310.15(B)(1) and 2

[dwsmith]

A student inquired as to why an example in a Mike Holt video chose table 1 instead of 2. 90c wire 100 F ambient, 4 conductors 75c terminals. Not on a roof. Both 1 and 2 ( rooftop) say you can use either table. Why would we use one over the other. Its not clear to me what the difference is except of the calculation used to derive the tables.

 

[augie47]

My Table numbers are different but I believe your question applies to the (2) ambient Tables.
The one for 30°C corresponds to 310.15(B)(16) whereas the 40°C applies to 3109.15(B)(18) with different insulations

 

[Carultch]

The tables are all based on the same equation of sqrt((Tc - Ta)/(Tc-T0)), where Tc is the temperature rating of the conductor, Ta is the ambient temperature (or ambient temperature with an applicable adder), and T0 is the standard test ambient temperature of the conductor that corresponds to its published or tabulated ampacity. One table is for T0 = 30 Celsius, and the other table is for T0 = 40 Celsuis. In the scope of work that I do, I've never used a type of conductor where T0 = 40 C, since Table 310.16 ampacities are all based on 30 C as the ambient temperature.

If you perform your calculations with this formula, you can forget about the two tables you mentioned entirely. The tables give you the same results for the worst-case-scenario temperature within each 5-Celsius temperature range. You are permitted to either use the square root formula, or the temperature correction factor tables, whichever you see fit.

 

[dwsmith]

Thank you, I ran the numbers through the equation after my post and found the equation gave me a multiplier if near one using table 2 (100F falling in the range of 36-40C falls outside of the range of the second table.) When I used table one I came up with the same ampacity as the formula.

 

[dwsmith]

My Table numbers are different but I believe your question applies to the (2) ambient Tables.
The one for 30°C corresponds to 310.15(B)(16) whereas the 40°C applies to 3109.15(B)(18) with different insulations

Yes your right and thank you for clarification, after some research someone directed me to see that table 1 is @ 30C and used for corrections of 310.16 and 17. Table 2 is 40c and used for corrections of tablel 310.18-20 Both are in the notes section.