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:
Figures:
The following
figures represent Underground electrical duct bank configurations:

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:

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

1.1.C Temperature Categories
Each of The two main sections
for conductor material types is divided into three temperature categories:
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:

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 (Multiconductor 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,
THW2, THWN2, RHH, RWH2, USE2, XHHW, XHHW2, ZW2

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:
B For Aluminum Conductors:

Note#3
Copper conductors of the same
size have three different allowable ampacities. The same is true for aluminum
(and copperclad 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:
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 multiconductor 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
 Multiconductor 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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