highpowered
Member
- Location
- los angeles
Just curious to everyones thoughts on the proper column to size your wire from? I would love to use 90 degree column is this correct. My base wire is thhn/thwn-2 for 250 amp sub-panel i would use 4/O indoors.
choosing wire ampacity from NEC tables based on tempurature
- Thread starter highpowered
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- New Jersey
- Occupation
- Journeyman Electrician (retired)
Take a look at 110.14(C).
highpowered
Member
- Location
- los angeles
I dont understand what they mean by not to exceed lowest temp rating of any connected termination, conductor or device? So if a sub panels terminals are rated 60degrees I have to size wire to that? I dont remember ever seeing that on the panel label but then again I never looked for it. I always thought it was for temperature the location the wire was ran in. Whats the standard terminal temp for panels if any?
highpowered
Member
- Location
- los angeles
oops read further i see
- Location
- New Jersey
- Occupation
- Journeyman Electrician (retired)
That's it. If they're 75? C then you can use the 75? C ampacity. For all intents and purposes 90? C is only a starting point for derating.
highpowered
Member
- Location
- los angeles
so its 75degrees for 100 amps and over #1 or larger. in what instance would you use the 90degree column then?
- Location
- Massachusetts
Almost never directly, you can use it as a starting point for derating.
highpowered
Member
- Location
- los angeles
good to know. I never used it to derate what about continuous load factor ? could I use 90 degree then?
- Location
- Connecticut
- Occupation
- Engineer
Continuous load factor is figured BEFORE the application of any adjustment or correction factors (see 215.2(A)(1)). Using a higher temperature rated conductor is permitted to be used for ampacity adjustment, correction or both (110.14(C)). So the continuous load factor should be figured at the ampacity based on the termination temperature rating (or the lowest device/termination rating in the circuit.)
For example, if you had a 200A load that was continuous, per 215.2(A)(1) you would need a minimum conductor size that had an ampacity (before any adjustment/correction) of 200 * 1.25 = 250A. A 90deg 4/0 or a 75deg 250mcm has sufficient ampacity, but if the terminations are rated 75 deg you would have to use the 250mcm as the minimum size.
Now lets consider if the ampacity had to be corrected for 120degF ambient (0.75 for 75deg conductors or 0.82 for 90deg conductors.) If you used a 75 deg conductor for the feeder (say THWN) then the corrected ampacity would be 255*0.75=191A. This ampacity is too small for the 200A load, so you would need to increase the conductor size OR you could consider using a 90deg rated conductor. Using 250mcm THWN-2, the corrected ampacity would be 290*0.82=238A. This ampacity is large enough for the 200A load, so it would be acceptable.
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