 Conductor/Cable Terms,
 Conductor/Cable Insulation Materials Terms,
 Conductor/Cable Tests Terms,
 Conductor/Cable Manufacturing Process Terms.
Also, in Article " Conductor Ampacity Calculation – Part Two ", I explained the following points:
 Measurement Units of Conductors Cross Section Area,
 Conversions between different Measurement Units of Conductors Cross Section Area,
 Standard Sizes of Conductors,
Today, I will explain the methods for Conductor Ampacity Calculations as follows.
The need for Ampacity
calculation
When the ampacity of conductors is not accurate, the conductors
may carry more currents than their rating and become overloaded, so they will
heat up and short out.
Note: As the conductor heats up the current carrying capacity
goes down.

When do we need Ampacity
calculation?

Rule#1: Methods for Conductors Ampacity Calculations as per NEC code
As per NEC Article 310, Methods for Conductors Ampacity
Calculations will depend on the voltage rating of these conductors, it is
divided to:

Part one: Methods for Ampacity Calculations of Conductors Rated 0–2000 Volts
Rule#2: Methods for Ampacity Calculations of Conductors Rated
0–2000 Volts
As per
310.15(A)(1), The allowable Ampacities for conductors rated 02000 Volts
shall be permitted to be determined by two methods:

First Method: Conductor Ampacity Calculations from Tables as provided in 310.15(B)
In this method, I will explain the following points:
 General Overview of Allowable Ampacity Tables for conductors rated 0 to 2000 volts,
 General rules controlling the conductor ampacity calculations,
 Factors affecting conductor ampacity.
1 General Overview of Allowable Ampacity Tables for conductors rated 0 to 2000 volts
The Allowable Ampacities for conductors rated 0 to 2000 volts are specified in the following tables:
 Table 310.15(B)(16)
 Table 310.15(B)(17)
 Table 310.15(B)(18)
 Table 310.15(B)(19)
 Table 310.15(B)(20)
 Table 310.15(B)(21)
To download a PDF file that includes The Allowable Ampacities for conductors rated 0 to 2000 volts, click on the link.
The above tables need to be modified to meet existing installation conditions as per the following tables:
 TABLE 310.15(B)(2)(A)
 TABLE 310.15(B)(2)(b)
 TABLE 310.15(B)(3)(a)
 TABLE 310.15(B)(3)(c)
 TABLE 310.15(B)(7)
These tables will be explained later.
1.1 Allowable Ampacities Tables Construction:
For example, Table 310.15(B)(16) which provides Allowable Ampacities of Insulated Conductors Rated Up to and Including 2000 Volts, 60°C Through 90°C (140°F Through 194°F), Not More Than Three CurrentCarrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30°C (86°F).
Table 310.15(B)(16) will consists of:
1.1.A Conditions for
table application
These conditions are existing in the table description at the
top of the table (see below image), and in table 310.15(B)(16), these conditions are:

1.1.B Conductor Material Sections
Two main Sections for
conductor material type are existing (see above image):

1.1.C Temperature Categories
Each of The two main sections
for conductor material types is divided into three temperature categories (see below image):
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 18 AWG to 2,000 kcmil (see below image):

1.2 Notes for Allowable Ampacities Tables:
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
Ampacity tables, particularly
Table 310.15(B)(16), do not take into account all the many factors affecting
ampacity.
If loads are not calculated
in accordance with the requirements of Article 220, the table ampacities,
even when corrected in accordance with ambient correction factors and the
notes to the tables, might be too high. Especially where many cables or
raceways are routed close to one another underground.

Note#4
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 85 amperes (A). The maximum allowable ampacity of the same 3 AWG copper
conductor with a temperature rating of 75°C is 100A. If the temperature
rating of the 3 AWG copper conductor is 90°C, the allowable ampacity is 115A
.

Note#5
All The Allowable Ampacities
Tables refer to table 310.104(A) (see below image) 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#6: 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 (see Table 310.13). 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 Allowable Ampacities Tables:
Step#1: Determine the proper table to use based on the existing wiring method conditions
For example, if we have a 6 AWG Type THWN copper wire in free air based on an ambient air temperature of 30° C (86° F), which table we will use?
Yes, it is table 310.15(B)(17), because tables Table 310.15(B)(16) and table 310.15(B)(18) don’t be used with single conductor in free air. Don’t use table TABLE 310.15(B)(19) because the ambient air temperature is 30° C, and THWN insulation is not listed in it.
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 310.15(B)(17).
Step#3: Locating 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: Reading the Ampacity Value
Now read the amapcity of the conductor, you should read 95.
Another example:
Another example:
What is the allowable ampacity of a nonmetallicsheathed cable containing three 12 AWG Type THW2 Copper wires based on an ambient air temperature of 30° C(86° F)?
Solution:
Step#1: Determine the proper table to use
Because this cable containing three conductors, use table Table 310.15(B)(16). Don’t use table 310.15(B)(17) or table TABLE 310.15(B)(19) because there are three conductor in a cable. Don’t use table 310.15(B)(18) because the ambient air temperature is 30° C, and THW2 insulation is not listed in it.
Step#2: Determine the proper section in the selected table based on the conductor material type (Copper – Aluminum or Copper Clad Aluminum)
Because these are copper conductors, we will use the section in the left side of table Table 310.15(B)(16).
Step#3: Locating the Ampacity Value
In the table section selected above, move down to the given conductor size, which is 12 AWG , the amapcity value are existing in the 12 AWG Row.
Now, move to the right in the 6AWG row until you reach the column for THW2 insulation for a copper conductor. This will be the 90° C (194° F) column.
Step#4: Reading the Ampacity Value
Now read the amapcity of the conductor, you should read 30.
This method for using Allowable Ampacity Tables is a general method and it will be refined in the next article " Conductor Ampacity Calculation  Part Four".
In the next Article, I will continue explaining the Conductor Ampacity calculations from Tables as provided in 310.15(B) . Please, keep following.
This method for using Allowable Ampacity Tables is a general method and it will be refined in the next article " Conductor Ampacity Calculation  Part Four".
In the next Article, I will continue explaining the Conductor Ampacity calculations from Tables as provided in 310.15(B) . Please, keep following.
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