ambient temp correction factors 310.16

[rick5280]

Re: ambient temp correction factors 310.16

Isn't it a fact that the higher the air around a conductor, the harder it would be for that air to "bleed" away the heat generated by the conductor? A conductor with a temp of 100 degrees, at an ambient temp of 100, will not cool off. Any ambient above that will severely limit the cooling, while an ambient below that will help bleed off the heat. I believe that this temp is what the code is addressing.
The fact that the heat will not be removed from the conductor is the basis for reducing the ampacity of that conductor. If a conductor can not bleed off it's excess heat, the insulation will be damaged.
Sure if the temp raises, then drops, the ambient will also drop, but will it drop enough to help in the cooling of that conductor? Enclose that conductor within steel or plastic conduit, and the heat will be trapped within the conduit. Enclose the raceway within a room with elevated heat and you have another heat trap. Enclose the room in an area that has 54 degrees during the hottest part of the year, and what do you get? Who cares????, the room and raceway will stay elevated for an extended period of time, and if long enough time has passed without cooling, the conductor's insulation will be damaged!
Ambient temp is the air around the conductor, not around a town.
Just my thoughts. Real interesting thread though!!

Rick Miell

 

[Guest]

Re: ambient temp correction factors 310.16

If you used the maximum temp theory then youd have to drrate everything in arizona by what?or texas or calif or florida? food for thoght eh?

 

[jtb]

Re: ambient temp correction factors 310.16

I agree with IWIRE here. Go with Worst Case temp and be safe.

 

[jtb]

Re: ambient temp correction factors 310.16

Just a wacky sidetrack here....

To reduce temperature in an enclosure, they make a device called a Vortex Cooler. You run compressed air thru it, and the coil inside the enclosure extracts the heat.

Would it be possible to manufacture a listed fitting or conduit type and devices to cool conduit runs thru hot ambient regions like this?

What about running cooling air thru the conduit itself? I know, not listed for use, etc, and problems with blowing crud thru the conduit, etc.

Just a thought.

 

[iwire]

Re: ambient temp correction factors 310.16

Originally posted by joe stewart:
If you used the maximum temp theory then you'd have to derate everything in Arizona by what? or Texas or Calif or Florida? food for thought eh?

I am with you, as I said I do not think many of us do this.

Bob

[ March 28, 2003, 05:27 AM: Message edited by: iwire ]

 

[charlie b]

Re: ambient temp correction factors 310.16

Originally posted by iwire:Charlie, I understand what you are saying about a limited time produces limited damage, but if you never go above the temperature rating the insulation will last even longer.

That?s true, and it?s a good deal for the user. I once worked in an industry that required replacement of a component that had exceeded its ?qualified life,? even if there were no physical evidence that it had deteriorated. Fortunately, we don?t have to replace cables that we install in buildings, simply because a specified time has elapsed. I suspect that over 90% of what we install will remain well below 30C for its entire life. As a result, it will far outlive any time periods listed in the manufacturer?s literature.

If the code wanted to allow us to go above the temperature ratings for a certain time period wouldn't they have specified a time, in the same way they specified 3 hours as a "continuous" load.

I wish they would specify a time; that would give the clarity that I think the code presently lacks.

The code?s only function is to keep us all safe. The only practical way for it to do so is to stay as simple as possible. The equations related to cable degradation as a function of time and temperature are not taught in most college EE programs, and are unknown to many EE-PEs. I?ve seen them, but never used them in a real design project. But I am certain that the code doesn?t intend that we derate a cable because the air conditioning failed one day, with the result that the building?s ambient temperature reached 31C for one half hour before they fixed the A/C.

I can think of two simple ways the code could be revised to make it clear how much time must be spent above 30C, before the deratings apply. One is to state that the ?ambient temperature? in the table refers to the year-round average for the area, as reported by the weather services. Another is to state that a cable whose ambient exceeds 30C for more than XXX days a year (I?d suggest 60) must be subjected to the derating factors.

I?m not fond of the notion of ?we know it?s wrong, but everyone does it, and no inspector has called us on it, and we don?t have any better ideas, so we have no choice but to continue doing it.? The code should be revised to help us out here.