Ampacity Adjustments for Conductors

Instructions on how to find ampacity of a given conductor

Step 1

Count your Current Carrying Conductors to get your bundling correction factor.
You have to count your neutrals unless they only handle unbalanced loads from a multiwire circuit that doesn't contain non-linear loads. Find your correction factor from the table at 310.15(B)(2)(a)
Number of
Current Carrying
% of temperature
corrected ampacity
< 3 100%
4-6 80%
7-9 70%
10-20 50%
21-30 45%
31-40 40%
>41 35%

Step 2

Find your conductor's  maximum capacity.
Look at tables 310.16 to 310.20 and find you conductor for the desired location.
Make sure your using the column that corresponds to the temperature rating of your insulation(60c, 75c, ect)

Step 3

Find your temperature correction factor.
If this conductor shall be used in a location subject to direct sunlight on or above a rooftop then you need to add to the ambient temperature according to 310.15(B)(2)(c).  At the bottom of the table find your adjusted ambient temperature. In the column marked with the temperature rating of your insulation (such as 60c, 75c, ect) you'll find your temperature correction factor.

Adjusted ambient temperature = ambient temperature + value from 310.15(B)(2)(c)

Step 4

Put it all together.
Amps you can use = (Amps of wire at 30c) x temperature correction factor x bundling correction factor

Instructions on how to figure out what size conductor to use for a given ampacity

Everything is the same as above except you need to divide by your correction factors
'Full Ampacity' = (Amps you need / bundling correction factor) / temperature correction factor
Then find a wire that can handle the 'Full Amps'

  Some average outdoor ambient temperatures for selected cities during the
period of June to August are shown below. These temperatures are based on
ASHRAE Handbook of Fundamentals 2 percent design temperatures and are
measured in the shade, not in the sun.
                              ̊F                                      ̊F
         City                                  City
 Birmingham, AL             94.0       St. Louis, MO                 94.4
 Phoenix, AZ               109.5       Las Vegas, NV               107.4
 Bakersfield, CA           102.7       Newark, NJ                    92.1
 Los Angeles, CA            78.6       Albany, NY                    87.3
 Long Beach, CA             86.9       Islip, NY                     87.3
 Palm Springs, CA          110.8       Akron/Canton, OH              87.1
 Wilmington, DE             90.5       Providence, RI                88.3
 Atlanta, GA                92.3       Knoxville, TN                 91.4
 Chicago, IL                90.6       Houston, TX                   96.0
 Baltimore, MD              92.5       Charleston, WV                89.7
 Boston, MA                 89.6

Copies of the Tables are available from the website of Houston Wire & Cable.

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copywrite & by Jason Wilson