MyCleveland
Senior Member
- Location
- Cleveland, Ohio
NEC 310.15(B)(3)(c) TEXT title: Raceways and Cables Exposed to Sunlight on Roof-Tops
Going over this article...AGAIN! I have went through some past threads and remain unsure that I have a complete understanding on when and what is a compliant action to take.
OLD Threads...
"Rooftop conductor derating.": Feb 4, 2016
'don_resqcapt19'...provides a CMP write up I believe is related to the 2017 change.
"Derating Factor": Sept 20, 2017
Seems to imply if your raceway is 7/8" or greater above the roof line then you are done...no derating regardless of what wire type you use.
"How do you know the ambient temperature of a roof?": Nov 17, 2021
This thread seems to imply the same thing and also brings up reference to a temperature guide publication located at "https://www.copper.org/applications/electrical/building/pdf/rooftop.pdf". Tom Baker seemed to be familiar with this document.
Can we come to a consensus on what is the proper approach to this design issue?
Scenario 1: Raceway install at or above 7/8" on a roof. Insulation may be ANY acceptable wire for your application.
a. NO adjustment for what is known to be a higher ambient than the provided ampacity tables? Simply use ampacities as provided in wire tables.
b. Make adjustment for ambient, but not required to add additional 33deg C penalty from 310.15(B)(3)(c). Pull your temperature value from the
(LINK) publication referenced above from the column headed "Max Temp".
c. Make adjustment for exterior temperature adder per 2017 Handbook "Calculation Example" pulling your temperature value from the (LINK)
publication referenced above from the column headed "2% Design Temp" and not required to add additional 33deg C penalty from 310.15(B)(3)(c).
Scenario 2: Raceway install below 7/8" on a roof. Insulation may be ANY acceptable wire for your application.
a. Make adjustment for ambient, and required to add additional 33deg C penalty from 310.15(B)(3)(c). Pull your temperature value from the (LINK)
publication referenced above from the column headed "Max Temp".
b. Make adjustment for exterior temperature adder per 2017 Handbook "Calculation Example" pulling your temperature value from the (LINK)
publication referenced above from the column headed "2% Design Temp" and required to add additional 33deg C penalty from 310.15(B)(3)(c).
Scenario 3: Raceway install below 7/8" on a roof. Insulation will be XHHW-2.
a. NO adjustment for what is known to be a higher ambient than the provided ampacity tables? Simply use ampacities as provided in wire tables.
b. Make adjustment for ambient, but not required to add additional 33deg C penalty from 310.15(B)(3)(c). Pull your temperature value from the
(LINK) publication referenced above
from the column headed "Max Temp".
c. Make adjustment for exterior temperature adder per 2017 Handbook "Calculation Example" pulling your temperature value from the (LINK)
publication referenced above from the column headed "2% Design Temp" and not required to add additional 33deg C penalty from 310.15(B)(3)(c).
I realize the wording in NEC 2020 was revised, I do not believe it changed the impact of 2017. I do not have a NEC 2023.
Going over this article...AGAIN! I have went through some past threads and remain unsure that I have a complete understanding on when and what is a compliant action to take.
OLD Threads...
"Rooftop conductor derating.": Feb 4, 2016
'don_resqcapt19'...provides a CMP write up I believe is related to the 2017 change.
"Derating Factor": Sept 20, 2017
Seems to imply if your raceway is 7/8" or greater above the roof line then you are done...no derating regardless of what wire type you use.
"How do you know the ambient temperature of a roof?": Nov 17, 2021
This thread seems to imply the same thing and also brings up reference to a temperature guide publication located at "https://www.copper.org/applications/electrical/building/pdf/rooftop.pdf". Tom Baker seemed to be familiar with this document.
Can we come to a consensus on what is the proper approach to this design issue?
Scenario 1: Raceway install at or above 7/8" on a roof. Insulation may be ANY acceptable wire for your application.
a. NO adjustment for what is known to be a higher ambient than the provided ampacity tables? Simply use ampacities as provided in wire tables.
b. Make adjustment for ambient, but not required to add additional 33deg C penalty from 310.15(B)(3)(c). Pull your temperature value from the
(LINK) publication referenced above from the column headed "Max Temp".
c. Make adjustment for exterior temperature adder per 2017 Handbook "Calculation Example" pulling your temperature value from the (LINK)
publication referenced above from the column headed "2% Design Temp" and not required to add additional 33deg C penalty from 310.15(B)(3)(c).
Scenario 2: Raceway install below 7/8" on a roof. Insulation may be ANY acceptable wire for your application.
a. Make adjustment for ambient, and required to add additional 33deg C penalty from 310.15(B)(3)(c). Pull your temperature value from the (LINK)
publication referenced above from the column headed "Max Temp".
b. Make adjustment for exterior temperature adder per 2017 Handbook "Calculation Example" pulling your temperature value from the (LINK)
publication referenced above from the column headed "2% Design Temp" and required to add additional 33deg C penalty from 310.15(B)(3)(c).
Scenario 3: Raceway install below 7/8" on a roof. Insulation will be XHHW-2.
a. NO adjustment for what is known to be a higher ambient than the provided ampacity tables? Simply use ampacities as provided in wire tables.
b. Make adjustment for ambient, but not required to add additional 33deg C penalty from 310.15(B)(3)(c). Pull your temperature value from the
(LINK) publication referenced above
from the column headed "Max Temp".
c. Make adjustment for exterior temperature adder per 2017 Handbook "Calculation Example" pulling your temperature value from the (LINK)
publication referenced above from the column headed "2% Design Temp" and not required to add additional 33deg C penalty from 310.15(B)(3)(c).
I realize the wording in NEC 2020 was revised, I do not believe it changed the impact of 2017. I do not have a NEC 2023.