Conductor Ampacity Calculation – Part Five


In Article " Conductor Ampacity Calculation – Part Four ", I explained the rule that controlling and refining the Conductor Ampacity selection from NEC Tables. 


Also, 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 correction Factors affecting conductor ampacity Tables as follows.


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





First Method: Conductor Ampacity Calculations from Tables as provided in 310.15(B)



3- Correction Factors Affecting Conductor Ampacity Tables:



In the previous articles, we learn how to select the correct allowable ampacity table for a certain application/ wiring method from 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) 


This selection process of the correct allowable ampacity table depends on knowing the following variables:

  1. Voltage,
  2. Cable configuration,
  3. Electrical duct configuration,
  4. Ambient Temperature.

After selecting the correct allowable ampacity table, the ampacity value of a certain application/ wiring method will be determined based on the following variables:

  1. Conductor material,
  2. Insulation type,
  3. Terminal temperature.



For example: 



To select Table 310.15(B)(16), the variables must be as follows:

  1. Voltage: Rated Up to and Including 2000 Volts 
  2. Cable configuration: Not More Than Three Current-Carrying Conductors in a Raceway or Cable. 
  3. Electrical duct configuration: one Raceway, Cable, or Earth (Directly Buried) 
  4. Ambient Temperature: 30°C (86°F)


And to determine the exact value for the ampacity, the following variables must be known:


1- Conductor material: Copper Or Aluminum Or Copper-Clad Aluminum

2- Insulation type:

  • Types TW, UF 
  • Types RHW, THHW, THW, THWN, XHHW, USE, ZW 
  • Types TBS, SA, SIS, FEP, FEPB, MI, RHH, RHW- 2, THHN, THHW, THW-2, THWN-2, USE-2, XHH, XHHW, XHHW-2, ZW-2 

3- Temperature Rating of Conductor: 60°C (140°F), 75°C (167°F), 90°C (194°F)

4- The Terminal temperature as discussed in Article 
 " Conductor Ampacity Calculation – Part Four ".


But if we have an application/ wiring method with different variables than that specified in the allowable ampacity tables, for example:

  • An application/ wiring method with Ambient Temperature: 37°C.
  • An application/ wiring method with Cable configuration: More Than Three Current-Carrying Conductors in a Raceway or Cable. 
  • An application/ wiring method with Electrical duct configuration: More Than One Conduit, Tube, or Raceway 


In this case, depending upon your application / wiring method, you might need to apply the following correction / adjustment factors:

  1. Ambient Temperature Correction Factors as in Table 310.15(B)(2)(A) and Table 310.15(B)(2)(b), 
  2. More Than Three Current-Carrying Conductors in a Raceway or Cable as in Table 310.15(B)(3)(a), 
  3. Ambient Temperature adjustment for Circular Raceways Exposed to Sunlight on Rooftops as in Table 310.15(B)(3)(c) 
















To download a PDF file that includes the Correction / Adjustment Factors Tables, click on the link. 




Now, we will discuss these Correction / Adjustment Factors one by one as follows:


1- Ambient Temperature Correction Factors






Rule#1: Ambient Temperature Correction Factors

Ampacities for ambient temperatures other than those shown in the ampacity tables shall be corrected in accordance with Table 310.15(B)(2)(a) or Table 310.15(B)(2)(b), or shall be permitted to be calculated using the following equation:




Where:
I’ = ampacity corrected for ambient temperature
I = ampacity shown in the tables
Tc = temperature rating of conductor (°C)
T’a = new ambient temperature (°C)
Ta = ambient temperature used in the table (°C)






2- Adjustment Factors



2.A More Than Three Current-Carrying Conductors in a Raceway or Cable






Rule#2: More Than Three Current-Carrying Conductors in a Raceway or Cable

The allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(3)(a) Where:

  1. The number of current-carrying conductors in a raceway or cable exceeds three, or
  2. Single conductors or multiconductor cables are installed without maintaining spacing for a continuous length longer than 600 mm (24 in.) and are not installed in raceways






Important!!!
Each current-carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor.




Important!!!
Where conductors of different systems (power, lighting, control, etc.) are installed in a common raceway or cable, the adjustment factors shown in Table 310.15(B)(3)(a) shall apply only to the number of power and lighting conductors  Neglecting any control and signal conductors in the same raceway or cable.






Rule#3: Cases where Adjustment factors shall not apply

Adjustment factors shall not apply for the following cases:

Case#1: Conductors in raceways having a length not exceeding 600 mm (24 in.),

Case#2: underground conductors entering or leaving an outdoor trench if those conductors have physical protection in the form of rigid metal conduit, intermediate metal conduit, rigid polyvinyl chloride conduit (PVC), or reinforced thermosetting resin conduit (RTRC) having a length not exceeding 3.05 m (10 ft), and if the number of conductors does not exceed four.( see below image)

Case#3: Type AC cable or to Type MC cable under the following conditions:
The cables do not have an overall outer jacket.
Each cable has not more than three current-carrying conductors.
The conductors are 12 AWG copper.
Not more than 20 current-carrying conductors are installed without maintaining spacing, are stacked, or are supported on“bridle rings.”








Important!!!
on the contrary to case#3 in Rule#3 above, An adjustment factor of 60 percent shall be applied to Type AC cable or Type MC cable under the following conditions:
  1. The cables do not have an overall outer jacket.
  2. The number of current carrying conductors exceeds 20.
  3. The cables are stacked or bundled longer that 600 mm (24 in) without spacing being maintained.






Important!!!
Where conductors are installed in cable trays, the provisions of 392.80 shall apply (This will be explained later).




2.B More Than One Conduit, Tube, or Raceway

Spacing shall be maintained between individual conduits in groups of conduit runs from junction box to junction box because of the need to separate the conduits where they enter the junction box, to allow room for locknuts and bushings. (No adjustment factor will be applied)

2.C Ambient Temperature adjustment for Circular Raceways Exposed to Sunlight on Rooftops








Rule#4: Ambient Temperature adjustment for Circular Raceways Exposed to Sunlight on Rooftops

the adjustments shown in Table 310.15(B)(3)(c) shall be added to the outdoor temperature to determine the applicable ambient temperature for application of the correction factors in Table 310.15(B)(2)(a) or Table 310.15(B)(2)(b) Where conductors or cables are installed in circular raceways exposed to direct sunlight on or above rooftops.(see below image)









Important!!!
In Rule#4 above, the closer the conduit is to the roof, the greater the ambient temperature adjustment.




Example#1:

What is the Allowable ampacity for a wiring method where conduit with three 6 THWN-2 conductors with direct sunlight exposure that is ¾ inch above the roof?



Solution:

Step#1: As per TABLE 310.15(B)(3)(c) for Distance Above Roof to Bottom of Conduit = ¾ inch




So, 40 °F to be added to the correction factors at table 310.15(B)(2)(A).


Step#2: Assuming an ambient temperature of 90°F , the temperature to use for conductor correction = 90°F + 40°F = 130°F ,


Step#3: As per table 310.15(B)(16), THWN-2 conductor has temperature rating 90°C and have allowable ampacity = 75A




Step#4: As per table 310.15(B)(2)(A), the adjustement factor for Ambient Temperature130°F and Temperature Rating of Conductor 90°C = 0.76




Step#5: So in this wiring method, the 6 THWN-2 conductor ampacity = 75 A X 0.76 = 57 A




Example#2:

Determine the ampacity of each of 10 current carrying no. 10 THW conductors installed in the same IMC, with an ambient temp of 105°f, connecting to terminals rated at 60°C.



Solution: 






Step#1: Find the no. 10 THW on table 310.15(B)(16), we find it in the 75°C column, however the terminals are rated at 60°C, so we will use the ampacity from the 60°C column; which will be 30 amps.
Step#2: As per table 310.15(B)(2)(A), the adjustement factor for Ambient Temperature105°F and Temperature Rating of Conductor 75°C = 0.82 


Step#3: The problem mentioned that we have 10 current carrying conductors installed in the same IMC, for this we will use table TABLE 310.15(B)(3)(a)

No. of current carrying conductors = 10, so the adjustment factor = 50%.


Step#4: Multiply the current rating of no. 10 THW Conductor from step#1 by the two correction/adjustment factors from steps# 2 & 3

Rating of each no. 10 THW Conductor = 30 x 0.82 x 50% = 12.3 A







As a summary, many corrections may be made for any conductor ampacity as follows:

  • First correction is based on its terminals temperature.
  • Second correction is based on existing ambient temperature
  • Third correction is based on existing of conditions requiring application of adjustment factors as discussed above.






In the next Article, I will explain some Special Cases for Conductor Ampacity Calculations. Please, keep following. 






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