# Ambient Temperature Correction Factors Tables

#### OldManFysh

##### Member
Studying for my journeyman's test and I can't seem to figure out when to use Table 310.15(B)(1) or Table 310.15(B)(2) when adjusting for ambient temp.

I know its likely an easy answer, but its escaped me

#### don_resqcapt19

##### Moderator
Staff member
One is based on a 30°C ambient and the other based on a 40°C ambient. Look at the headings of the ampacity tables for the ambient associated with that table (if you are using the 2017 or older). If you are using the 2020 look to sections 310.16 through 310.21 for that information.

#### OldManFysh

##### Member
Thanks for the help Don, but i still don't follow.

If the ambient temp is 42C / 108F which one of the two tables would I use and why.

thanks again for any help

#### david luchini

##### Moderator
Staff member
Thanks for the help Don, but i still don't follow.

If the ambient temp is 42C / 108F which one of the two tables would I use and why.

thanks again for any help
Compare the ambient temperature listed in the heading of Table 310.15(B)(16) vs. Table 310.15(B)(18).

The correction factor table 30C vs 40C will depend on the ambient temperature basis from the ampacity table you are using.

#### OldManFysh

##### Member
........I'm an idiot. Thanks for ya'lls help.

#### Carultch

##### Senior Member
It's based on the same formula, except done in incremental ranges for the value of Ta. You will get similar-ish results whether you use the formula, or whether you use the table, and you can use either in practice. In an exam, the answer key is likely based in the expectation that you'll use the tables.
sqrt((Tc - Ta)/(Tc - T0))

Where
Tc is the rating of the conductor
Ta is the air temperature to which you are correcting the ampacity
T0 is the basis ambient temperature at which the conductor has its original ampacity rating

T0 is most commonly 30C. Some types of conductors that are less common have their original ampacities determined for T0=40C. If you rarely use the 40C rating, mark your code book clearly to indicate to avoid using that table by mistake.