310.16

[iwire]

Morning George, let start by saying I wish the NEC was more clear about this.......in other words I am not that confident I have the real answer here. :lol:

Where in the NEC tells us we can use the average temperature? :?:

It doesn't. Since you brought it up, I looked closer:

310.10, FPN No.1, (1) Ambient temperature ? ambient temperature may vary along the conductor length as well as from time to time.

Unfortunately, it doesn't provide much assistance in decision making. :?

Nope :?

IMO we have to use the maximum expected temperature.

If so, we're still okay. A 14-2 NM cable in a 140?F attic has an ampacity of 17.25 amps. The same cable in a 158? attic has an ampacity of 14.5 amps. Provided the load does not exceed 14.5 amps for less than three hours, or 11.6 amps for over three hours, then we're still okay.

No we are not OK in the 158 F attic, you can not roll up to the next higher breaker on that 14.5 amp cable if it supplies a multi-outlet circuit suppling receptacles as most dwelling unit circuits are.

When derating conductors for fill do you use the highest ampacity a conductor may carry or do you use the average ampacity the conductor may carry?

Neither. The expected load does not factor into derating, until you have finished the derating process. Once we're done derating, then we can turn our attention back to the load, and overcurrent protection.

Correct just as it is when figuring for ambient temperature.

To me it is the same issue we are dealing with heat damaging the insulation.

But the damage isn't a foregone conclusion. I think what's messing with us is that we're trying to determine mathematically if a conductor is going to be damaged under certain conditions. If we assume that the damage is guaranteed, then we would be inclined to follow the "max temperature" approach.

I have a hard time with that statement.

If I accept what you say than I should also be able to apply this averaging to the actual current I run through the conductor.

Why not run 30 amps through a 14 AWG for 5 minutes an hour the other 55 minutes the load is 2 amps.

I know for a fact the insulation will survive that but it is against the code to do so.

Who are we to second guess the NEC on the Ambient temps?

Bob

 

[pierre]

Bob posted
"No we are not OK in the 158 F attic, you can not roll up to the next higher breaker on that 14.5 amp cable if it supplies a multi-outlet circuit suppling receptacles as most dwelling unit circuits are."

Most people take for granted that 240.4(B) can be applied here, but as Bob has pointed out, it will not be applied to branch circuits supplying multioutlets.

 

[George Stolz]

I agree with Bob about your calculations, I kind of got a chuckle of how you handled that.

I figured it would get a laugh. I was attempting to get the thought across, mostly, more than some hard numbers.

Specifically how, I am not sure, but it cannot be good over time. Have this happen over a couple of summers and I am sure it is not good.

Exactly. It seems as though we're given cause for concern, but no recipe for determining when our concern is warranted. It really seems as though the NEC has given us half the puzzle here.

No we are not OK in the 158 F attic, you can not roll up to the next higher breaker on that 14.5 amp cable if it supplies a multi-outlet circuit suppling receptacles as most dwelling unit circuits are.

The circumstance I am thinking of is a furnace, or some other singular utilization equipment, in the attic itself. Other cables would likely be buried under the insulation, as opposed to exposed to the full ambient of the attic.

Who are we to second guess the NEC on the Ambient temps?

When did they offer us a first guess?

 

[iwire]

George I am not sure what you mean by 'half the puzzle'.

All of our conductor selections are based on heat, the heat the insulation and equipment can tolerate.

I believe I read a copper 14 AWG will carry about 200 amps before it becomes a fuse.

Any table that rates a 14 AWG less than the 200 amps is based on the heat someone has determined the insulation can tolerate.

So whether we are derating the conductor because of raceway fill or we are derating the conductor for ambient temperature the goal is the same.....keep the conductor temperature within its limits. I can not see how heat damage from ambient is any less of an issue than heat damage caused by 'overcrowding'.

Bob

 

[George Stolz]

We are not told to derate conductors based on how many will be in the conduit this week, versus next week. There's no sense to that. The amount of conductors will remain in the conduit until it is changed. It is a constant that can have rules readily applied to it.

Ambient temperature is a variable. Therefore, unless they give us a method for determining the value to be used, we're up a creek without a paddle.

At 6:00am the temperature is different than 6:00pm on the same day.

There are three methods to determine an average: mean, median, and mode. They can produce very different answers, and they are all relevant methods depending on which method best suits the conditions.

We have half the puzzle: we have rules to apply once we've decided on the ambient temperature. We have no basis for how to find that number.

The reason we can't load a 14 to 200 amps is because they've given us specific rules prohibiting it. We hardly have a vague rule for this problem.

 

[iwire]

We are not told to derate conductors based on how many will be in the conduit this week, versus next week. There's no sense to that. The amount of conductors will remain in the conduit until it is changed. It is a constant that can have rules readily applied to it.

Your right, the number of conductors is a constant however the loading of the conductors is a variable that may change moment to moment, therefore the heat generated by the current flow changes moment to moment. We are required to assume all conductors will be loaded to the max.

Example: Say I run a bunch of lighting circuits in the same raceway, the connected load on each circuit is 10 amps. The conductor we must use will not be based on the 10 amps, it will be based on the OCPD that we use regardless of the fact the circuit conductors will never be loaded past 10 amps each.

We are not allowed to average this heat produced by the current why would you think we could average the heat from the ambient?

Why do think different rules should apply to the same problem?

Why do you think the heat generated by the conductor is more of a concern than the ambient temperature?

Not being a wise guy just don't understand why you look at the heat differently. :?

Heat is heat regardless of the source.

IMO you think think this way as you have never been shown any other way.

Its the "We never do that, it works fine and always passes inspection" argument. :lol:

BTW, I hardly ever see anyone apply the temperature adjustment.

 

[George Stolz]

Not being a wise guy just don't understand why you look at the heat differently. :?

I'll try to line up my argument better. Stand by.

IMO you think think this way as you have never been shown any other way.

I used to think the same as you are now, but a discussion here changed that. Now I get to review the issue again!

BTW, I hardly ever see anyone apply the temperature adjustment.

No kidding.

All the same, I'd like to attempt to gain understanding of such things.

 

[iwire]

BTW, I hardly ever see anyone apply the temperature adjustment.

No kidding.

:lol:

No kidding.

I just wanted to keep things fun, I forget about the temperature adjustments as well. :lol:

Let me ask this, would you feel comfortable using the 1.08 multiplier temperature adjustment for conductors in an ambient temperature of 77 F or less.

77 F or less is very likely to be more than the average temperature in a modern building.

How do think 310.10 affects this discussion?

 

[pierre]

FPN # 2 that follows 310.10 is our precurser to the changes that will come for ampacity corrections for ambient temperatures. The studies are being performed as we speak. I am not sure if they will be ready for '08, if not most likely the '11 NEC.
George, then the "other half" of the puzzle will be completed for you. Although it may not make you happy.

 

[macmikeman]

Did my question get answered?. No kidding, I am still just as confused as when I asked it. Wasn't attics and so forth the reason they went from old nm to nm-b a number of years ago? At least that is what the local inspectors were telling us at the time of the change. By the way, I get the need for de-rating when used in higher ambient, but I am still under the impression that the insulation must also be rated for the enviroment its used in, hence the 90 deg rating. By the way George, my experience in my backyard and before in Fort Misery Florida is that attics are pretty hot all darn year long. Up to a few years ago they were at least tolerable here because insulation wasn't a requirement until recently.

 

[George Stolz]

Here goes nothing...

Why do you think the heat generated by the conductor is more of a concern than the ambient temperature?

Because of the hard and fast rules we have for the one type of heat (240.4, 310.15(B)(2)(a)) compared to the lazy language of 310.10, with it's half-asleep FPN's trying to compensate.

FPN No. 1: 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.

"...ya know, if you can fit it in, or, uh, well..."

Compare that with...

240.4 Protection of Conductors. Conductors ... shall be protected against overcurrent in accordance with their ampacities specified in 310.15...

They shall be protected according to "X" or else! Small men with big hammers with pound on your toes! Very direct! Very non-controversial.

...the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(2)(a).

Reduce it! Bam! Here's when! Where are those small men?

Believe it or not, that was unrehearsed. I did not fulfill my promise to align my argument better.

Let me ask this, would you feel comfortable using the 1.08 multiplier temperature adjustment for conductors in an ambient temperature of 77 F or less.

Not at all. But obviously, if we can't nail down with authority when we have to use this principle to our disadvantage, we sure ain't gonna feel too hot about trying to make use of it to our favor, ey?

FPN # 2 that follows 310.10 is our precurser to the changes that will come for ampacity corrections for ambient temperatures. The studies are being performed as we speak. I am not sure if they will be ready for '08, if not most likely the '11 NEC.

The ROC (6-37) that heralded the change certainly had a fair amount of arguing to it. I notice though, that the TCC didn't have a hold on the issue. Who is performing the studies, if not at the NFPA's direction?

Do you think it'll be rooftop (sunlight) specific, or more broad?

Wasn't attics and so forth the reason they went from old nm to nm-b a number of years ago?

That does make sense - looking at the correction factors table below 310.16, true 60?C cable couldn't be used, the ampacity would be close to zero. Was the old NM cable 60? or 75? rated?

 

[iwire]

Why do you think the heat generated by the conductor is more of a concern than the ambient temperature?

Because of the hard and fast rules we have for the one type of heat (240.4, 310.15(B)(2)(a)) compared to the lazy language of 310.10, with it's half-asleep FPN's trying to compensate.

George what part of 310.10 is half asleep?

I don't have the section in front of me but as I recall it says more or less

Don't be using them wires above the temperature ratings

Kind of a 'hard and fast rule' to me.

 

[George Stolz]

I don't have the section in front of me but as I recall it says more or less

Don't be using them wires above the temperature ratings

Kind of a 'hard and fast rule' to me.

Here:

310.10 Temperature Limitation of Conductors. No conductor shall be used in such a manner that its operating temperature exceeds that designated for the type of insulated conductor involved. In no case shall conductors be associated together in such a way, with respect to type of circuit, the wiring method employed, or the number of conductors, that the limiting temperature of any conductor is exceeded.

There's no meat. It looks like a dandy preface to a rule, but without a rule being present. The second sentence is brought to practical application by 310.15(B)(2)(a). What language provides practical application for the first sentence?

Sample conversation:
"Hey, buddy, I am going to have to cite you for violating 310.10."
"Oh?"
"Yeah, you're running through that attic. 'No conductor shall be used in such a manner that it's operating temperature is exceeded.'"
"I'm not."
"Sure you are. Attics are hot."
"Stick a thermometer up there right now, it's 95?F."
"Well, all right then. Carry on."

 

[iwire]

George I am at a loss you seem to look at the ambient heat as a separate issue then the heat from the current on the conductor. :?

That makes no sense whatsoever.

 

[pierre]

George
"Sample conversation:
"Hey, buddy, I am going to have to cite you for violating 310.10."
"Oh?"
"Yeah, you're running through that attic. 'No conductor shall be used in such a manner that it's operating temperature is exceeded.'"
"I'm not."
"Sure you are. Attics are hot."
"Stick a thermometer up there right now, it's 95?F."
"Well, all right then. Carry on." "



"Well, all right then. Carry on." - a cop out or unknowing inspector.

This is a good example of using a statement to get around issues.



It could go more like this:
" I know it is 95 F now, but in a couple of months that attic may well reach 120 F for hours/days at a time. 310.10 mentions we are not to have conductors used where temperatures are exceeded. Please take a look at the correction factor section of Table 310.16 and make the appropriate corrections, as the ambient temperature of Table 310.16 is 86 F. Thank You."

Heat/temperature is our worst enemy.

 

[iwire]

Sample conversation:
"Hey, buddy, I am going to have to cite you for violating 310.10."
"Oh?"
"Yeah, you're running through that attic. 'No conductor shall be used in such a manner that it's operating temperature is exceeded.'"
"I'm not."
"Sure you are. Attics are hot."
"Stick a thermometer up there right now, it's 95?F."
"Well, all right then. Carry on."

That is nuts.

Can I do that with the the load on the conductors?

"Your overloaded'

"No I am not, the load is off right now."


Lets keep this simple.

I have a circuit running through a room with a constant ambient temperature of 160 F the load on the circuit is 100 amps non-continuous.

What size conductor do I need?

4/0 copper 90 C.

(260 * .41 = 106 amps)


Now things change, 1/2 the day the temp drops to 40 F or an average temp of 100 F each day.

Can I now reduce the conductor to 3 AWG due to the 100 F average temp.

(110 * .91 = 100 amps

No.

Because each time the temp goes up to 160 F and the 100 amp load is applied the conductor is overloaded per table 310.16

This rise in temp causes all the same violations of article 240 as not derating when more than 3 CCCS are in a raceway or simply ignoring 310.16 in general.

Bob

 

[George Stolz]

Well, all right then. Carry on.

- a cop out or unknowing inspector.

It could go more like this:
" I know it is 95 F now, but in a couple of months that attic may well reach 120 F for hours/days at a time.

Or, it could go like this:
"I know it's 95? now, but in a couple of months that attic may well reach 185?F for hours/days at a time. I'm not passing this installation."

What do you do?

 

[pierre]

Maybe you can locate the run so as not to be in the attic. If the job is sheetrocked or it has to be install in the attic, then I would say there is an issue and you may need to install a new cable with larger conductors. The issue I see with that is there is still a high temperature consideration. I would be curious and probably call the manufacturer to see what they have to say about such high ambient temperatures and what their suggestion may be. Maybe even try to get it in writing.

 

[George Stolz]

Maybe my exaggeration went unnoticed. Would you think it fair that the inspector thinks the ambient is 185?F, despite your belief that 120? or 140? would be more accurate?

 

[pierre]

A good inspector should be aware of local situations. The problem is there are too many inspectors who either do not care or are on an ego trip. If it were not for the good inspectors, it would be a horrible situation.

I did see your exaggeration :wink: