Table 310.15(B)(2)(a) vs table 310.15(B)(2)(b)

[askDano]

Table a is for ambient temperature of 30 deg C and table b is for 40 deg C yet each table covers the complete ambient temperature range. when do you apply each table? Table b has more conductor/terminal temperature ratings but when I have a terminal that is rated for 75 deg C and a ambient temperature of 120 deg F which table do I use and why?




Mike Holt example uses table a when ambient temperature is 100 deg f

 

[texie]

Table a is for ambient temperature of 30 deg C and table b is for 40 deg C yet each table covers the complete ambient temperature range. when do you apply each table? Table b has more conductor/terminal temperature ratings but when I have a terminal that is rated for 75 deg C and a ambient temperature of 120 deg F which table do I use and why?






Mike Holt example uses table a when ambient temperature is 100 deg f

Note that the conductor ampacity tables are not all based on the same ambient temperature. For example, the table we use most often is 310.15(B)(16) and note in the title it is based on 30 degree C. You then use Table 310.15(B)(2)(a) for ambient correction as it is for ampacities based on 30 degree C.