Tables 310.15(B)(2)

[augie47]

As pappy used to say "I need someone to 'spalin this to me".
It's probably elementary but I can't get my brain wrapped around it.
I am still under the '08 Code but a lot of my review is '11 (and now '14).
With the '11 Code our 310.15(B)(16) ambient adjustments now reference 310.15(B)(2) which would be great for me if there were not (2) tables.
To help describe my confusion, let me take (3) #3 THHN conductors in a raceway.
If I install use 310.15(B)(2)(a) and I install those conductors in a 104? degree ambient my ampacity will be 0.91 X 115 or 105 amps.
If I use Table 310.15(B)(2)(b) at 104? ambient my amapcity is back to 115 amps

My feeble mind is telling me if I install a conductor at a higher ambient I get more ampacity so
obviously I'm not understanding something.

Can some one calrify this is simple terms and old man can understand

 

[Dennis Alwon]

I have never had the need to use the second table. I know there was a thread on this question with an explanation on when you would use T. 310.15(B)(2)(b) but I don't recall. Obviously one is based on 30C and the other table on 40C. I'll see if I can find that thread

 

[augie47]

I have never had the need to use the second table. I know there was a thread on this question with an explanation on when you would use T. 310.15(B)(2)(b) but I don't recall. Obviously one is based on 30C and the other table on 40C. I'll see if I can find that thread

My search skills are on par with my understanding skills this morning..... my search didn't do well
This is one of those days when I recognize it's about time to find a rocking chair

 

[Dennis Alwon]

I don't believe you can use Table (b) if you use the ampacity of the conductors from T. 310.15(B)(16) because that is based on 30C not 40C. Now when do you use 40C-- IDK

 

[Dennis Alwon]

I found one thread but only a few responses and Trevor appears to say what I said above

http://forums.mikeholt.com/showthread.php?t=160778&highlight=Table+310.15(B)(2)(b)

 

[Dennis Alwon]

My search skills are on par with my understanding skills this morning..... my search didn't do well
This is one of those days when I recognize it's about time to find a rocking chair

The search feature on the forum sticks IMO. Select the single search and type Table 310.15(B)(2)(b)

Sometimes it is easier to do a search with the article or table number thru google.

Let me show you old man-- Click Here

 

[Dennis Alwon]

Here is an art. on ecm about how to choose. http://www.ecmag.com/section/codes-standards/sizing-conductors-part-v

 

[Smart $]

... Now when do you use 40C-- IDK

When one of the Tables 310.15(B)(18) through (21) are used. Note in their titles they are based on an ambient of 40?C.

 

[Dennis Alwon]

When one of the Tables 310.15(B)(18) through (21) are used. Note in their titles they are based on an ambient of 40?C.

That I know but how do we determine if it is 40C or 30C. Is that the temp of the equipment used, ie breakers etc?

 

[Smart $]

That I know but how do we determine if it is 40C or 30C. Is that the temp of the equipment used, ie breakers etc?

Wherever the conditions of installation match the table title... you use the correction table which matches the "based on ambient temperature" in the title.

 

[Dennis Alwon]

Again I know that, but when do they conditions match 40C. Can you give an example of when you use 40C

 

[david luchini]

Again I know that, but when do they conditions match 40C. Can you give an example of when you use 40C

You don't use 40C or 30C. You use the table that matches your condition.

For instance, 3 THWN in a raceway - Use table 310.15(B)(16).

3 THWN supported on a messenger - Use table 310.15(B)(20).

 

[Dennis Alwon]

You don't use 40C or 30C. You use the table that matches your condition.

For instance, 3 THWN in a raceway - Use table 310.15(B)(16).

3 THWN supported on a messenger - Use table 310.15(B)(20).

Gotcha-- I should have checked to see the titles in the tables. I* was assuming there was a 40C table for raceways

 

[david luchini]

Gotcha-- I should have checked to see the titles in the tables. I* was assuming there was a 40C table for raceways

There is a 40C table for raceways, but its for use with Conductor Types Z, FEP, PFA, et. al., at higher conductor temperature ratings.

Table 310.104(A) tells you that the higher temperature ratings are for "special applications" where design conditions require maximum conductor operating temperatures above 90C.
I've never come across a "special application" so I'm not sure where it would apply.

 

[augie47]

Thanks, guys

 

[don_resqcapt19]

When Mike did his video for the 2014 code, his advice was just to "cross out" the larger [T310.15(B)(2)(b)]of the two tables in the code book. The larger table is for adjustments to ampacity tables that used 40?C as the base for the current. We rarely use the tables based on 40?C so cross it out and just look at the smaller[310.15(B)(2)(a)] of the two tables for your temperature correction factor.

 

[Smart $]

...
I've never come across a "special application" so I'm not sure where it would apply.

When Mike did his video for the 2014 code, his advice was just to "cross out" the larger [T310.15(B)(2)(b)]of the two tables in the code book. The larger table is for adjustments to ampacity tables that used 40?C as the base for the current. We rarely use the tables based on 40?C so cross it out and just look at the smaller[310.15(B)(2)(a)] of the two tables for your temperature correction factor.

You can if you are like David... but I've ran into these 'special applications' in power plant and steel mill work.

 

[GoldDigger]

As pappy used to say "I need someone to 'spalin this to me".
It's probably elementary but I can't get my brain wrapped around it.

My feeble mind is telling me if I install a conductor at a higher ambient I get more ampacity so
obviously I'm not understanding something.

Can some one calrify this is simple terms and old man can understand

Not to take away from all of the enlightening responses, but I have not seen anyone address this part directly.
If the maximum temperature of the insulation is 90C and you start with the unloaded wire at 40C you will not be able to push as much current before getting to 90C than if you started at 30C.
Your statement above is like saying that the hotter the air temperature around a motor the more I can load it without burning it out. It just ain't so.

 

[augie47]

I'm clear on it now... one must read the headings

 

[Carultch]

As pappy used to say "I need someone to 'spalin this to me".
It's probably elementary but I can't get my brain wrapped around it.
I am still under the '08 Code but a lot of my review is '11 (and now '14).
With the '11 Code our 310.15(B)(16) ambient adjustments now reference 310.15(B)(2) which would be great for me if there were not (2) tables.
To help describe my confusion, let me take (3) #3 THHN conductors in a raceway.
If I install use 310.15(B)(2)(a) and I install those conductors in a 104? degree ambient my ampacity will be 0.91 X 115 or 105 amps.
If I use Table 310.15(B)(2)(b) at 104? ambient my amapcity is back to 115 amps

My feeble mind is telling me if I install a conductor at a higher ambient I get more ampacity so
obviously I'm not understanding something.

Can some one calrify this is simple terms and old man can understand

310.15(B)(2)(a) is for wires that are factory tested at an ambient temperature of 30C or 86F. This is most of your general wire types that are part of your favorite Table 310.15(B)(16).

310.15(B)(2)(b) the huge table that jumps right out at you off the page, is for wires that are factory tested at an ambient temperature of 40C or 104F. It is rare that you will use this table. If it doesn't apply to your scope of work, I recommend drawing a huge X through it, to save you the mistakes I've been through. Exotic wire types, like medium voltage and nickel-plated.