Table310.15(B)(2)(a) or Table 310.15(B)(2)(b)

[paul]

Why two tables and when do you use one or the other? I'm trying to figure out what correction facters I need for a run on blocks across the roof. ASHRA tells me to use a design temp of 84°F with a maximum temp of 101°F.

 

[Dennis Alwon]

Use table (a) I never quite understood Table (b)

 

[charlie b]

Table (a) assumes an ambient of 30C, and gives correction factors for up to 85C, for conductor insulation systems rated up to 90C.. Table (b) assumes an ambient of 40C, and gives correction factors for up to 225C, for conductor insulation systems rated up to 250C. Both are said to have been derived from the same equation 310.15(B)(2). However, as an example, looking at the row for an ambient of 50-55C and the columns for insulation systems of 60, 75, and 90C, the tables do not show the same correction factors. So I am confused.

 

[paul]

Table (a) assumes an ambient of 30C, and gives correction factors for up to 85C, for conductor insulation systems rated up to 90C.. Table (b) assumes an ambient of 40C, and gives correction factors for up to 225C, for conductor insulation systems rated up to 250C. Both are said to have been derived from the same equation 310.15(B)(2). However, as an example, looking at the row for an ambient of 50-55C and the columns for insulation systems of 60, 75, and 90C, the tables do not show the same correction factors. So I am confused.

You and me, both. Since this job is in your back yard, west side of Queen Anne Hill, what table would you use?

 

[charlie b]

Like Dennis, I would only pay attention to table (a). But the tricky part is deciding what to use as the "ambient" temperature. The "design temp" of 84 would give you a correction factor of 1.0. What is not clear is whether to use the "maximum" temp of 101, given that the conductor will not experience that temperature value for more that short periods of time. In the Seattle area, we seldom see summer temperatures above 90F. But the impact of sun light directly hitting the conductor is a different matter.

 

[infinity]

I'm trying to figure out what correction facters I need for a run on blocks across the roof. ASHRA tells me to use a design temp of 84°F with a maximum temp of 101°F.

What code section are you apply that needs correction factors?

 

[paul]

What code section are you apply that needs correction factors?

That's the first part of this puzzle I had to put together. I was instructed by an inspector to get the numbers from ASHRA, then apply them to correct table in 310.15. Which, in turn, led me here.

 

[infinity]

That's the first part of this puzzle I had to put together. I was instructed by an inspector to get the numbers from ASHRA, then apply them to correct table in 310.15. Which, in turn, led me here.

I thought that maybe he was asking for correction factors based on the temperature adders in previous additions of the NEC which involved measuring the height of the raceway above the roof. In the 2011 NEC those adders were in T310.15(B)(3)(c) in the 2017 they have been removed if your raceway is 7/8" or greater above the roof.

 

[texie]

Why two tables and when do you use one or the other? I'm trying to figure out what correction facters I need for a run on blocks across the roof. ASHRA tells me to use a design temp of 84°F with a maximum temp of 101°F.

Take a look at the heading for table 31015(B)(16). Note it says "Based on Ambient Temperature of 30C (86F)". So you use Table 315.15(B)(2)(a).
Note some of the other ampacity tables are based on 40C. Then you would use the b correction table.

 

[Carultch]

Why two tables and when do you use one or the other? I'm trying to figure out what correction facters I need for a run on blocks across the roof. ASHRA tells me to use a design temp of 84°F with a maximum temp of 101°F.

If your wire type in question is not part of 310.15(B)(16), and instead has its ampacity in a different table that assumes a 40C ambient for its default ampacity, then you would use table 315.15(B)(2)(b). This is a rare situation for most of us, so most often you'll use 315.15(B)(2)(a). Both tables are based on the same formula in 310.15(B)(2), that can be used as an alternative method.

 

[Dennis Alwon]

If your wire type in question is not part of 310.15(B)(16), and instead has its ampacity in a different table that assumes a 40C ambient for its default ampacity, then you would use table 315.15(B)(2)(b). This is a rare situation for most of us, so most often you'll use 315.15(B)(2)(a). Both tables are based on the same formula in 310.15(B)(2), that can be used as an alternative method.

So THHN is also in Table 310.15(B)(20) which is based on 40C. So when would you assume 40C? Basically can you give an example of the rare situation