Conductor Ampacity Calculation – Part Three


In Article " Conductor Ampacity Calculation – Part One ", I listed the definitions for related terms which were included under the following sections: 

  1. Conductor/Cable Terms, 
  2. Conductor/Cable Insulation Materials Terms, 
  3. Conductor/Cable Tests Terms, 
  4. Conductor/Cable Manufacturing Process Terms. 

Also, in Article " Conductor Ampacity Calculation – Part Two ", I explained the following points:
  1. Measurement Units of Conductors Cross Section Area, 
  2. Conversions between different Measurement Units of Conductors Cross Section Area, 
  3. 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?

  1. When modifications are done to existing circuits,
  2. On all new designs, the conductors/feeders ampacity should be checked.







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:

  1. Methods for Ampacity Calculations of Conductors Rated 0–2000 Volts,
  2. Methods for Ampacity Calculations of Conductors Rated 2001 to 35,000 Volts.


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 0-2000 Volts shall be permitted to be determined by two methods:

  1. Tables as provided in 310.15(B) or
  2. Under engineering supervision, as provided in 310.15(C).




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 Current-Carrying 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. Conductors Rated Up to and Including 2000 Volts
  2. Actual conductor temperature ranges from 60°C Through 90°C (140°F Through 194°F)
  3. Not more than three current-carrying conductors in a raceway, cable or earth (directly buried).
  4. An ambient temperature of 30°C (86°F).







1.1.B Conductor Material Sections

Two main Sections for conductor material type are existing (see above image):

  1. One for copper,
  2. 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 (see below image):

  1. 60°C (140°F),
  2. 75°C (167°F) and
  3. 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 18 AWG to 2,000 kcmil (see below image):

  1. The right left one beside copper conductors column is for copper conductors sizes,
  2. The right one beside aluminum or copper-clad aluminum conductors column is for aluminum or copper-clad aluminum conductors sizes.









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:
  • #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

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 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 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:


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 (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:


What is the allowable ampacity of a non-metallic-sheathed cable containing three 12 AWG Type THW-2 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 THW-2 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 THW-2 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. 


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