Lower Ambient Temp Correction 310.16-20

[rexowner]

I may be missing something in 310.15 etc.

What if the ambient temp falls outside the ranges on the
bottom of tables 310.16 - 310.20?

If they are on the high side, it is apparent that one should
not use a wire with the given temperature rating.

On the low side, however, they appear to be undefined.
Using 310.18 as an example, the text says, "for ambient
temperatures other than 40 deg C .... the appropriate
factor (in the table). Since the multipliers are all less than
one, and the multiplier is only one if the ambient is exactly
40 deg c, it appears, lower temperatures are undefined.

E.g. let's say, trying to apply 310.18, and the ambient temp
is 30 degrees C. If one is supposed to just use the number
for 40 degrees C, it does not appear that the Code actually
says to do this.

Another example, would be applying the more common
310.16 at ambient less than 21 degrees C -- this appears
to be undefined by the Code.

Am I missing something?

Thanks.

 

[charlie b]

I seldom use any table other than 310.16, and that one has temperature corrections higher than 1.0. But I have never tried to take advantage of higher ampacities in installations for which the ambient temperature is below 30C. I didn't want to be called upon to prove the temperature never gets higher than that value. The simple truth is that the cooler the wire, the longer it will take for its insulation system to fail. So it is better to be cooler, but that fact does not often work to your advantage.