ambient temp correction factors 310.16

[Guest]

310.16 tables allow a corection factor of 1.05 on use and 1.04 on thhn for ambients of 21-25 cel. I am located in seattle where the average temp is 24 degrees cel as shown in several references. (54 deg far). has anyone had any experience using this in thier ampacity calcs?

 

[rick5280]

Re: ambient temp correction factors 310.16

Joe, this is somewhat confusing. For the most part, the ambient correction factors do not apply until you reach a high temp, above 86?F. Ambient is not the same as average, and applies mainly to indoor or room temps, and underground.

If you were to use the correction factors you mentioned, the size of conductor might be a little smaller, but what will happen when the temp. goes up above the 25?C you mentioned?

Rick Miell

 

[nosparks]

Re: ambient temp correction factors 310.16


Rick just a point to think about here. I do not interpret 310.16 corection factors being exclusive to temperatures above 30 deg. C or 86 deg F. I look at it like correction factors are for temps other than 30 deg. C or 86 deg. F.

Joe, Rick has a point here also. You say that the avg. temp is only 54 deg. F. I have never been to Seattle, but I have to believe that it gets warmer in that area. But you would know better than me. I would use the correction factors in areas such as walk in coolers or in much colder year round climates. Only 54 deg... really?

 

[Guest]

Re: ambient temp correction factors 310.16

I got that info about the "avereage " from the weather service. The 1.04 or 1.05 multiplier allows the ampacity of the conductors to be increased slightly. E$speciaslly usfull in sizing underground conductors where the temp is lower as well i think.

 

[bphgravity]

Re: ambient temp correction factors 310.16

I would not consider the air temperature that is present at any given moment in the area of the country that you live. If the wiring you are installing is going to be outdoors or in non-conditioned areas, the maximum possible ambient temperature must be considered. Every state in the nation has reached 100 degrees F at some point in history. I believe the correction factors at below 30 degrees C and 86 degrees F are reserved for areas that are conditioned or cooled consistently. This also means that the entire run of the conductors have to be in this area from source to final termination. This is unlikely. For good design and ensurity of proper full current carrying capability, I would not increase the allowed amapacity of my wires in any case. I would only apply correction factors to above 30 degrees C or 86 degrees F.

 

[charlie b]

Re: ambient temp correction factors 310.16

The code has never been clear on the following point: ?Does the ?ambient temperature? related to correction factors mean ?peak? or ?year round average? or something else?? I am inclined to believe that it means ?year round average.? That is based on my understanding of (1) The way temperature influences the rate degradation of insulation systems, and (2) The purpose of ampacity limits (that being to establish a limit on insulation degradation). If you add ten degrees to the ambient temperature surrounding a cable, and keep it there for the entire life of the cable, you will cut the cable?s lifetime in half. But if you only keep the ?plus ten degrees? for a few days or weeks, and keep the temperature ten or more degrees below the specified 30C the rest of the year, the net effect is to extend the cable life by twice or even more.

I too live in the Seattle area, and am prepared to accept the assertion that our year round average temperature is 54F. Yes the summer does bring hotter days. But the two days a year that the temperature exceeds 85F is more than offset by the two days a year that there is snow on the ground.

All that notwithstanding, I would not take advantage of a lower average ambient temperature to use a smaller cable for the same job.

 

[dereckbc]

Re: ambient temp correction factors 310.16

Is the 4 or 5 percent enough to make a difference?

 

[bphgravity]

Re: ambient temp correction factors 310.16

I can imagine that if your total calculated load was right at a point where you could almost drop one conductor size, the 4 or 5 percent could mean alot. I would rather be at the low end of the conductors capability and not right at the top of it's capability.

 

[bennie]

Re: ambient temp correction factors 310.16

Ambient means surrounding. I would use the maximum temperature that could surround the conductor.

 

[charlie b]

Re: ambient temp correction factors 310.16

Originally posted by bennie: Ambient means surrounding. I would use the maximum temperature that could surround the conductor.

Yes, my dictionary confirms that ?ambient? means surrounding. But the definition has no words that carry the connotation of ?maximum.? It is certainly a conservative move to use the maximum, but I believe it to be unnecessary, and inappropriate. As a specific example, if I design a cable run such that it would feel the full range of Seattle?s outdoor temperatures, figuring two to four days per year above 30C and 360+ days well below 30C, I would not derate the cable on the basis of those two to four days. And if this choice were to be challenged by an inspection authority, I could produce a physical and mathematical argument to justify that position.

But my main point is that I believe the code to be unclear on this topic. Does anyone know of a code reference that resolves this question?

 

[iwire]

Re: ambient temp correction factors 310.16

IMO the code is clear on this, 310.15(A)(1) Ampacitys of conductors to be determined by tables or engineering supervision.

If we chose to use Table 310.16 the "correction factors" are clearly part of that table or it would have a separate article number.

It says "For Ambient temperatures other than 30C (86F) multiply the ampacitys shown by the appropriate factor shown below"

No where does it say average ambient, or ignore the correction factors if it only gets hot once in a while.

So "if this choice were to be challenged by an inspection authority", I do not think you would have any defense.

I could produce a physical and mathematical argument to justify that position.

Now if you do that you have left the table and are going "engineering supervision" something the average electrician can not do.

Now that I got that out I do not and I do not know any electricians that derate for using wires in outside air temps in this area even though we can reach 95F a many days, the only time I have used it is in boiler rooms or compressor rooms that I know will run hot.

My point is I think the code is clear on this, but enforcement does not happen.

 

[bennie]

Re: ambient temp correction factors 310.16

This brings to mind the old trick question...

What is the ampacity of 500 kcmil cu TW in an ambient temperature of 140?F. ?

The insulation temperature should not be exceeded.

 

[charlie b]

Re: ambient temp correction factors 310.16

Originally posted by iwire: It says "For Ambient temperatures other than 30C (86F) multiply the ampacitys shown by the appropriate factor shown below" No where does it say average ambient, or ignore the correction factors if it only gets hot once in a while.

But nowhere does it say ?maximum? either. And nowhere does it say that one second (or one day, or one week, or one month) at 31C during the lifetime of a cable is sufficient to warrant use of the derating factors.

The ambient temperature of any location varies second-by-second, hour-by-hour, day-by-day. At which exact second of which exact hour of which exact day will you measure the temperature, so that you will have a number upon which you can base your decision of whether or not to derate? If you choose to interpret the code as intending maximum temperature, no statement in the code would prove you wrong. If I choose to interpret the code as intending average temperature, no statement in the code would prove me wrong. That, by definition of the word ?unclear,? makes the code unclear.

 

[bphgravity]

Re: ambient temp correction factors 310.16

In my opinion, using a yearly average temperature is like using an average total load. When sizing the service, you take maximum possible load to be served, not average of the loads served. Therefore, I also feel when making corrections for ampacity, maximum potential needs to be considered, not averages.

 

[iwire]

Re: ambient temp correction factors 310.16

Originally posted by charlie b:
The ambient temperature of any location varies second-by-second, hour-by-hour, day-by-day. At which exact second of which exact hour of which exact day will you measure the temperature, so that you will have a number upon which you can base your decision of whether or not to derate? If you choose to interpret the code as intending maximum temperature, no statement in the code would prove you wrong. If I choose to interpret the code as intending average temperature, no statement in the code would prove me wrong. That, by definition of the word ?unclear,? makes the code unclear.

IMO If they meant average they would have said average, they think about the wording a great deal.

They do not say minimum ambient either, but by your logic I could use the lowest temperature the conductors will be exposed to and figure from there?

 

[don_resqcapt19]

Re: ambient temp correction factors 310.16

bp,
I'm sure that in your area it would be common to have the attic temperature reach 141 or 150+. Do you use a 0.58 correction factor for all conductors installed in attics? What about rooftop conduits exposed to the hot Florida sun? I would expect that the interior temperature of these conduits would be even higher than within an attic space.
Don

 

[bennie]

Re: ambient temp correction factors 310.16

A circuit draws 24 amps for one hour per day. Can I average this to one amp and size wire accordingly? I hope not!

 

[bphgravity]

Re: ambient temp correction factors 310.16

Don, you have hit on severe problem in my area. South West Florida does indeed have temperatures that rise into the high 90's for several months. However, the travel agencies and tourists centers will tell you the average temperature is 75 degrees. Do all contractors or myself always derate for these conditions. No. Does this present a problem later down the line. Yes. Florida building code considers maximum atttic temperatures at 120 degrees F. Many communities required minimum 20-amp rated wire for lightning and general branch-circuits, but the recent adoption of the Unified Code has removed this restriction. Many AHJ do not enforce these corrections so many don't bother. This doesn't make it right, but that is the reality of things.

 

[charlie b]

Re: ambient temp correction factors 310.16

Originally posted by iwire: They do not say minimum ambient either, but by your logic I could use the lowest temperature the conductors will be exposed to and figure from there?

That is not my logic. In fact, I am not relying on logic, but rather on physics. Ampacity limits exist for one purpose only: to protect the cable?s insulation system. Too much current means too much heat generated within the wire, with the result that the insulation gets subjected to too high a temperature.

Consider the following as a laboratory experiment (i.e., no load calcs, no 80% rule, no inspections, no NEC; just a lab coat, a test oven, and a thermometer). If you were to run 110 amps continuously (i.e., for decades on end) through a #1 type TW wire in a room that is maintained (throughout all those decades) at a constant 30C, the I*2R heat generation would raise the temperature of the wire no more than 30 degrees above the ambient 30, with the result that the insulation system would be subjected to no more than the 60C for which it is rated. You could run the same experiment on a #1 type THW, but this time run 130 amps through it (again, for decades on end). The I*2R heat generation would raise the temperature of the wire no more than 45 degrees above the ambient 30, with the result that the insulation system would be subjected to no more than the 75C for which it is rated. Run 150 amps through a #1 type THHN, and its insulation system would be subjected to no more than the 90C for which it is rated. That is the meaning of the ampacity limits given in Table 310.16.

As temperature is raised, the rate at which the insulation system degrades increases ?exponentially,? a term that I will loosely translate as ?faster than linear.? Consider, for example, a type TW cable. Suppose for the moment that a manufacturer guarantees that its insulation system will be good for a 40 year life. That means that you could keep it at a constant 60C (through any combination of ambient temperature and I*2R heat generation), and it would maintain an adequate insulation resistance for the entire 40 years. Now raise the temperature to 70 degrees. Will it fail instantly? No. But it would only last about 20 years, before its insulation system had degraded to the point that it was no longer adequate. Now raise the temperature to 80 degrees. Will it fail instantly? No. But it would only last 10 years at that temperature. Raise the temperature to 90 degrees, and it would last only 5 years. Raise the temperature to 100 degrees, and it would last only 2.5 years. Keep going at higher and higher temperatures, and eventually the insulation?s useful life will be limited to a few seconds, as it cooks, cracks, or even catches fire.

Now here?s the point: If a cable is at 31C (88F) for one summer month, it will degrade faster than it would have at 30C; it will have lost some of its 40 year life. But if you then keep it below 20C (68F) for most of the Fall and Spring and all of the Winter, during these times it will degrade slower than it would have at 30C. The result is that it could have its useful life extended beyond the original 40 years.

So yes, the use of ?average ambient? makes sense, from a physics perspective. Now I would like the NEC to say that, so that we don?t have to guess or argue, and so that there will be no question of non-compliance or enforcement.

 

[iwire]

Re: ambient temp correction factors 310.16

So to keep it short, If a inspector did call me on this I do not think I would impress him with telling him they mean average temperature, I would have to fix it.

Or I call you, and ask you to show the inspector the math and sign it with EE after your name and the inspector says it's OK because now it is "Under Engineering Supervision"

I still think the code is clear but not enforced.

310.15(A)(2) "Where more than one calculated or tabulated ampacity could apply for a given circuit length , the lowest value shall be used.

There is an exception about less than 10' or 10% can be figured at the higher ampacity.

Table 310.16 "Allowable Ampacities of Insulated Conductors"

The word "Maximum" is not here, but we all know that is what they mean.