Ampacity correction based on 90? column??

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Igor Rauschen

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I?m preparing for the NABCEP certification using the solar photovoltaic system guide. I?m not quite understanding why the authors?s comment to article 690.8 states that "when performing conductor ampacity correction and adjustment calculations, use the conductor ampacity listed in the 90?C column of table 310.15(B)(16) for RHH/RHW-2/USE-2 and PV wire". Isn?t that a violation of code rule 110.14 (C)(1)?

Hope you can clear this up.

Thanks
Igor
 
You can use the 90C to start but the final ampacity cannot be more than the weakest link in the circuit. Generally that would be the equipment which is usually rated 75C. You can use the 90C for de-rating.

Read 110.14 carefully as it states if the equipment temp. is not known then you must use 60 C for 100 amps or less.
 
An interesting factoid in calculating ampacity is that XHHW-2 is exempt from the rooftop adder for temperature derating (exception to 310.15(B)(3)(c)).
 
Dennis Alwon said:
You can use the 90C to start but the final ampacity cannot be more than the weakest link in the circuit. Generally that would be the equipment which is usually rated 75C. You can use the 90C for de-rating.

Read 110.14 carefully as it states if the equipment temp. is not known then you must use 60 C for 100 amps or less.
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Thank you Dennis
after watching Mikes video I found it in article 310.15(B) "Temperature correction and adjustment factors shall be permitted to be applied to the ampacity for the temperature rating of the conductor, if the corrected and adjusted ampacity does not exceed the ampacity of the temperature rating of the termination in accordance with the provisions of 110.14(C)".

So derate the conductors ampacity rating based on the temperature rating of the conductor and then double check with the ampacity of the temperature rating of the terminals.
 
Igor Rauschen said:
Thank you Dennis
after watching Mikes video I found it in article 310.15(B) "Temperature correction and adjustment factors shall be permitted to be applied to the ampacity for the temperature rating of the conductor, if the corrected and adjusted ampacity does not exceed the ampacity of the temperature rating of the termination in accordance with the provisions of 110.14(C)".

So derate the conductors ampacity rating based on the temperature rating of the conductor and then double check with the ampacity of the temperature rating of the terminals.
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I find it easier to do it the other way around. Start by calculating 75C terminations with no derates, then see if it can work for wire insulation with derates. This also gives you clues as to the minimum local size, in the event that you have to reduce the wire at a termination.

Also remember the mostly academic rule, that 100A and less terminations are rated 60C unless listed and labeled otherwise. Very important when taking an exam, rarely an issue in practice (although it does come up occasionally).
 
ron said:
Nice adder to 2014
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And an excellent opportunity for a "gotcha" on the NABCEP exam. The difference between an exam and the real world, of course, is that in the real world you would not be penalized nearly as much for adding the derate where it wouldn't be needed.

Real world: Is this wire big enough? I have a ton of it in the truck.
Exam world: Is this wire the absolute minimum size that would be compliant?
 
ggunn said:
And an excellent opportunity for a "gotcha" on the NABCEP exam. The difference between an exam and the real world, of course, is that in the real world you would not be penalized nearly as much for adding the derate where it wouldn't be needed.

Real world: Is this wire big enough? I have a ton of it in the truck.
Exam world: Is this wire the absolute minimum size that would be compliant?
Click to expand...

Are NABCEP exams always based on the latest NEC, regardless of whether or not it applies in your state?
Or is it based on the latest NEC that applies in your state?
 
ggunn said:
And an excellent opportunity for a "gotcha" on the NABCEP exam. The difference between an exam and the real world, of course, is that in the real world you would not be penalized nearly as much for adding the derate where it wouldn't be needed.

Real world: Is this wire big enough? I have a ton of it in the truck.
Exam world: Is this wire the absolute minimum size that would be compliant?
Click to expand...


Real world: I'm on the borderline in the calculation between whether or not this wire is code compliant. It's not worth risking the possible fight with the inspector, I'm just going to use the next larger size.

Exam world: This wire better be the absolute minimum code compliant.


Real world: You ever tried fitting 4 wires at 500 kcmil through a 3" conduit? If I didn't use anything other than 4" or larger, it would've never gone through.

Exam world: This is what the code says, and that's what it says. This is the correct answer, and everything else both larger and smaller is wrong.
 
Carultch said:
Are NABCEP exams always based on the latest NEC, regardless of whether or not it applies in your state?
Or is it based on the latest NEC that applies in your state?
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The one coming up is the first to use the 2014 NEC. When they change they let us know, but it is the same exam for everyone, held at the same time down to the minute, and strictly proctored.
 
Not quite:

The NABCEP PV Installation Professional Exam to be administered on October 4, 2014 and April 11, 2015 will be based on the 2011 NEC; Exams administered after April 2015 will be based on the 2014 NEC.
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