Underground steam tunnel-high temp wiring for wet locations (?)

[jtinge]

We have underground steam tunnels on our campus that typically have an ambient temperature of over 125°F (52°C). What high temperature wire is available, for wet locations, that can be used for branch circuit wiring in the tunnels and still comply with NEC, Chapter 3 wiring methods? A engineering firm designing modifications to the ventilation systems for the tunnels has proposed the use of SRML (silicone rubber motor lead) cable for branch circuit wiring; however, as I read Art. 310, this is not a permitted wiring method. Anyone know of a permitted wiring type that can be used in lieu of SRML for this application?

 

[zbang]

THWN/THWN-2? (75c deg wet)
XHHW? (90c deg wet)

Roofs and attics can easily get hotter than that 55c, and I don't think the tunnels would count as "wet", only as "damp". Any reason the usual wiring methods can't be used?

 

[petersonra]

If I understand it correctly, you need to correct the conductor ampacities for higher than normal ambient temperatures in accordance with table 310.15(B)(2)(a).

It says you have to derate conductors rated for 90 Deg C by a factor of .76 for 52 Deg C ambient.

So a 1/0 Copper THHN2 conductor would have a starting ampacity of 125 Amps instead of 170.

I would be inclined to use normal wiring methods and just upsize the conductors as needed.

When does one use table 310.15(B)(2)(b)?

 

[jtinge]

Intent is not to use SRML for higher ampacity, just to tolerate the high ambient temperature better than the typical THWN conductors. My inclination is to require the use of the XHHW wiring as well. Thanks for the feedback.