Proper use of Table 310.15 B(2)(b).

[pbazua]

I´m an associate professor in the Instituto Tecnológico de Los Mochis, Mx, and my class is on Electrical Installations. I don't find examples to clarify the use of this table 310.15 B(2)(b) on Ambient temperature Correction factors based on 40°C (104 °F). Would you be so kind to share on this topic?
Kindly regards.
IME Luis Pedro Alcántar Bazúa.

 

[don_resqcapt19]

The ampacity tables are all based on the conductors being in either a 30°C or a 40°C ambient. Where the ambient temperatures exceed those values, the ampacity of the conductor is required to be reduced. Table 310.15(B)(2) is for messenger supported conductors and the ampacity of the conductors in that table are based on an ambient of 40°C. Using that table, if you have a 8 AWG copper conductor in the first, 75°C, column, its ampacity is 57 amps. If you are installing that conductor in a 50°C ambient, you have to "correct" the ampacity. Using Table 310.15(B)(2)(b) we find that the correction factor for a 75°C conductor in a 50°C ambient is 0.85. The corrected ampacity is 57 x 0.85 or 48.45 amps. That is the maximum current carrying capacity of the conductor under that condition and the load and the overcurrent protective device should not exceed the corrected ampacity. Note the ampacity tables that are most commonly used have the conductor ampacities based on 30°C and the correction factors for those conductors are found in Table 310.15(B)(2)(a).

 

[pbazua]

I thank you don_resqcapt. I'm dealing with a bad translation of the Table 310.15(B)(2), our actual Code doesn't mention what is it for: messenger supported conductors.
I appreciate your attention.
Pedro.

 

[Carultch]

I thank you don_resqcapt. I'm dealing with a bad translation of the Table 310.15(B)(2), our actual Code doesn't mention what is it for: messenger supported conductors.
I appreciate your attention.
Pedro.

"Messenger" means a structural wire that supports the group of current-carrying wires as they travel through air as (probably overhead) wiring. The messenger wire would likely be galvanized steel or aluminum.

One place you see this construction method, is in a cable-supported traffic signal setup. There are steel wires supported at the top of vertical poles, which crosses the road intersection, and supports each traffic signal fixture. You'll notice a set of support rings along one of the cables, that carry the power wiring through open air, from the poles to the fixtures. The wiring method in this example, is messenger-supported conductors.

 

[RichB]

The ampacity tables are all based on the conductors being in either a 30°C or a 40°C ambient. Where the ambient temperatures exceed those values, the ampacity of the conductor is required to be reduced. Table 310.15(B)(2) is for messenger supported conductors and the ampacity of the conductors in that table are based on an ambient of 40°C. Using that table, if you have a 8 AWG copper conductor in the first, 75°C, column, its ampacity is 57 amps. If you are installing that conductor in a 50°C ambient, you have to "correct" the ampacity. Using Table 310.15(B)(2)(b) we find that the correction factor for a 75°C conductor in a 50°C ambient is 0.85. The corrected ampacity is 57 x 0.85 or 48.45 amps. That is the maximum current carrying capacity of the conductor under that condition and the load and the overcurrent protective device should not exceed the corrected ampacity. Note the ampacity tables that are most commonly used have the conductor ampacities based on 30°C and the correction factors for those conductors are found in Table 310.15(B)(2)(a).

Just try and convince a designer that you can "uprate" the conductor when the ambient temp is low !!!!

 

[charlie b]

Just try and convince a designer that you can "uprate" the conductor when the ambient temp is low !!!!

Speaking as a design engineer, I believe you can. But I would never choose to do so.

 

[RichB]

Speaking as a design engineer, I believe you can. But I would never choose to do so.

LOL That's what I was saying!! I brought it up once just to mess with our designers--Watching the smoke start coming out of ears was great--then I showed them where it said that and that I was messing with them--I had to buy doughnuts the next meeting!!