bare or covered conductor shall be equal to the
lowest temperature rating of the insulated conductors for the purpose of
determining ampacity.

310.15(B)(4): Neutral
Conductor
This section discuss considering the neutral conductor as a
current carrying conductor or not.
If it will be a current carrying conductor it will be counted
when applying the Adjustment factor of table B.310.15(B)(2)(11) where is More
Than Three CurrentCarrying Conductors in a Raceway or Cable.

Definition:
Nonlinear Load: A load where the wave shape of the
steadystate current does not follow the wave shape of the applied voltage.

Note#3
The asterisk (**) means that These
adjustment factors in table B.310.15(B)(2)(11) for Number of conductors from 10 to 85, include the effects of a load diversity of
50 percent.

Note#4
The ampacity limit for 10
through 85 currentcarrying conductors is based on the following equation:
where:
A1 = ampacity from Table
310.15(B)(16), Table 310.15(B) (18), Table B.310.15(B)(2)(1), Table
B.310.15(B)(2) (6), or Table B.310.15(B)(2)(7) multiplied by the appropriate adjustment
factor from Table B.310.15(B) (2)(11).
N = total number of
conductors used to select adjustment factor from Table B.310.15(B)(2)(11),
E = number of conductors carrying
current simultaneously in the raceway or cable,
A2 = ampacity limit for the
currentcarrying conductors in the raceway or cable
For more than 85 conductors,
special calculations are required that are beyond the scope of this table.

1.3 How to use Tables group#3 (Table B.310.15(B)(2)(11))
Example# 1:
Calculate the ampacity limit for twelve 14 AWG THWN currentcarrying conductors (75°C) in a raceway that contains 24 conductors that may, at different times, be current carrying.
Solution:
Step#1: determine tha ampacity for 14 AWG, THWN, Copper conductors (75°C) in a raceway.
Use table Table 310.15(B)(16), you get ampacity reading = 20 A
Step#2: apply the above equation to get the adjusted ampacity , given the following data:
A1 = ampacity from Table 310.15(B)(16) from Step#1 = 20 A
N = total number of conductors = 24
E = number of conductors carrying current simultaneously in the raceway or cable = 12
SO,
Example# 2:
Calculate the ampacity limit for eighteen 14 AWG THWN currentcarrying conductors (75°C) in a raceway that contains 24 conductors that may, at different times, be currentcarrying.
Solution:
Step#1: as in Example#1 above.
Step#2: apply the above equation to get the adjusted ampacity , given the following data:
A1 = ampacity from Table 310.15(B)(16) from Step#1 = 20 A
N = total number of conductors = 24
E = number of conductors carrying current simultaneously in the raceway or cable = 18
SO,
Fourth: Figures Group#1: adjustment figure for Thermal Resistivity and Load Factor
This figures group include one figure only which is Figure B.310.15(B)(2)(1).
Using Figure B.310.15(B)(2)(1)
All three tables included in
group#2A have same values of thermal resistivity (RHO) and Load Factor (LF) which
are as follows:
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).

Tables Group#2A
This tables group includes the following tables:

Fifth: Figures Group#2A: Detail Figures
This figures group include one figure only which is Figure B.310.15(B)(2)(2).
Using Figure B.310.15(B)(2)(2)
All tables included under
group#2 are used in Accordance with electrical duct bank configurations
included in Figure B.310.15(B)(2)(2). (see below table)

Tables Group#2: tables for conductors installed in underground
electrical duct bank or directly Buried tables
This tables group can be
divided to two subgroups as follows:
1 Tables Group#2A: tables for
conductors installed in underground electrical duct bank which includes:
2 Tables SupGroup#2B: tables
for conductors directly Buried in earth which includes:

Table B.310.15(B)(2)(5)

Table B.310.15(B)(2)(6)

Table B.310.15(B)(2)(7)


Allowable Electrical Duct Configurations from Figure
B.310.15(B)(2)(2)

Detail 2 ,3 and 4

Detail 1 ,2 and 3

Detail 1 ,2 and 3

Table B.310.15(B)(2)(8)

Table B.310.15(B)(2)(9)

Table B.310.15(B)(2)(10)


Allowable Cable Installation Configurations from Figure
B.310.15(B)(2)(2)

Detail 5 and 6

Detail 7 and 8

Detail 9 and 10

Note#1
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#2
For two and four electrical
duct installations with electrical ducts installed in a single row, see
B.310.15(B)(5).

In the next Article, I will explain how to use the last figures group#3 included in NEC Annex B. Please, keep following.
Post a Comment
Leave a comment to help all for better understanding