Ampacity Rating of Conductor at 60 Degree C rated Termination

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shortcircuit2

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What is ampacity rating of #12 AWG Copper with THWN-2 rated insulation at 60 Degrees Celsius?
Should the correction factors in table 310.15(B)(1) be used?
Using ampacity of #12 THWN-2 from Table 310.16...
Example: 30amps x .71= 21.3amps

What is ampacity rating of #12 AWG Copper with THWN rated insulation at 60 Degrees Celsius?
Using the correction factors in table 310.15(B)(1)
Using ampacity of THWN from Table 310.16...
Example: 25amps x .58=14.5amps

(wet location application) 2020 NEC
 
This question sounds a bit oxymoronic. Perhaps you can add some clarification. THWN-2 is 90C rated, and THWN is 75C rated. The single H means 75C, and the "-2" means dual rated for 75C and 90C. Type TW with no H's, would be 60C rated.

The only way the 60C rating would come in to play is in the termination ratings, where derate factors don't apply. The main value of having 90C rated wire despite terminations that are rated less, is that it gives you headroom for your derate calculations.

Additionally, where are they getting the problem parameters that determine 0.71 and 0.58? This question doesn't mention any bundling (qty of wires in raceway) or ambient temperature in excess of 30C.
 
Carultch said:
Additionally, where are they getting the problem parameters that determine 0.71 and 0.58? This question doesn't mention any bundling (qty of wires in raceway) or ambient temperature in excess of 30C.
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0.71 and 0.58 are temperature correction factors. The post mentions 60 degrees C.
 
No, you do not use the correction factors as shown to keep to a 60C limit.

If the requirement is to keep the conductors below 60C then you simply use the 60C column.

If you have a 90C conductor connected to a 60C terminal then the 60C column applies at that location.

If that same conductor goes into a conduit with others and you need to derate or adjust then you work from the 90C column to determine the ampacity for the portion in the conduit.

Jon
 
Dennis Alwon said:
Remember the 60C termination would only be on equipment that isn't marked. All new equipment is 75C
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If you are taking an exam or if it is an academic problem, you can't assume 75C, unless there is a statement that specifies this. You have to go by 60C for 100A and less, unless the exam specifies otherwise.
 
shortcircuit2 said:
In short, how do we calculate the amperage of a given wire type so that it does not exceed a 60C degree termination limit?
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Look up the size in the column for terminations, either the 60C or 75C column as applicable. Write down the number. Call this A.
Look up the size in the 90C column, or the column applicable to the wire's own temperature rating. Write down the number. Call this B.

Leave the number A alone. No derate calculation applies to it. If derate factors apply to your problem (we'll call d), apply them to B.

Which number is smaller? A or d*B? That becomes your ampacity.
 
Carultch said:
If you are taking an exam or if it is an academic problem, you can't assume 75C, unless there is a statement that specifies this. You have to go by 60C for 100A and less, unless the exam specifies otherwise.
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I don't believe I said anything about an exam. I was just giving real life info...If it is an exam he is taking then he seems to know 60C is the correct temp
 
Thanks guys. I well past exams ;)

The columns in table 310.16 (2020) are specific to the (high) allowable ampacity for a given conductor insulation. I am aware it is practiced and taught to look to the 60C column and say this is the 60C termination ampacity for 90C wire (or 75C wire).

But I do not see any written reference in the Code that points to this practice.

Should equation 310.15(B) be utilized? Probably not.

Thanks again.
 
shortcircuit2 said:
Thanks guys. I well past exams ;)

The columns in table 310.16 (2020) are specific to the (high) allowable ampacity for a given conductor insulation. I am aware it is practiced and taught to look to the 60C column and say this is the 60C termination ampacity for 90C wire (or 75C wire).

But I do not see any written reference in the Code that points to this practice.


Thanks again.
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110.14(C)(1)
 
david luchini said:
110.14(C)(1)
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Table 310.16 does not list a 60C degree temperature rating for THWN-2, it only lists a 90C degree temperature rating.

For example: 110.14(C)(1)(a)(2) says - Conductors with higher temperature ratings, provided the ampacity of such conductors is determined based on the 60C (140F) ampacity of the conductor size used. (period) It does not say per the 60C column in 310.16 ( which is practiced and taught)

This reference to "ampacity of conductor" is used elsewhere also.

So, back to the question. How is the ampacity calculated to comply with 110.14(C)(1)?
 
shortcircuit2 said:
So, back to the question. How is the ampacity calculated to comply with 110.14(C)(1)?
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Reading through this thread I'm a little confused. Are you asking for a formula to calculate the ampacity of a conductor or the method used to determine ampacity of a conductor? I think that would be beyond the scope of the NEC. that may be more of a question for UL testers.
 
shortcircuit2 said:
Table 310.16 does not list a 60C degree temperature rating for THWN-2, it only lists a 90C degree temperature rating.
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Insulation does not an ampacity rating.

The conductor (copper or aluminum) has an ampacity rating for a given allowable temperature, and that rating depends on that temperature only, not on the insulation type.

Then each insulation type has a maximum allowable temperature, which tells you which of the conductor ampacities you can use for that conductor + insulation combination.

Table 310.16 column headings are just a reminder to you of the temperature limits of various insulation types.

Cheers, Wayne
 
shortcircuit2 said:
For example: 110.14(C)(1)(a)(2) says - Conductors with higher temperature ratings, provided the ampacity of such conductors is determined based on the 60C (140F) ampacity of the conductor size used. (period) It does not say per the 60C column in 310.16 ( which is practiced and taught)
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Your answer is in red. Table 310.16 gives you the ampacity 60C, 75C and 90C for each conductor SIZE.

See also 310.15(A), Information Note No. 1; (1), and 110.14(C).
 
The ampacity of conductors is calculated from the Neher McGrath equation, which factors in the heat generated in the wire, the thermal resistance of the surroundings, and ambient temperature. I believe that under the NEC one can still use this equation to determine ampacity 'Under Engineering Supervision'.

The table that us mere mortals use is based on this equation along with very conservative assumptions about the conditions of application.

-Jon
 
Thanks again for guiding this mere mortal towards a better understanding of the ampacity tables. The Neher-McGrath equation hits the nail on the head Jon.

Here is some info on the development of table 310-16:


Quote from article:

"There are ampacity tables in the National Electrical Code that are sufficient for most installations. However, the tables in the NEC are very crude approximations and therefore include a substantial safety margin."
 
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