30 c and 40 c ambient temp correction confusion

[dm9289]

My understanding is wire temp is a combination of ambient and heat produced by current and when calculated some things did not seem to add up.

For a residential house lets say ambient temp 30c and there is a situation that requires temp correction like a superhot attic 115f using RHW cable 75c column
310.15B16 #3awg copper RHW is 100 amp correction .75 from 310.15(B)(2)a 100*.75=75 amp.

For a normally hotter place like a factory with temp of 40c same situation
310.15B16 #3awg copper RHW is 100 amp correction .85 from 310.15(B)(2)b 100*.85= 85amp

So this is telling me under higher ambient temp i can put more current on conductor?????
There has to be a factor I'm missing help???????

 

[wwhitney]

Ampacities from (2017) 310.15(B)(16) are all based on 30C ambient. So for temperature correction starting from those values, you always use 310.15(B)(2)(a).

310.15(B)(2)(b) is only used with ampacity tables based on 40C ambient, such as Table 310.15(B)(18), should you be using one of those insulation types.

Cheers, Wayne

 

[dm9289]

Ampacities from (2017) 310.15(B)(16) are all based on 30C ambient. So for temperature correction starting from those values, you always use 310.15(B)(2)(a).

310.15(B)(2)(b) is only used with ampacity tables based on 40C ambient, such as Table 310.15(B)(18), should you be using one of those insulation types.

Cheers, Wayne

OK so your saying once you exceed a normal ambient temp of 30c, 150c rated conductors are going to be the lowest rated for use in that area

 

[wwhitney]

No, not at all.

What I'm saying is that the "based on 30C" and 40C part of the titles of Tables 310.15(B)(2)(a) and (b) refers to the specification in the title of Tables 310.15(B)(16) through (21). If you only ever use the conditions and insulation types in 310.15(B)(16), you'll only ever use Table 310.15(B)(2)(a).

To go back to your example, #3 copper RHW has 75C insulation and a 75C ampacity of 100A. If you use it in a residence where the ambient is 30C, its ampacity is 100A. If you use it in a factory where the ambient is 40C, that 100A value is based on 30C, so you use the correction factor from 310.15(B)(2)(a): 0.88. So now the conductor has an ampacity of 88A. If you instead use it in an attic where the ambient is 50C, the correction factor is 0.75, so the ampacity is now 75A.

Cheers,
Wayne

P.S. The use of 90C insulation is beneficial when you have to derate for ambient temperature. The final answer is still generally limited to the 75C ampacity based on the connections at the ends. But when derating you can start with the 90C ampacity.

So #3 RHW-2 has a 75C ampacity of 100A and a 90C ampacity of 115A. If used at 30C ambient, the ampacity is limited to 100A based on the connections at the end. If used at 40C ambient, the ampacity is the lesser of 100A or 0.88 * 115A = 101.2A, so you can still use it for 100A. If used at 50C ambient, it's now the lesser of 100A or 0.75*115A = 86A.

 

[dm9289]

Im still missing it so let me try this example and maybe you can straighten me out
#1 THHN 90 c table 145amp

30c location like dwelling boiling attic 52c 145*.76 from 310.15(B)(2)a=110.2 amp
40c location like factory high bay area 52c 145*.84 from 310.15(B)(2)b=121.8 amp

Now do you see what im getting at although only difference is ambient you can get way more ampacity

 

[wwhitney]

30c location like dwelling boiling attic 52c 145*.76 from 310.15(B)(2)a=110.2 amp
40c location like factory high bay area 52c 145*.84 from 310.15(B)(2)b=121.8 amp

OK, here's the problem. If the dwelling attic is 52C, it doesn't matter what temperature the rest of the dwelling is, that doesn't come into the process. Likewise if the factory area is 52C, it doesn't matter what temperature the rest of the factory is.

The 30C on 310.15(B)(2)(a) refers to the fact that the ampacities are from a table based on 30C ambient, like 310.15(B)(16). It has nothing to do with ambient in the rest of the building. With 310.15(B)(16) you always use 310.15(B)(2)(a), never 310.15(B)(2)(b).

So the above should read:

(temperature elsewhere irrelevant) boiling attic 52C, 145A * 0.76 from 310.15(B)(2)(a) = 110.2A
(temperature elsewhere irrelevant) high bay area 52C, 145A * 0.76 from 310.15(B)(2)(a) = 110.2A.

Cheers, Wayne

 

[dm9289]

OK, here's the problem. If the dwelling attic is 52C, it doesn't matter what temperature the rest of the dwelling is, that doesn't come into the process. Likewise if the factory area is 52C, it doesn't matter what temperature the rest of the factory is.

The 30C on 310.15(B)(2)(a) refers to the fact that the ampacities are from a table based on 30C ambient, like 310.15(B)(16). It has nothing to do with ambient in the rest of the building. With 310.15(B)(16) you always use 310.15(B)(2)(a), never 310.15(B)(2)(b).

So the above should read:

(temperature elsewhere irrelevant) boiling attic 52C, 145A * 0.76 from 310.15(B)(2)(a) = 110.2A
(temperature elsewhere irrelevant) high bay area 52C, 145A * 0.76 from 310.15(B)(2)(a) = 110.2A.

Cheers, Wayne

Thank you but when would one use the 40C table? It appears only to be when using high temp conductors like Type Z or FEP

 

[don_resqcapt19]

Im still missing it so let me try this example and maybe you can straighten me out
#1 THHN 90 c table 145amp

30c location like dwelling boiling attic 52c 145*.76 from 310.15(B)(2)a=110.2 amp
40c location like factory high bay area 52c 145*.84 from 310.15(B)(2)b=121.8 amp

Now do you see what im getting at although only difference is ambient you can get way more ampacity

The ampacity tables and the associated adjustment and correction factors are based on conditions where the heat from all sources will not increase the temperature of the conductor enough to damage the insulation, so of course a conductor with a much higher temperature rating can carry more current than a conductor with a lower temperature rating without causing thermal damage to the insulation.

To the question about where your use the 40°C ampacity correction table, you use it with the values from Tables 310.18 through 310.21. The most commonly used tables are 310.16 and 310.17, but you use the 30°C ampacity correction table for those ampacities.