Table 310.16 has all the ampacities and at the bottom it has correction factor's on ambient air temp. This is all used when derating, load calculation's and so forth correct? Now how often is it actually used and when it refer's to ambient air temp does it mean for the lug's or the temp of the air around it? I'm just a little confused on how it is used. Can someone clarify this.
ambient air temp.
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jtester
Senior Member
- Location
- Las Cruces N.M.
Re: ambient air temp.
The derating factors should be considered in every ampacity determination. They apply to the air around the conductors. Table 310.16 is based on conductors in an 86 degree F ambient. Here in southern New Mexico we see 110 degrees, and so have to adjust for that higher ambient temperature. Running conduits past heat sources in a building, like boilers, heaters. etc., could raise the ambient temperature also.
There is considerable debate whether ambient means the air surrounding the conduit or the air inside the conduit before current is present. Does solar heat gain have to be included in a conduit on a roof, for example?
I hope that helps, at least all except for the sun part.
Jim T
The derating factors should be considered in every ampacity determination. They apply to the air around the conductors. Table 310.16 is based on conductors in an 86 degree F ambient. Here in southern New Mexico we see 110 degrees, and so have to adjust for that higher ambient temperature. Running conduits past heat sources in a building, like boilers, heaters. etc., could raise the ambient temperature also.
There is considerable debate whether ambient means the air surrounding the conduit or the air inside the conduit before current is present. Does solar heat gain have to be included in a conduit on a roof, for example?
I hope that helps, at least all except for the sun part.
Jim T
- Location
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- Semi-Retired Electrical Engineer
Re: ambient air temp.
In my view (and the reason for my confusion on this topic), the unresolved issue is whether the temperature value we use for de-rating purposes needs to be the highest temperature that we ever expect to see (no matter for how short a duration) or some type of year-round average. I favor the year-round average concept, and I think the laws of physics will agree with me. I have described the basis of my viewpoint in other threads. But the NEC is not clear on this point, and I don?t think it will likely change anytime soon.
[ January 14, 2005, 10:38 AM: Message edited by: charlie b ]
I know there has been some considerable debate, and I have been a participant in some of it. But I have not heard this as being a point of debate. If the conduit is surrounded by air (i.e., not buried in concrete), and if there is not yet any current, then the air temperature inside and the air temperature outside will quickly come to equilibrium. When you turn on the current and the wires start to heat up, the air temperature inside will exceed that outside, and heat will be transferred to the surrounding air. The ability of the system to dissipate that heat will depend largely on the initial temperature outside the conduit. For that reason, I see it as being clear that the NEC uses ?ambient temperature? in the sense of the air temperature outside the conduit.
In my view (and the reason for my confusion on this topic), the unresolved issue is whether the temperature value we use for de-rating purposes needs to be the highest temperature that we ever expect to see (no matter for how short a duration) or some type of year-round average. I favor the year-round average concept, and I think the laws of physics will agree with me. I have described the basis of my viewpoint in other threads. But the NEC is not clear on this point, and I don?t think it will likely change anytime soon.
[ January 14, 2005, 10:38 AM: Message edited by: charlie b ]
jtester
Senior Member
- Location
- Las Cruces N.M.
Re: ambient air temp.
quote
The air temperature inside and the air temperature outside will quickly come into equilibrium...
If equilibrium means they stop fluctuating, then I agree. If it means equal, I disagree. A local inspector put a thermometer in an empty conduit on a roof here in southern New Mexico, and measured over 130 degrees inside the conduit, on a day where the ambient was in the low 90's. Solar heat gain is the culprit.
Clearly the temperature that the conductor would be at before the input of current is 130, not 90. This will occur for as long as 6 hours or so most days, for over 4 months.
I would suggest there is no one way suitable for the entire country. Just like areas have different loading zones for line strength calculations, or lightning stroke maps, we could use a map, showing high solar gain, medium gain, or low solar gain factors. We could leave it up to the local AHJ. We could leave it alone. Seems like there is something in the 2005 recognizing conductor ambients higher than conduit ambient temps. I will look for it.
Jim T
quote
The air temperature inside and the air temperature outside will quickly come into equilibrium...
If equilibrium means they stop fluctuating, then I agree. If it means equal, I disagree. A local inspector put a thermometer in an empty conduit on a roof here in southern New Mexico, and measured over 130 degrees inside the conduit, on a day where the ambient was in the low 90's. Solar heat gain is the culprit.
Clearly the temperature that the conductor would be at before the input of current is 130, not 90. This will occur for as long as 6 hours or so most days, for over 4 months.
I would suggest there is no one way suitable for the entire country. Just like areas have different loading zones for line strength calculations, or lightning stroke maps, we could use a map, showing high solar gain, medium gain, or low solar gain factors. We could leave it up to the local AHJ. We could leave it alone. Seems like there is something in the 2005 recognizing conductor ambients higher than conduit ambient temps. I will look for it.
Jim T
Re: ambient air temp.
Charlie's post
I would tend to agree with you statement. However Jtester's statement
[ January 14, 2005, 04:34 PM: Message edited by: bob ]
Charlie's post
Charlie
I would tend to agree with you statement. However Jtester's statement
makes me think that there are areas that may need to make special adjustments. When you have temps in the 100 degree range that last 8 hours a day and last from June thru September that has to affect the cable. You would think that the cable MFGs would provide input as to how their insulation reacts to these extreme inviornments.
[ January 14, 2005, 04:34 PM: Message edited by: bob ]
wirenut1980
Senior Member
- Location
- Plainfield, IN
Re: ambient air temp.
[ January 14, 2005, 04:53 PM: Message edited by: wirenut1980 ]
I like this idea! If there are maps used for estimating something as unpredictable as lightning, then there should have no problem putting something together like jtester is suggesting. Let me know when you have something put together, Jim.
[ January 14, 2005, 04:53 PM: Message edited by: wirenut1980 ]
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