Derating XHHW at 75 degrees

[Howard Burger]

T310.13 tells me XHHW is rated at 75 degrees in wet locations. We are planning on pulling 10 ccc underground for a bull rail, which would require 50% derating. Since XHHW is 75 degree wire in wet locations, can I still use th 90 degree column of T301.16 to derate the #10 from 40 to 20 amps, or do I need to use the 75 degree value? I see XHHW is shown under 'types' at the top of both the 75 and 90 degree columns.

 

[kbsparky]

Are you using XHHW-2?

IF so, then you can use the 90 degree column in wet locations when calculating your loads and wire sizes ...

 

[Howard Burger]

I don't know what type X was planned; I take it with the -2 indicating 90 degree that we could use that column to derate, otherwise if we use just XHHW we use the 75?

 

[jusme123]

I don't know what type X was planned; I take it with the -2 indicating 90 degree that we could use that column to derate, otherwise if we use just XHHW we use the 75?

what you stated is correct

see table 310.13(A)

XHHW is on page 143 of '08 code (make sure to look under 'application provisions')

 

[Howard Burger]

thank you

thank you

Thank you both for your comments. The owner of the company and the lead journeyman didn't know about -2, either. I printed out a coment by jdsmith on a 9/27 thread (http://forums.mikeholt.com/showthread.php?t=129867&highlight=thhn+xhhw) which explained important differences bewtween thhn and xhhw. This forum is a fantastic resource. Thanks again for your commentds.