# Thread: Table 310.15(B)(2)(a) vs. 310.15(B)(2)(b)

1. Junior Member
Join Date
May 2012
Location
Texas
Posts
4

## Table 310.15(B)(2)(a) vs. 310.15(B)(2)(b)

All the examples in Mike's exam prep book use the 86 degree F table, when would be a situation to use the 104 degree F table? Thanks.

2. Originally Posted by MWayne
All the examples in Mike's exam prep book use the 86 degree F table, when would be a situation to use the 104 degree F table? Thanks.
Good question... for which I do not know the answer. I found the initiating ROP, which I quote in part below...

Substantiation: Acceptance of this new section will harmonize the ampacity
correction factors for various ambient temperatures between the NEC and the
CEC, consolidate the ampacity correction and adjustment factors into a single
section [310.15(B)], and consolidate seven ambient temperature correction
tables into two tables.
The equation proposed to be added is the same equation that is permitted in
310.60(C)(4) to calculate the ampacity correction factors for various ambient
temperatures based on the conductor temperature ratings in the tables. The term
ÄTD was omitted from the equation since it is not necessary to include it for
cables rated below 46kV as the temperature rise due to dielectric heating is
insignificant compared to the conductor losses. This equation also appears in
3.4.1 of IEEE STD 835, IEEE Standard Power Cable Ampacity Tables.
Since the NEC is used internationally, the lower and higher ambient
temperatures were added to provide the appropriate ampacity correction factors
for colder and warmer climates.
Some of the ambient temperature ranges were revised to 5ºC increments to
harmonize with the proposal submitted for the CEC, to correlate with existing
NEC Tables 310.20 and B310.3, and to provide consistency with the 30ºC
table.
The ampacity correction factors for 85ºC (185ºF) that are included in Table
B.310.3 were not included in the revised table since, due to environmental
restrictions, paper and lead cable is no longer commonly used in applications
covered by the NEC. The ampacities remain in Table B.310.3 for reference
in existing installations and the ambient temperature correction factors can be
calculated using the equation in 310.15(B)(2).
This Proposal was generated by the NFPA/CSA NEC/CEC Ampacity
Harmonization Task Group...

3. You would use 310.15(B)(2)(a) when correcting ampacities from Tables 310.15(B)(16) and 310.15(B)(17), both of which are based on an ambient temp of 86degF.

You would use 310.15(B)(2)(b) when correcting ampacities from Tables 310.15(B)(18), 310.15(B)(19) and 310.15(B)(20), all of which are based on an ambient temp of 104degF.

4. One may ask why the tables are based on different temperatures. I think because the tables that have a higher temp. are in free air where they will like be dealing with higher temperatures.

5. Originally Posted by Smart \$
Good question... for which I do not know the answer. I found the initiating ROP, which I quote in part below...

Substantiation: Acceptance of this new section will harmonize the ampacity
correction factors for various ambient temperatures between the NEC and the
CEC ......
This Proposal was generated by the NFPA/CSA NEC/CEC Ampacity
Harmonization Task Group...
I used to have a good opinion of our neighbors to the north. After dealing with the new tables, not so much.

6. Originally Posted by ActionDave
I used to have a good opinion of our neighbors to the north. After dealing with the new tables, not so much.
IMO we shouldn't be harmonizing anything with the CEC. If they want harmony then they should change their code to match the NEC.

And while we're at it lets put all of the metric conversions after the American standards in the codebook.

7. Originally Posted by Dennis Alwon
One may ask why the tables are based on different temperatures. I think because the tables that have a higher temp. are in free air where they will like be dealing with higher temperatures.
That's heavy dude.

8. Originally Posted by infinity
IMO we shouldn't be harmonizing anything with the CEC. If they want harmony then they should change their code to match the NEC.

And while we're at it lets put all of the metric conversions after the American standards in the codebook.
Same problem with the NESC. Silly foreigners!

9. Originally Posted by infinity
IMO we shouldn't be harmonizing anything with the CEC. If they want harmony then they should change their code to match the NEC.
I couldn't agree more.

Roger

10. Junior Member
Join Date
May 2012
Location
Texas
Posts
4
Thanks everyone for the info. Is it safe to assume to always use (a) when taking a test unless notified otherwise?

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•