another question for an electrician

[Steven Triggs]

I've been looking through the derating information in the NEC, and I see that you are supposed to compensate for ambient temperature by multiplying the value from 310.16 by a correction factor. I understand the reasoning behind that.

Here's my question. How do you determine what ambient temperature to use? In reality, it would be different in exterior walls, interior walls, and attics. Do people take all of this into account? Is there a publication somewhere that says something like, "In region A, ambient temperatures for attics are X degrees, exterior walls are Y degrees, and interior walls are Z degrees?" I did a bunch of searching online for such a thing, but couldn't find it.

Even if there isn't a publication, is there an electrician here who works in or near Metro Atlanta and can tell me which temperatures I should use for those three locations?

Thanks.

[Kyle Kraft]

Not being sarcastic, but why is this an issue? Wire is rated for certain temperatures, for example up to 90* C. Other wire types based on the insulation are rated for higher or lower service temperatures.

For your shop use 14ga. for 15 amps, 12ga. for 20 amps, 10ga. for 30 amps, etc.

Can you imagine the dilemmas this would cause if the electrician had to take into consideration the temperature of each environment in an average house, and specify a different conductor for each??

Unless you're wiring up a blast furnace I wouldn't worry too much about it.

[Brian Pliska]

He's correct. Ambient temperature, as mentioned in the NEC, specifically refers to the insulation on various types of wire and the highest temperature it is rated to still safely carry current. Like the man said, you needn't worry about it unless your running circuits in a foundry or maybe even hell. Up here at ground level, in a residence or commercial building, THHN wire will handle all ambient temps on the Earth's surface as 90 degrees C is equivalant to about 194 degrees F. Relax!

[Jim Becker]

Steven may be stuffing conduit and is just trying to make sure that he does all the calculations correctly...while it may or may not matter, the question is interesting!

[Von Bickley]

Here's my question. How do you determine what ambient temperature to use? In reality, it would be different in exterior walls, interior walls, and attics. Do people take all of this into account?


Do people take all of this into account? NO

I did electrical work for many years and never took temperature readings when pulling wire. :) :) :)

[Steven Triggs]

I am stunned by the responses. Temperature rating of wire has nothing to do with ambient temperature. The temperature rating of the wire is a rating of what safe temperature the wire can operate at without damage to the insulation. Clearly, 194 degrees farenheit, the rating applied to NMB while calculating derating, is well above ambient temperature. This however has nothing to do with what I'm talking about. The temperature rating is a property of the wire. The derating compensatiion factor takes into account the ambient temperature, because at a given load, the electrical current will add a certain "amount" of temperature to the ambient temperature, resulting in the temperature that the wire reaches. In other words, if a certain load, say 20 amps, will generate a certain amount of heat, that will result in a net temperature of the wire higher if the ambient temperature is higher.

If you review the NEC's "correction factors" in table 310.16, you will find, for example, that an ambient temperature of 141-158 degrees will cause a 12/2 romex cable to only be sufficient for 17.4 amps. It wouldn't be unreasonable to think that temperatures in the attic might reach 141 degrees in the summer time.

I'm surprised that my intererst in obtaining valid information so as to follow the guidelines set out in the NEC have been met with such a negative response. I'm simply trying to make certain I do everything safely and properly. Just because something is commonly done a certain way, or in this case perhaps ignored, doesn't make it right.

[Andrew Williams]

I'm with you Steven. There is no reason not to find more knowledge, if it is out there. Why not just do a test with a thermometer on your house in those locations?

[John Terefenko]

Just because you followed what the code said you still maybe wrong. Your city or county or whatever may have their own rules that you have to follow also. I think if you gave an idea as to what you are trying to do instead of asking a general question it maybe easier to answer. This is not a knock on your question but many times electrical questions are asked but there are many answers because of the application.

[Kyle Kraft]

Yes, Steve you did pose a very interesting question. My response was based on the assumption that you wanted to get some machinery up and running to make sawdust, not consuming calculator batteries:D . Sorry if I offended you or any others:( .

[ROY DICK]

Steven,

The correction factors in this table do not apply, because 12-2 romex does not have "more that 3 current-carrying conductors."


Roy

[Steven Triggs]

Why not just do a test with a thermometer on your house in those locations?

I thought about this, but two problems come to mind:
1. Does ambient temperature, as used by the NEC, mean average across the year, or on the hottest day of the year?
2. In either case, I'd have to collect temperature readings in each of the locations for a year, to find average, or throughout the summer, to find peak.

I was just hoping this data might already exist. When you build a structure, you account for wind speed and snow load (for roof design), mean annual temperatures (for insulation values), and so on. This information is easily available. For example, when I look on my county web site, it lists several facts relevant to construction in my area:


Ground Snow Load Pg = 5 psf.
Wind Velocity V = 90 mph
Seismic Performance Category for Group I and II is “B”. For Group III it is “C”.
Energy Code = Climate Zone 7A-3499 HDD.
Winter Design Temperature = 22 degrees.
Weathering = Moderate.
Air Freezing Index = 58
Mean Annual Temperature = 61.7 degrees.
Frost line depth = 12 inches (minimum per code)
Termite=Very heavy.
Decay = Moderate to Severe

[Steven Triggs]

The correction factors in this table do not apply, because 12-2 romex does not have "more that 3 current-carrying conductors."

I believe you are mistaken.


Table 310.16
The first section of the table is headed as follows:
Allowable Ampacities of Insulated Conductors Rated 0 Through 2000 Volts, 60°C Through 90°C (140°F Through 194°F), Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30°C (86°F)
The second section of the table is described as:

For ambient temperatures other than 30°C (86°F), multiply the allowable ampacities shown above by the appropriate factor shown below.


So, I believe you are supposed to look up the conductor in the first section, find the ampacity, and then multiply it by the factor in the second section that corresponds with the ambient temperature where the wire will be installed.

[ROY DICK]

Article 334 non-metalic sheathed cable:types nm, nmc, and nms. (Romex)

Section 112 "Conductor insulation shall be rated at 90°C(194°F)", in which case that is not even on the correction factors chart.

2005 edition of the National Electrical Code

So this does not apply or required according to the code.(Not including any local jurisdiction).

Roy

[Rick Christopherson]

If you are doing this as just an exercise of your mind, then this is fine, but if you are going through this for the purpose of planning out your home/shop electrical system, then this is not fine. This information is put into the NEC so that a certified engineer can push the limitations and built-in margins found in the standard look-up tables.

The purpose of this section is to account for heat dissipation in situations where the standard look-up table is not desired to be used. You will note that the maximum ampacities listed in the previous table are higher than the standard circuit protection ampacities listed elsewhere in the NEC. Furthermore, these ampacities do not take into account voltage drop, which is why the values are higher.

Even if you went through the trouble of calculating this temperature derating, the circuit ampacity is still limited elsewhere to the common 15, 20, and 30 amp circuit protections, unless of course you were going to manually go through all of that information too.

Is it your intention to install 20 to 25 amp breakers on a 14 gauge circuit (or 25 to 30 amp on 12 gauge circuits)? If not, then you are misreading the NEC. On the other hand, if you were going to be installing this wiring into a hostile environment where the ambient temperature is close to the temperature rating of the insultation, then you may find a situation where this ampacity limitation exceeds the normal ampacity limits, but this would require very high ambient temperatures.

As to your original question, you would use the maximum ambient temperature, not an average.

[Rick Christopherson]

Article 334 non-metalic sheathed cable:types nm, nmc, and nms. (Romex)

Section 112 "Conductor insulation shall be rated at 90°C(194°F)", in which case that is not even on the correction factors chart.

2005 edition of the National Electrical Code

So this does not apply or required according to the code.(Not including any local jurisdiction).

RoySorry Roy, but this is a misinterpretation of the specification. This value is the maximum permissible temperature of the insulation, and is not directly related to the ambient temperature of where the conductor is located. The higher the ambient temperature, the less efficiently the conductor can dissipate the heat generated, which will eventually result in the conductor/insulation exceeding it temperature rating. Any time current is flowing through a conductor, it will generate heat. If that heat cannot be properly dissipated, then the temperature will continue to rise.

Consider a non-realistic example: If you encased a cable in an extremely efficient insulation that prevented any heat dissipation, then even 1 amp of current flowing through the wire would eventually cause the temperature of the cable to exceed the rating of the cable. Likewise, the higher the ambient temperature, the easier it would be for normal current flowing through the conductor to reach its maximum rated temperature.

[Chuck Lenz]

Your looking for highly technical advise on wireing with little information on what your actually doing. Then you ask it in a wood working forum and redicule anyone that has a response. I find it rather irritating and a tad bit disrespectful.

[Steven Triggs]

Your looking for highly technical advise on wireing with little information on what your actually doing. Then you ask it in a wood working forum and redicule anyone that has a response. I find it rather irritating and a tad bit disrespectful.

Wow, I think you are blowing this a bit out of proportion. I didn't ridicule anyone. I said I was dissapointed that I asked a question in an effort to "follow the rules" and basically was told I was being ridiculous. I found it rather "disrespectful" that I asked a legitimate question in an effort to learn something, and several people took it as an oppurtunity to tell me what a waste of time it is.

As to "asking a highly technical question", all I actually asked was how "ambient temperature" is typically determined and applied.

I asked it in a wood working forum because many of the people who post in here are electricians. You might notice the title of the orginal post.

I truly am sorry if you, or anyone else, thinks I'm being rude here. It isn't my intention by any means. However, I just don't understand why I've basically been yelled at and called an idiot for trying to learn. I thought that was why we are all here, to learn and share.

Seriously, could one of the long-term and respected members back me up on this? Doesn't it seem like I was kind of attacked here?

Thanks, and I hope to still enjoy this community. I hope I haven't somehow made myself into a blacksheep here...

[Steven Triggs]

Is it your intention to install 20 to 25 amp breakers on a 14 gauge circuit (or 25 to 30 amp on 12 gauge circuits)? If not, then you are misreading the NEC.

Rick,
No, I'm not planning to go over the normal 15, 20, and 30 amp ratings for 14, 12, and 10 gauge wires.

Where I got concerned was that according to that second part of the table, ambient temperatures of 141-158 F require a deration factor of 0.58. If this factor is applied to the 30 amp value pulled from the 90 C column for 12 gauge wire, it would net an ampacity of 17.4. If I understand it correctly, you are supposed to use the adjusted amount if it is lower than 20 for 12 gauge. I may be wrong, but I was thinking attics around here (near Atlanta, GA) reached into the 140s during the peak of summer. So, my concern is that a 12 gauge wire may not be sufficient for a 20 amp circuit if part of it's run is through the attic space.

Thanks!

[Jason Roehl]

Steven, Rick is an electrician (or former?) who has provided much valuable info here and elsewhere on the internet. I think you need to re-read his post. He said that portion of the NEC you are trying to use does not apply because you are using 10-, 12- and 14-gauge Romex on their "normal" 30A, 20A and 15A breakers, respectively. Where the derating would start to happen is if you were trying to push the limits of the wire by maybe using 40A, 30A or 25A and 25A or 20A breakers respectively. Basically, I think he's saying it applies to engineers/electricians in industrial situations, not residential.

[Rod Sheridan]

Hi, conductors certainly do have to be de-rated for ambient temperatures higher than 30C.

I don't have a copy of the NEC, in Canada we use the Canadian Equivalent which is almost identical.

A conductor has a given insulation temperature rating, based upon the insulation material used. The insulation, will not be a good conductor of heat, so the conductor will be hotter than the exterior insulation temperature.

The exterior insulation will have to conduct the thermal energy to the surrounding atmosphere, to cool the conductor. Obviously, as the ambient temperature rises, the insulation will not be able to maintain the conductor temperature at the safe value for the insulation, unless the conductor produces less heat.

To produce less heat, you have to reduce the current.

A 90C insulated wire will be de-rated as per the table below

Ambient Temp Conductor Ampacity

30C 1.00

40C .90

45C .85

50C .80


You can look up other derating factors for different temperatures in the NEC, the above from the CEC Part 1.

Note that your normal home panel breaker is only rated at 80% continuous load, so that de-rating will take care of the 50C ambient temperature de-rating for your conductors.

When specifying conductor size, load current, ambient temperature, insulation rating, cable spacing and voltage drop are all calculations that your electrical contractor should be performing to maintain compliance with the NEC or CEC.

Regards, Rod.

[Jim Becker]

Folks, Steven asked a legitimate question..."How do you determine what ambient temperature to use?" relative to the section of the NEC he was reading. Let's try to be helpful rather than taking him to task for asking the question in the first place.

Jim
SMC Moderator

[Joe Chritz]

Surprisingly enough this came up with my electrical inspector when I was building my house. I was concerned about a section where I had a large number (roughly half) of the circuits all in a small section of attic space.

Being a weekend warrior electrician at best I called him and discussed it. His answer was that unless the wires are bundled or encased in conduit I didn't have to derate any of the circuits.

Ultimately the interpretation is up to the final inspector, unless you want a long battle.

I am however inclined to believe that the "normal" household wiring gear is set to account for any reasonable contingencies in heat, even in hot climes.

Joe

[Steven Triggs]

Joe,
That is good to know. If you specifically discussed this and the inspector said "ok", sounds like it probably isn't a big deal.

Thanks!

[Dennis Peacock]

Now can we all go pull some wire, hook up some tools, and go have fun???
This is not the place to debate the NEC.

[Rick Christopherson]

Steven, Rick is an electrician (or former?) (Actually, I'm an Electrical Engineer) who has provided much valuable info here and elsewhere on the internet. I think you need to re-read his post. He said that portion of the NEC you are trying to use does not apply because you are using 10-, 12- and 14-gauge Romex on their "normal" 30A, 20A and 15A breakers, respectively. Where the derating would start to happen is if you were trying to push the limits of the wire by maybe using 40A, 30A or 25A and 25A or 20A breakers respectively. Basically, I think he's saying it applies to engineers/electricians in industrial situations, not residential.Actually, if he truly is reaching temperatures above 140 degrees, then he is starting to get into the area of the normal limitations. However, it is not likely that attic temperatures within the wiring area truly jump into this range of temperatures.

Circuits that are simply passing through the attic are generally run on the tops of the framing members, which are then covered with insulation. Even an inch of insulation is enough to keep the wire ambient temperature below 140 degrees. The attic space is going to get hot, but unless the dwelling is poorly engineered from a thermal efficiency basis, it should not get into this temperature range.

In normal residential dwellings, it is rare that the environmental condition reach such high levels, and that is why most electricians have never considered this form of derating. It isn't that the derating is not needed, but for most applications, it is not the limiting factor. When taken to extreems, then it may become the limiting factor.

[Steven Triggs]

Rick,
Thanks for the post. That makes me feel much better. I was just thinking of the temperature in the attic space above the insulation, but you're right, the wires on the joists will be under a lot of insulation and so should be much cooler.

[Eugene A. Manzo III]

Steve it sounds like you didn't need much help at all sounds like your very well read. Ambient temperature I always believed is the temperature at the time in the immediate suroundings (Maybe I'm wrong)? so it may vary all the time with temperature changes in the area.

[Jason Roehl]

Well, now I've GOT to know what attic temps can be... :D

I've been in attics on a sunny day, near noon, when the outside temp was 80ish. I couldn't tell you what the temp was, but it was take-your-breath-away-when-you-first-enter hot. Kind of like a closed car on a hot day. Now, we don't get as hot here in IN as the South, but I'm sure we'll hit 95 or better at some point this summer, so I'll have to report back.

Ok, Rick, so you're a EE--that tells me you probably at least know the THEORY behind much of the NEC...oh, and you should be able to do any calculations it might require in a given situation...;)

[Steven Triggs]

I've been in attics on a sunny day, near noon, when the outside temp was 80ish. I couldn't tell you what the temp was, but it was take-your-breath-away-when-you-first-enter hot.

Jason,
I like the way you put that "take-your-breath-away-when-you-first-enter hot".

One summer during high school, I worked for a heating and air company. Man, it sucks climbing around in an attic on a summer day. We had a strictly enforced rule that said we had to take a 5 minute break (out of the attic) for every 10 minutes we spent in the attic. I think the temperatures really are sufficient to cause a major concern for heat stroke and other fun stuff...

[Steven Triggs]

Holy crap! I just found a couple articles on the subject of attic temperatures:

http://www.factsfacts.com/MyHomeRepair/ventilation.htm
This one claims that attic temps can easily reach 150-160 when the outside temperature is 95 to 97. (note that in Georgia, we usually have a few days of above 100 each summer)


http://www.stretcher.com/stories/06/06may15e.cfm
They claim that it is considered "ok" for attics to reach 180 just below the roof sheathing. And they say that a lot of material testing is done in Phoneix and Las Vegas, where it is common for attics to hit 150.