So both the service entrance conductor to the 100A panel and the subpanel feeder are #2 AL? My eyes must be getting bad … wait, I’m not getting old, it’s a distorted picture. Yea, thats the ticket … :mrgreen:
Chances are it is a simple as it seems. The installer when to lowes or home depot and the dude with the orange or blue vest said it was ok to supply this panel with 2 AWG AL…only to find out on a municipal inspection that this 2 AWG AL was only good for 90A per table 310.16 and informed him/her that 310.15(B)(6) could not be used.
Thus they changed the breaker to a 90A rated device and move on as I am sure all terminals involved are at least 75 degree rated.
I was surprised when I saw they changed the NEC provisions for interior SE cable to force you to use the lower 60C ampacity values, similar to NM cables, by removing the 338.10.B.4 exception.
Was the exception wrong all these years, are they making SE cable differently now, or perhaps significant de-rating in say hot attics wasn’t being done (say #2 AL in 120°F attic is 100A x 0.82 = 82A, which is lower than the 75C value of 90A )?
Yes, this one catches many guys off guard because it can radically change the way installations have been done for decades. From what I remember there is absolutely no technical substantiation for this change. Maybe someone thought that since SE and NM are similar in design and construction that they should both be sized using the 60° C ampacity.
I think that change might be rescinded in the 2014 code. If it were it would not add any substantiation to why it changed in the first place.
Thought you guys might know, cause from what I remember nobody seemed to have a straight answer as to the technical reason when they were working on the local code adoptions a while back.
And it also appears that the 2009 and 2012 editions of the IRC electrical part do not have the requirement to use the 60C values for type SE cables … it only applies to type NM. And the IRC is a stand alone code, meaning you are not required to go to the NEC. Maybe an oversight, but thats the way it seems to read.
You guys loose me. After all these years I think I still need about three times as much time and exposure to any Electrical issue to comprehend than any other subject. I still get gel brain.:eek:
Since there are some heavy hitters in this particular thread, may I ask without seeking to chang the subject for some good resources to bring one up through theory and practice? Say from an intermediate level, other than the INACHI electrical and advanced electrical courses which I am aware of.
Thanks
For resources outside of NACHI, Mike Holt has some great products. I have several of his books which are well written and have very good graphics to explain things. Also he has a code forum with great information. You can simply browse through the threads or participate if you have specific question or comments.
www.mikeholt.com
If you want to get a better understanding of the NEC then I would also recommend to NEC Handbook which is the full NEC code book with explanatory graphics, photo’s and commentary on a wide range of code sections. Here’s a sample graphic:
Try these two references … “The Professional Inspector’s Resource Guide to Electrical Inspections” by Douglas Hansen and “Code Check - Electrical” by Redwood Kardon.
Also Mike Holts website/board and books are really good resources, and I think one of his books is really good for getting familiar with the NEC called “Mike Holt’s Illustrated Guide to Understanding the NEC Volume 1”. It’s packed with really good diagrams like the one RM posted to explain the code.
Great Stuff! Thanks!
They seem to go back and forth on some issues … like the previous changes related to service wire ampacities … :roll:
Yes like the most recent code cycle (2011) where they changed the ampacities of #14 and #12 conductors to match the Canadian electrical code. Since when do we care about the Canadian electrical code? Sorry all of our Canuck friends. 
I understand it is a good code so why not use the best there is… Roy
Easy there. If ya rile up enough people here we might just have to invade “Canadian Bacon” style … http://www.invadecanada.us/
Our Best friends where here from Florida for ten days .
They love shopping in Canada She loads up with spices and special tea from our Bulk food store .
She spent about $800;00 on things she can not get in USA .
She gets it for many others .
This should last till December when we will take her down some things she needs .
We have been doing this for over 30 years .
Gosh it would be great if she still lived in NJ .
Strange how that bacon is so popular here and not there.
Thanks for the memories… … Roy
Have you ever watched Rick Mercer Report?
This will give you an idea why we have a problem.
In some areas US is way ahead of us and in others it is behind.
I agree with the most stringent applications.
Nice to see color in you money now. LOL
Yea, that is strange … :shock: … But all kidding aside my favorite place to visit was Montreal … 
Was that the reason they gave?
It seems that the original ampacity table 310.16 was originally derived from the CEC. So they lowered the ampacities of a few condcutors to be in harmony with the CEC. IMO this is a worthless change because the substantiation has no real safety value. The old ampacities were fine the way that they were. Here’s part of the ROP:
Sounds like the values are a little different from rounding off equations, and changes to both the NEC and CEC are being made so they are consistent. May not be such a bad thing.