90 Degree Column In Ampacity Table

[Alwayslearningelec]

A few questions.

1. 310.16 table is now 310.15(B)(16)? As of what code cycle? 2011?
2. When your sizing conductors and need to use ampacity adjustment and correction you can then use to the 90 degree column to get the new ampacity?

Hope this was one of my better structured posts? Try not to be hasty when posting, think out my question an write it properly.

 

[ron]

Table 310.16 was only known as Table 310.15(b)(16) from 2011 NEC through the 2017 NEC because it turned back to Table 310.16 in 2020 code.
310.15(A) is the answer to your 90 deg col question.

 

[Dsg319]

A few questions.

1. 310.16 table is now 310.15(B)(16)? As of what code cycle? 2011?
2. When your sizing conductors and need to use ampacity adjustment and correction you can then use to the 90 degree column to get the new ampacity?

Hope this was one of my better structured posts? Try not to be hasty when posting, think out my question an write it properly.

#2- yes as long as you have 90 degree rated conductors.

Example. 9ccc with all having 20amp OCPD. Thhn at 90 degree is 30amps.

Adjustment factor for 9ccc is 70%.

30amps x .70= 21amps.

Now throw a wrench in that. You have a set of #10awg conductors needing protected at 30amp OCPD still with 9ccc.

#10awg 90degree conductors rated at 40amps.

40x.70=28amperes. Still complaint (next size up rule)
as long as your load does not exceed that or as long your continuous load does not exceed 24amperes.

That’s how I’ve always understood it.

 

[Alwayslearningelec]

I meant to expand on my question #2

......Where I get a little confused with this is there are TWO tables you must refer to when doing a correction factor. 310.15(B)(1)(1) and then 310.15(B)(16). So, first, I would go to 310.15(B)(16) and use the 90 degree column? Then which temperature column would I use for the multiplier in 310.15(B)(1)(1)?? 75 degree or 90 degree? So if your always using the 90 degree column when doing correction( and adjustment) when would one ever use the 60 or 75 degree columns for correction multiplier?

 

[Alwayslearningelec]

Table 310.16 was only known as Table 310.15(b)(16) from 2011 NEC through the 2017 NEC because it turned back to Table 310.16 in 2020 code.

Thanks. What made then go back?

 

[infinity]

Thanks. What made then go back?

I chalk it up to never ending stupidity but I'm somewhat angry when it comes to the dumb changes in the NEC.

 

[Tulsa Electrician]

I meant to expand on my question #2

......Where I get a little confused with this is there are TWO tables you must refer to when doing a correction factor. 310.15(B)(1)(1) and then 310.15(B)(16). So, first, I would go to 310.15(B)(16) and use the 90 degree column? Then which temperature column would I use for the multiplier in 310.15(B)(1)(1)?? 75 degree or 90 degree? So if your always using the 90 degree column when doing correction( and adjustment) when would one ever use the 60 or 75 degree columns for correction multiplier?

Some of the listed conductors would not qualify for the 90 c table. This is when you use 60 or 75 to start then move to ambient.
An example would be if you look at the table for THWN.
Is it listed as 75c
In order to use the 90c is has to have -2.
For me the words condition of use is the key.
Using the THWN, let's say it is a THHN / THWN conductor.
This conductor has a 75c and a 90c rating. So which to use?
The KEY, condition of use. Based on the letters and the location the conductor is installed will be you quide to choice.
THWN would be a 75 c location  Wet. The THHN rating is for DRY only.
If the conductor was in raceway above grade in a dry location 90c can be used. If the conductor was installed in a wet or combination of wet and dry you would use 75c to start.

If the conductor had -2 than you you could 90c to start for both wet and dry locations.

Another item of importance with the letter designators are continues use. This was mentioned above so don't read past it. You would need to consider the condition of use.
A designation of -2 is contiuous use wet or dry at 90c. For the T HHN, it is rated for 90c c continuous use Dry location only. The (HH) of the THHN tells you continuous use.
These letters will direct you to the correct column to start with.
Reviewing the table you will see that the 60c has no H letters. So any conductor without this (H) letter has to start at the 60 c table.

Read thru part 2 of 310 installation.
310.10 will help shed light on letter designators. I will also post a pic which hopefully will help future on choosing the correct column.
Then read 310.15 looking close at (3).

See 110.14 and look at (C). While your there I would recommend read all of 110.14.

This should set you on. The right path to make a decision on using the correct column for the conductor used for a given installation.

As you move to step 2 for ambient you can now easily choose the correct column to use based on the header. Temperature Rating of the conductor.
If your using 90 c for step one use 90c for step two.
If the example THWN/ THHN is use in a wet location you use 75c in step one and in Step 2. If this conductor is used in a dry location you can use 90c for step one and 90c for step two.
Now that this is done go to step three number of conductors.
Now you have the final conductor ampacity.

Step 1: Conductor ampacity based on insulation type.

Step 2: Correction of ambient

Step 3: Adjustment of # of
conductors.

Final ampacity for conductor based on it's insulation type use correct column from table.

Hope this helps.

 

[Alwayslearningelec]

#2- yes as long as you have 90 degree rated conductors.

Example. 9ccc with all having 20amp OCPD. Thhn at 90 degree is 30amps.

Adjustment factor for 9ccc is 70%.

30amps x .70= 21amps.

Now throw a wrench in that. You have a set of #10awg conductors needing protected at 30amp OCPD still with 9ccc.

#10awg 90degree conductors rated at 40amps.

40x.70=28amperes. Still complaint (next size up rule)
as long as your load does not exceed that or as long your continuous load does not exceed 24amperes.

That’s how I’ve always understood it.

Need some clarification.
You first Example you must be referring to #12, correct?

Regarding your #10 example. You can use a breaker with higher ampacity than the wire ampacity? Wouldn't that cause the wire to overheat although 2 amps very negligible.

Aslo, not clear on how continuous load comes into play.

 

[Alwayslearningelec]

Some of the listed conductors would not qualify for the 90 c table. This is when you use 60 or 75 to start then move to ambient.
An example would be if you look at the table for THWN.
Is it listed as 75c
In order to use the 90c is has to have -2.
For me the words condition of use is the key.
Using the THWN, let's say it is a THHN / THWN conductor.
This conductor has a 75c and a 90c rating. So which to use?
The KEY, condition of use. Based on the letters and the location the conductor is installed will be you quide to choice.
THWN would be a 75 c location  Wet. The THHN rating is for DRY only.
If the conductor was in raceway above grade in a dry location 90c can be used. If the conductor was installed in a wet or combination of wet and dry you would use 75c to start.

If the conductor had -2 than you you could 90c to start for both wet and dry locations.

Another item of importance with the letter designators are continues use. This was mentioned above so don't read past it. You would need to consider the condition of use.
A designation of -2 is contiuous use wet or dry at 90c. For the T HHN, it is rated for 90c c continuous use Dry location only. The (HH) of the THHN tells you continuous use.
These letters will direct you to the correct column to start with.
Reviewing the table you will see that the 60c has no H letters. So any conductor without this (H) letter has to start at the 60 c table.

Read thru part 2 of 310 installation.
310.10 will help shed light on letter designators. I will also post a pic which hopefully will help future on choosing the correct column.
Then read 310.15 looking close at (3).

See 110.14 and look at (C). While your there I would recommend read all of 110.14.

This should set you on. The right path to make a decision on using the correct column for the conductor used for a given installation.

As you move to step 2 for ambient you can now easily choose the correct column to use based on the header. Temperature Rating of the conductor.
If your using 90 c for step one use 90c for step two.
If the example THWN/ THHN is use in a wet location you use 75c in step one and in Step 2. If this conductor is used in a dry location you can use 90c for step one and 90c for step two.
Now that this is done go to step three number of conductors.
Now you have the final conductor ampacity.

Step 1: Conductor ampacity based on insulation type.

Step 2: Correction of ambient

Step 3: Adjustment of # of
conductors.

Final ampacity for conductor based on it's insulation type use correct column from table.

Hope this helps.

Thank you.
Doesn't HH mean high heat?
What are you referring to when you say "continuous use"? Continuous load?

 

[wwhitney]

Need some clarification.
You first Example you must be referring to #12, correct?

Yes, he must have left that out.

Regarding your #10 example. You can use a breaker with higher ampacity than the wire ampacity? Wouldn't that cause the wire to overheat although 2 amps very negligible.

See 240.4(B)

Cheers, Wayne

 

[don_resqcapt19]

Thanks. What made then go back?

Code users wanted the old table numbers back so other changes were made to get back to the old table numbers. The NEC style manual does not permit a table unless there is an associated section with the same number. In the old codes there was no section 310.16, so to comply with the style manual, the table numbers were changed for the 2011 code

When they brought back Table 310.16, they added section 310.16 that contains the application information that previously appeared in the heading of the table.

 

[wwhitney]

40x.70=28amperes. Still complaint (next size up rule)
as long as your load does not exceed that or as long your continuous load does not exceed 24amperes.

I agree with you that should be the rule, that a 24A continuous load is fine on a 28A conductor protected by a 30A breaker. A 28A conductor is protected by a 30A breaker per 240.4(B), and a 30A breaker should avoid nuisance tripping with a 24A continuous load. And a 28A conductor can carry 28A continuously, per the definition of ampacity.

But 210.19(A)(1)(a) actually requires the conductor to have an ampacity of 125% of the continuous load, or 30A in this case. So #10 Cu with 7-9 CCCs and an ampacity of 28A would be limited to a 22.4A continuous load.

Makes no sense, but that's what it says.

Cheers, Wayne

 

[augie47]

A simple rule of thumb to follow:
1. calculate the load (100% non continuous + 125% continuous)
2. Select a conductor that is rated for that load after applying any adjustment (ambient, fill, etc)
3. Choose your OCP device based on the rules in Art 240.

 

[infinity]

Makes no sense, but that's what it says.

I agree that some of this 125% stuff seems silly but what about this example makes no sense? If the 24 amp continuous load requires a 30 amp conductor are you saying that 28 amp should be close enough? I think that you're saying that even at 28 amps continuous the 28 amp conductor is, at least in the real world, fine.

 

[wwhitney]

But 210.19(A)(1)(a) actually requires the conductor to have an ampacity of 125% of the continuous load, or 30A in this case. So #10 Cu with 7-9 CCCs and an ampacity of 28A would be limited to a 22.4A continuous load.

The above is my mistake--the 125% factor for continuous load only applies to the uncorrected/unadjusted ampacity. After adjustment and correction, the conductor ampacity only needs to be as large as the load, no 125% required.

So #10 Cu 90C with 7-9 CCCs and an ampacity of 28A can be protected by a 30A breaker and carry a 28A non-continuous load or a 24A continuous load.

Cheers, Wayne

 

[wwhitney]

I agree that some of this 125% stuff seems silly but what about this example makes no sense? If the 24 amp continuous load requires a 30 amp conductor are you saying that 28 amp should be close enough?

I messed up this particular example, but the basic idea is that physics-wise a 24A continuous load only requires a 24A ampacity conductor, as ampacity is a continuous rating. And if you use a 100% rated OCPD, then the NEC recognizes that and all is well.

But if you don't use a 100% rated OCPD, the NEC requires you both to upsize the OCPD by 125% and upsize the (uncorrected) ampacity by 125%. The latter is overly conservative--it should be enough to upsize the conductor as required to ensure that the OCPD adequately protects it per 240.4. In particular 240.4(B) should be allowed.

Classic example is 48A continuous load (EVSE), 6/2 NM (55A ampacity), and 60A OCPD.

Cheers, Wayne

 

[infinity]

The above is my mistake--the 125% factor for continuous load only applies to the uncorrected/unadjusted ampacity. After adjustment and correction, the conductor ampacity only needs to be as large as the load, no 125% required.

So #10 Cu 90C with 7-9 CCCs and an ampacity of 28A can be protected by a 30A breaker and carry a 28A non-continuous load or a 24A continuous load.

Thanks for the clarification. For some reason the after adjustment and correction part always trips me up.

 

[wwhitney]

Thanks for the clarification. For some reason the after adjustment and correction part always trips me up.

Well, the CMP has written that section wrong, and not understood my and Don's PIs to fix it. 210.19(A)(1)(a) uses the word "ampacity" and that, per the definition, already means adjusted and corrected (the definition refers to conditions of use).

But the use of the words "after adjustment and correction" in 210.19(A)(1)(b) makes it clear that what they mean in 210.19(A)(1)(a) is "before adjustment and correction." Given the definition of ampacity, the modifying phrase (with "before") belongs in (a), not in (b) (with "after").

Cheers, Wayne