table 310.15(B)(2)(a) and (b)...2011 NEC

[eric dolphy]

hello

please take a moment to differentiate the uses of (and, the necessity for) the two tables...

for example, B2a is based upon 30 deg. C, whereas B2b is presupposes 40 deg. C.

further, B2a has listings/factors for both 30 and 40 deg. C--why not then simplify (Thoreau) and rely on one, comprehensive table and adjust/factor accordingly...?

it's likely obvious, but fatigue necessitates the post...


thank you

(studied with MH in Ft. Lauderdale, c. 1988)...

 

[augie47]

310.15(B)(2)(a) relates to Tab;es 310.15(B)(16) & (17) where the ampacities listed as based on 30°C whereas 310.15(B)(2)(b) related to 310.15(B)(18)-(21) where the ampacties are based on 40♠C.

I don't see the 40 deg in 310.15(B)(2)(a) that you mention.

 

[Carultch]

hello

please take a moment to differentiate the uses of (and, the necessity for) the two tables...

for example, B2a is based upon 30 deg. C, whereas B2b is presupposes 40 deg. C.

further, B2a has listings/factors for both 30 and 40 deg. C--why not then simplify (Thoreau) and rely on one, comprehensive table and adjust/factor accordingly...?

it's likely obvious, but fatigue necessitates the post...


thank you

(studied with MH in Ft. Lauderdale, c. 1988)...

310.15(B)(2)(a) is the most common in terms of the scope of work that applies to most applications, and is for the standard wire types that are lab tested at 30C ambient to achieve the indicated ratings in table 310.15(B)(16).

The other table is for wires that are lab tested at 40C ambient to achieve the ampacity ratings in tables 310.15(C)(##), which are more specialized wire types. I've never used this table, and I recommend drawing a big X through it, to avoid using the wrong table by mistake.

You did ask why, and unfortunately there isn't a real answer. It probably has to do with the history of development of the NEC, where one table existed first, and the additional table came in later as the specialized wire types were added after the general table for common wire types had existed for possibly decades. In the code making process, it is easier to add a table than to edit an existing one.

 

[eric dolphy]

thanks again, Augie and Carultch...

I don't see the 40 deg in 310.15(B)(2)(a) that you mention.

Possibly a misread on my part, but there appears an ambient temperature 'range' for B2a in my 2011 edition, to wit:

36-40 deg C, followed by three columns of factors for 60, 75 and 90 deg conductors...

Regarding the puzzlement prompting the post, yes, indeed, I can put this one to bed--great deal, guys...