Ambient temp.
- Thread starter benaround
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hmspe
Senior Member
- Location
- Temple, TX
- Occupation
- PE
Arizona attics are hot, but not that hot....
At this ambient I'd either get a letter from the manufacturer or UL, or I'd use a cable from Table 310-18 instead of THHN.
Martin
At this ambient I'd either get a letter from the manufacturer or UL, or I'd use a cable from Table 310-18 instead of THHN.
Martin
Pierre C Belarge
Senior Member
- Location
- Westchester County, New York
THHN is rated for 194F
2-3 amps of current produces heat as a product of resistance. I do not know the temperature value 2-3 amps will produce, but it may be possible that 15F or more heat could be produced, now making the cable not suitable for this type of installation.
310.10
2-3 amps of current produces heat as a product of resistance. I do not know the temperature value 2-3 amps will produce, but it may be possible that 15F or more heat could be produced, now making the cable not suitable for this type of installation.
310.10
- Location
- Windsor, CO NEC: 2023
- Occupation
- Hospital Master Electrician
Frank, you've got to elaborate on this one. 
Can you make use of 310.15(A)(2) in this matter?
As Pierre grazed on ... should you?
Can you make use of 310.15(A)(2) in this matter?
As Pierre grazed on ... should you?
Pierre C Belarge
Senior Member
- Location
- Westchester County, New York
George
As Pierre grazed on
Are you calling me a cow?
As Pierre grazed on
Are you calling me a cow?
bphgravity
Senior Member
- Location
- Florida
I would question the 180?F value. Is this the worst case or typical temperature the cables will be exposed to?
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Table 310.16 only goes to 176F ambient, so I don't think that you could use 310.16 as the basis for operating THHN in a 180F environment.
However it is pretty easy to calculate that if the table had an 81-85C row, then the correction factor for THHN would be 0.29 .
This tells me that in a 180F environment, you could have on the order of 8 or 9 A flowing through 12ga THHN conductors without overheating them. But I think that you would need appropriate 'engineering supervision' to make the installation code compliant. I would also imagine some difficulties with design of the over-current protection.
Additionally, while THHN is rated for 90C, I would not be surprised if the conductors themselves can easily tolerate higher temperature. I regularly use wire that is rated as THHN and THWN and MTW; with a 90C rating as THHN but a 105C rating as MTW.
The big thing that would make me uncomfortable is Bryan's question: how well can you trust the 180F number? If the real value is 190F then the ampacity of the conductors is reduced still further, and if the real value is 200F, then the insulation would be overheated with no current flow at all. The other thing that makes me uncomfortable is that you are getting so close to the bleeding edge of the insulation rating that small errors in calculation or conditions might use up all of your safety margin.
-Jon
However it is pretty easy to calculate that if the table had an 81-85C row, then the correction factor for THHN would be 0.29 .
This tells me that in a 180F environment, you could have on the order of 8 or 9 A flowing through 12ga THHN conductors without overheating them. But I think that you would need appropriate 'engineering supervision' to make the installation code compliant. I would also imagine some difficulties with design of the over-current protection.
Additionally, while THHN is rated for 90C, I would not be surprised if the conductors themselves can easily tolerate higher temperature. I regularly use wire that is rated as THHN and THWN and MTW; with a 90C rating as THHN but a 105C rating as MTW.
The big thing that would make me uncomfortable is Bryan's question: how well can you trust the 180F number? If the real value is 190F then the ampacity of the conductors is reduced still further, and if the real value is 200F, then the insulation would be overheated with no current flow at all. The other thing that makes me uncomfortable is that you are getting so close to the bleeding edge of the insulation rating that small errors in calculation or conditions might use up all of your safety margin.
-Jon
Thanks to All for the replies.
The installation is in a new block (cement) making plant in the kiln area,
this is a 24/7 365 days a year operation. The temperature will be 180F via
a steam vapor generator,the control of the 8 isolated zones will be very
accurate.
The reason I asked for opinions is it is so close to the limit,194F, and I've
never really been that close before.
Maybe this will shed some more light!
Thanks agian,
The installation is in a new block (cement) making plant in the kiln area,
this is a 24/7 365 days a year operation. The temperature will be 180F via
a steam vapor generator,the control of the 8 isolated zones will be very
accurate.
The reason I asked for opinions is it is so close to the limit,194F, and I've
never really been that close before.
Maybe this will shed some more light!
Thanks agian,
Shockedby277v
Senior Member
- Location
- Michigan
Tell the kid to go get that wire cooler out of the truck! It's next to the wire stretcher.
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