The NEC in Alaska

[WhyDoYouAsk]

Since resi. insulation temp is fixed at 60? to 90? and
ampacity is determined by I² and amb. temp.,
can resi. wires and thermally operated CBs in Alaska legally carry more current?

 

[BJC]

Or less in Arizona??

 

[davidbeach]

Generally no credit for low ambient temperatures, and then only for conductors, not breakers. Definitely derating for elevated ambient temperatures.

 

[WhyDoYouAsk]

Thanks so far; this forum is great!

While we're on the subject, is 60? a nominal value (like a 2x4 is not 2.0" x 4.0") or does the insulation actually reach that temp (I'm hoping the second option is correct)?

 

[davidbeach]

60[°]C is the temperature at which the insulation fails. Operating temperature should be much lower.

 

[PHovnanian]

The NEC ampacity tables are calculated with very conservative assumptions about ambient temperatures and thermal resistances. Operating conductors under these conditions, conductors carrying their maximum ampacity will reach the max temps. However, the NEC throws an additional 80% derating factor into the equation, so its highly unlikely that they will outside of some abnormal condition.

It might be possible to exceed the table ampacities for installations designed under engineering supervision (as allowed by the NEC) if the engineer responsible has additional information about thermal characteristics or maximum ambient temperatures.

 

[WhyDoYouAsk]

"Fail"meaning its service lifetime is unduly shortened? Like many other chemical reactions I'm assuming service lifetime halves for each 10? rise above ambient.

 

[dpc]

"Fail" meaning the building burns down.

 

[alehman]

The NEC conductor tables have rating factors for various ambient temperatures. The tables are based on 30C ambient and rating factors are >1.0 for lower temperatures, but I've never seen a case where those were used.

Also you can't use larger than a 20A breaker on 12AWG or 30A on 10AWG, regardless of the numbers in the tables.

Don't forget that it gets warm in the summer, even in Alaska.

New rules in the 2008 NEC have specific temperature adjustments that must be applied to conduits installed on rooftops, due to solar heating.