Ambient temperature

[hardworkingstiff]

I'm installing parallel conductors for a service from an UG transformer to my panels mounted outdoors. Since the panels are outdoors, do I need to derate the conductors for ambient temperature? If so, what is the correct temperature to use? How about the effects of the sun beating down on the 3R/12 enclosure?

Thanks,
Lou

 

[iwire]

If so, what is the correct temperature to use?

That is the $10,000 question.

Should you design for the absolute worst case or can you 'wing' it a bit?

I have read on the forums that research has shown 140 F inside raceways on roofs.

If you where to use that temp as your ambient the breakers in those panels would also have to be derated.

It should be an interesting thread, IMO it is not an easily answered question.

Perhaps just move the equipment inside air conditioned space.

To get the thread rolling I will say IMO the NEC requires we design for the highest ambient temperature the conductors will ever be exposed to.

I don't see that being done nor do I think it makes much senses to do so.

 

[derf48]

Ambient temperature selection is an area of the code where we have a requirement that has absolutely no guidance and there is no guidance available! I have personally called the code panel members and the wire manufacturers for assistance in the selection of ambient temperature. Each and every answer was "I don't know where the derating schedule came from nor how to select an ambient temp for a geographical area".

The cable manufacturers did enlighten me a little on their products. They have a built in overload in the insulation, which means that the wire can be heated to 140 C for short duration up to 100 hours a year and not more than 1500 hours a lifetime based on 30 year life. and not be damaged. I used to teach that for ambient temperature selection you must use the highest recorded temp in your area in the last 10 years, usually available from the airport. But that is now changed in my mind, if it reaches 120 F for one hour every 10 years do I really need to use that as my maximum ambient temperature?

I presently work for a national company and we do work in the Western 2/3 of the United States. We have decided to use the ASHRA ambient temperatures as our starting point, and then depending on if the conductors are subject to extremes such as rooftop or direct sunlight, we will make changes based on those factors. Everytime we set parameters they are based on something, simply document what they are based on and then sleep a little better at night.

Fred

 

[bphgravity]

Just for your information, this proposal was accpeted again as it was for the 2005 NEC but ended up as a FPN to 310.10:

6-51 Log #3150 NEC-P06 Action: Accept in Principle
(310.15(B)(2)(c))
_______________________________________________________________________________________________
Submitter: Travis Lindsey, Travis Lindsey Consulting Services
Panel Meeting Action: Accept in Principle
Add 310.15(B)(2)(c) to read:

(c) Conduits Exposed to Sunlight on Rooftops. Where conductors or cables are installed in conduits exposed to direct
sunlight on or above rooftops, the adjustments shown in Table 310-15(B)(2)(c) shall be added to the outdoor
temperature to determine the applicable ambient temperature for application of the correction factors in Tables 310.16
and 310.18.

Table 310.15(B)(2)(c) Ambient Temperature Adjustment for Conduits Exposed to Sunlight On or Above Rooftops

Distance Above Roof to
Bottom of Conduit Temperature Adder

0 thru 13 mm (? in.) 33?C 60?F
Above 13 mm (? in.), thru 90 mm (3-? in.) 22?C 40?F
Above 90 mm (3-? in.), thru 300 mm (12 in.) 17?C 30?F
Above 300 mm (12 in.), thru 900 mm (36 in.) 14?C 25?F

 

[petersonra]

I'm installing parallel conductors for a service from an UG transformer to my panels mounted outdoors. Since the panels are outdoors, do I need to derate the conductors for ambient temperature? If so, what is the correct temperature to use? How about the effects of the sun beating down on the 3R/12 enclosure?

Thanks,
Lou

The tables in the code are based on every wire at maximum allowed current thus producing maximum heat. Since this almost never happens, not derating for ambient temperature conditions is probably almost always safe. It is clearly not code, but it is probably safe to do so.

Think of the millions of homes with NM cable laid in attics. Are there any documented cases of fires started by such installations?

Look at it this way. A wire that is at only 90% of its maximum current produces only 80% of the heat at its max current. At 75% of max current, it only produces just over half as much heat.

How many wires do you run that carry more than 75% of their max allowed current for any length of time?

The thing about conduits that are open to the air is they have more chance to radiate the heat from inside the conduit. Look at it this way. 120 deg F out on the roof. 90 Deg C conductors. Do you really think that it will be hot enough inside the conduit to boil water and thus exceed the temperature rating of the conductor insulation?

 

[charlie b]

To get the thread rolling I will say IMO the NEC requires we design for the highest ambient temperature the conductors will ever be exposed to.

Just to get it rolling faster, I disagree. But we've had this discussion before, and it won't get resolved this time either.

IMO the temperature that should be used for derating purposes is the year-round average. That opinion is based on the physics of the situation, as opposed to any language that appears in the code. Here is a simple summary of the physics:

Suppose a cable manufacturer is prepared to guarantee a 30 year life, based on a 30C ambient, and based on the ampacity limits of Table 310.16. If you were to expose the cable to an ambient temperature of 40C, FOR EVERY MOMENT of its entire lifetime (not just for an occasional hour or fifty during the summer), then the cable insulation would sustain degradation that would cut its lifetime in half. Every ten degrees more cuts the lifetime again in half.

However, during the spring and fall, the average ambient is well below the manufacturer's assumed 30C, and during the winter it is lower still. So during those months the rate at which insulation systems degrade will slow to a snail's pace. Operating the cable at ambient temperatures below 30C will extend the lifetime well beyond the guaranteed 30 years.

A cable is not going to fail instantly if it goes 10 or 20 or 30 degrees above the assumed 30C ambient. What happens is that during the time it is above 30, it degrades at an accelerated pace. The failure will not take place until after it degrades beyond the level to which the manufacturer tested it.

 

[bphgravity]

So how long are conductors supposed to last? Should they be designed and installed to last the lifetime of the building or structure or should we assume they will eventually have to be replaced at some point in the future?

 

[iwire]

Just to get it rolling faster, I disagree. But we've had this discussion before, and it won't get resolved this time either.

IMO the temperature that should be used for derating purposes is the year-round average. That opinion is based on the physics of the situation, as opposed to any language that appears in the code.

To be fair I did go on to say that I do not see it being done or that I think it makes sense to do so. (Rate the wire on the highest expected heat ever)

Outside of the NEC I agree with your thoughts on this issue.

My only problem is with the NEC part of this.

Between section 310.10, and it's FPN

FPN:The temperature rating of a conductor (see Table 310.13 and Table 310.61) is the maximum temperature, at any location along its length, that the conductor can withstand over a prolonged time period without serious degradation. The allowable ampacity tables, the ampacity tables of Article 310 and the ampacity tables of Annex B, the correction factors at the bottom of these tables, and the notes to the tables provide guidance for coordinating conductor sizes, types, allowable ampacities, ampacities, ambient temperatures, and number of associated conductors.

The principal determinants of operating temperature are as follows:
(1) Ambient temperature ? ambient temperature may vary along the conductor length as well as from time to time.
(2) Heat generated internally in the conductor as the result of load current flow, including fundamental and harmonic currents.
(3) The rate at which generated heat dissipates into the ambient medium. Thermal insulation that covers or surrounds conductors affects the rate of heat dissipation.
(4) Adjacent load-carrying conductors ? adjacent conductors have the dual effect of raising the ambient temperature and impeding heat dissipation.

and the ampacity tables "For ambient temperatures other than 30?C (86?F), multiply the allowable ampacities shown above by the appropriate factor shown below."

In my mind indicates the max., not avg temp. is what the NEC requires.

Of course you can always say 'engineering supervision' I can not.

 

[petersonra]

FPN:The temperature rating of a conductor (see Table 310.13 and Table 310.61) is the maximum temperature, at any location along its length, that the conductor can withstand over a prolonged time period without serious degradation.

I realize a FPN is not an enforceable part of the code but it does imply the intent. I don't know what they consider a prolonged period of time though.

 

[hardworkingstiff]

As usual, y'all are great.

Thanks.

 

[winnie]

Charlie B,

While I accept in principal the idea of using the _average_ ambient temperature to determine derating, I don't believe that it is appropriate to use the ordinary 'mean' type of averaging.

As you noted, conductors degrade much faster as they heat up; wire that is 10C hotter is 'aging' at double the rate. It seems to me that the 'average' that one should use is a weighted average that gives the same total 'aging' over a given time period. A conductor that spends 50% of the time at 120C and 50% of the time at 60C will age roughly 7x the rate of a conductor that is at a constant 90C.

To draw the analogy to 'root mean square' averaging, I would use the 'log mean exponential' average of ambient temperature to get the same average aging effects.

Here is a discussion of magnet wire temperature rating, probably somewhat different than building wire rating.

http://www.essexgroup.com/News_Media/White_Papers/thermalclass.pdf

-Jon

 

[Pierre C Belarge]

As Bob mentioned, this is a very interesting topic.

Because so few in our industry even understand ambient temperature effects, and the heat applied to the insulation of conductors, it has literally been ignored in the field except for a few... and I mean a few.


I believe that the ambient temperature has more effect than most will agree to, that is my OPINION. With that said, I think with some of the different studies presently being performed and the knowledge we already understand that some kind of positive language and guidance will be put in manufacturers info and the NEC. Right now the manufacturers are content with letting this sit until further notice.

I have seen the effects of heat on conductors, as most here probably have as well. I consider it luck and good design of insulation and installation practices that have kept this issue from becoming an issue.


The part that concerns me is not the lighter loaded circuits, but the ones that are heavily loaded and the ones that get added to as years go by. I guess we will have to wait and see how this all pans out. It will take courage on the part of the CMPs to institute some kind of requirement...