Conductor Ampacity Calculation – Part Four


In Article " Conductor Ampacity Calculation – Part Three ", I explained the following points: 

  1. The need for ampacity calculation, 
  2. When do we need ampacity calculation?, 
  3. Methods for Conductors Ampacity Calculations as per NEC code, 
  4. Methods for Ampacity Calculations of Conductors Rated 0–2000 Volts,
  5. First Method: Conductor Ampacity Calculations from Tables as provided in 310.15(B),
  6. General Overview of Allowable Ampacity Tables for conductors rated 0 to 2000 volts. 


Today, I will continue explaining the First Method: Conductor Ampacity Calculations from Tables as provided in 310.15(B) as follows.


For more information and good following, please review the following articles:





2- General rules controlling the conductor ampacity calculations

In article " Conductor Ampacity Calculation – Part Three ", I explained the first point: how to use the allowable ampacity tables, this was as a general method, but this general method is controlled and refined by the other rules as follows. 





Rule#1: Temperature Limitation of Conductors.

As per 310.15(A)(3), No conductor shall be used in such a manner that its operating temperature will exceed that designated for the type of insulated conductor involved.



The above rule said that:

  • Conductor with 60°C temperature rating must have ampacity from the 60°C column.
  • Conductor with 75°C temperature rating must have ampacity from the 60°C column or the 75°C column and according to Rule#2 in below. 
  • Conductor with 90°C temperature rating must have ampacity from the 60°C column or the 75°C column and according to Rule#2 in below or from the 90°C column after applying correction factors. 





Using The 90°C Column
Be careful when using the 90°C column because no equipment is listed and identified for use with 90°C conductors other than individual lugs, terminal bars and equipment listed for use on circuits over 600V.  The 90 ° C ampacity can be used for corrections only for factors:
Ambient Temperature
Number of Conductors





Rule#2: Selection Of Conductor Ampacity Based On Its Terminations’ Temperature Ratings

As per 110.14(C), Conductors must be sized in accordance with the lowest tem­perature rating of any terminal, device, or conductor insula­tion of the circuit.




Example#1:


A THHN conductor will have a 60°C termination on one end and a 75°C termination on the other, which ampacity column will be used?


Solution: 


The lowest tem­perature rating of conductor’s two terminals is 60°C.

So, the conductor’s ampacity must not exceed the rating listed in the 60°C column Conductors.




How to Select Conductor Ampacity Based On Its Terminations’ Temperature Ratings


Selection of the appropriate conductor amapcity column depends on the temperature rating of the termination (or connection) points as follows:

First: For Conductor/equipment Rated 100A or Less or marked for 14 AWG through 1 AWG


The selected ampacity column will be as follows:


1- Use the 60°C column.

Example#2:


What is the Maximum allowed ampacity for a single #2 THHN Conductor installed in a circuit.


Solution: 


Step#1: 110-14(c) (#14 thru #1), so Use the 60°C column

Step#2: use Table 310.15(B)(16), (60°C Column) and Read Ampacity

The conductor ampacity is 95 Amps



2- If any termination is either 60°C or unknown, Use the 60°C column regardless of the insulation rating of the conductor.

Example#3:


A THHN conductor will have a 60°C termination on one end and a 75°C termination on the other, which ampacity column will be used?


Solution:

Because one of the connection points has a 60°C rating, the conductor’s ampacity must not exceed the rating listed in the 60°C column Conductors.



3- If the equipment is listed and identified for use with certain conductor temperature rating, Use the column with temperature rating = the lowest temperature rating of conductor terminations. 




If Temp. rating of listed conductor is
Termination#1
Termination#2
Use column
75°C
75°C
75°C
75°C
75°C
75°C
60°C
60°C
90°C
75°C
75°C
75°C
90°C
75°C
60°C
60°C



Example#4:


A THHN conductor will have 75°C termination on one end and a 60/75°C termination on the other, which ampacity column will be used?


Solution: 


A temperature rating of 60/75°C means the equipment has been listed for both 60°C and 75°C conductors; therefore, it is permissible to use the 75°C rating if the installed conductor is rated at least 75°C. Because all of the connection points in this example have at least a 75°C rating, the conductor’s ampacity can be based on the 75°C column



4- Use the 75°C column for conductors supplying power to motor marked with a design letter B, C or D and the conductor temperature rating is not less than 75°C. 




If a motor marked with a design letter B, C or D use conductor with temp. rating
Use column
75°C
75°C
90°C
75°C



Example#5:


THHN conductors will have 75°C terminations on one end and a motor marked with a design letter D on the other end. After complying with the applicable requirements in Article 430, the conductors supplying power to this motor must have an ampacity of at least 55 amperes (A). What is the minimum size of THHN conductors required to supply power to this motor?


Solution: 





Motor with a design letter D and the THHN conductors will have 75°C terminations on the other hand.

So, we must use the 75°C column even if THHN is not listed in the 75°C column and listed in the 90°C column.

Using table 310.15(B)(16), for a 55 A motor, the smallest conductor with ampacity higher than 55 A is 6 AWG (with 65A ampacity).

So, Conductors supplying power to this motor must be at least 6 AWG.


Notes for example#5:


  • In table 310.15(B)(16), The allowable ampacity of an 8 AWG THHN conductor (in the 90°C column) is 55A. Although conductors with a temperature rating of 90°C can be installed, the ampacity must not exceed the 75°C ampacity. 
  • The ampacity of an 8 AWG conductor in the 75°C column is 50A. Because this motor requires a minimum ampacity of 55A, installing 8 AWG conductors is not permitted. 


Second: For Conductor/equipment Rated over 100A or marked for larger than 1 AWG


The selected ampacity column will be as follows:



1- Use the 75°C column.

Example#6:


THWN conductors (larger than 1 AWG) will supply power to a circuit rated greater than 100A, the conductors will have 75°C terminations on both ends. Which ampacity column will be used?


Solution: 


The ampacity can be based on the 75°C column because the conductors are larger than 1 AWG, and the circuit it is supplying is rated greater than 100A.



2- If the equipment is listed and identified for use with certain conductor temperature rating, Use the column with temperature rating = the lowest temperature rating of conductor terminations.

If Temp. rating of listed conductor is
Termination#1
Termination#2
Use column
75°C
75°C
75°C
75°C
75°C
75°C
60°C
60°C
90°C
75°C
75°C
75°C
90°C
75°C
60°C
60°C



Example#7:


What is the maximum ampacity for a 1/0 AWG THHN copper conductor fed from a 150A breaker? Assume an ambient temperature of 30°C and no more than three current-carrying conductors in the raceway. The conductors will have 75°C terminations on one end, but the temperature rating of the terminations on the other end is unknown.


Solution: 




We must not exceed the 75°C ampacity for this conductor. Although the temperature rating on one end is unknown, the ampacity of this 90°C conductor (since it is THHN) can be based on the 75°C column

Using table 310.15(B)(16), and Because the conductors are larger than 1 AWG and the circuit it is supplying is rated greater than 100A. This 1/0 AWG THHN copper conductor has a maximum ampacity of 150A.






Rule#3: Selection Of Ampicity For Multi-Ampacity Conductors

As per 310.15(A)(2), Where more than one ampacity applies for a given circuit length, the lowest value shall be used.




For example, in below image, The ampacity for No. 3 THHN (90°C) is 110A, but the correction factors of Table 310 reduce the conductor ampacity to only 96A because the lowest ampacity value = 110A x 0.87 = 95.7A and You round up to get 96A. 







Exception to Rule#2

A higher ampacity is permitted, if the length of the reduced ampacity doesn’t exceed 10 ft and is not longer than 10% of the total length of the circuit.



For example, in below image,, The ampacity for each No. 12 THHN is 30A, but the correction factors in Table 310-15(b)(2)(a) reduce the conductor ampacity by 50% to be 15A inside the panelboard but it will be 30 amps outside the panelboard.








Rule#4: Minimum Size of Conductors

As per 310.106(A), the minimum size of conductors shall be as shown in Table 310.106(A) see below image, except as permitted in other sections, such the following:

  • Small conductor sizes 18 and 16 AWG as permitted by 240.4(D)(1) and (2)
  • Flexible cords as permitted by Table 400.4
  • Fixture wire as permitted by 402.6
  • Motors rated 1 hp or less as permitted by 430.22(F)
  • Cranes and hoists as permitted by 610.14
  • Elevator control and signaling circuits as permitted by 620.12
  • Class 1, Class 2, and Class 3 circuits as permitted by 725.49(A) and 725.127, Exception
  • Fire alarm circuits as permitted by 760.49(A); 760.127, Exception; and 760.179(B)
  • Motor-control circuits as permitted by 430.72
  • Control and instrumentation circuits as permitted by 727.6
  • Electric signs and outline lighting as permitted in 600.31(B) and 600.32(B)






Notes for Rule#4:


  • The smallest size conductor permitted by the NEC for branch circuits, feeders, or services is 14 AWG copper or 12 AWG aluminum. 
  • Some local codes require a minimum 12 AWG for commercial and industrial installations. 
  • Conductors smaller than 14 AWG are permitted for: 

  1. Class 1 remote-control circuits,
  2. Fixture wire,
  3. Flexible cords,
  4. Motor control circuits,
  5. Non-power-limited fire alarm circuits,
  6. Power-limited fire alarm circuits.



As a guide when sizing conductors, the following table must always be used to determine the minimum size conductor:






In the next Article, I will explain the correction Factors affecting conductor ampacity Tables. Please, keep following.


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