Interpolation for Ambient Temp. Correction & Adjustment Factors

[Zyb]

Are we allowed to interpolate Table 310.15(B)(1) 2020 - Ambient Temp. Correction Factors & Table 310.15(C)(1) Adjustment Factors for More Than Three Current-Carrying Conductors? Thank you

 

[ramsy]

Are we allowed to interpolate

Please provide an example of what you’re proposing.

 

[Zyb]

Please provide an example of what you’re proposing.

no. of current carrying conductors = 12
IF 9 = 0.70, and 20 = 0.50
can i interpolate 12 = 0.65, and use 0.65 in my calculation

Ambient Temp = 36deg, 90deg wire
IF 35 = 0.96, and 40 = 0.91
can i interpolate 36 = 0.95, and use 0.95 in my calculation

 

[wwhitney]

Are we allowed to interpolate Table 310.15(B)(1) 2020 - Ambient Temp. Correction Factors

You would not linearly interpolate, but you can use formula 310.15(B). For correction factors near 1.0, linear interpolation may give you the same answer as the formula, but the formula is the correct way to do it.

For your example with 90C insulation at 36C instead of the 30C specified for the table values, you have an allowable 54C rise (90-36) instead of the specified 60C rise (90-30). So the correction factor is sqrt(54/60)=0.95. The sqrt occurs because the heat generated is proportional to the square of current (I²*R), while the heat dissipated is proportional to the temperature difference.

& Table 310.15(C)(1) Adjustment Factors for More Than Three Current-Carrying Conductors?

Clearly not, as the table specifies a full range of conductor sizes: 4-6, 7-9, etc. So you are stuck using the factor for whichever interval your installation falls within.

Cheers, Wayne