310.15(B)(3)(c) & Copper.Org

[fmtjfw]

That's not the primary purpose of Part III. The conductor sizing is for the purposes outlined in 250.4(A)(1), grounding.

Bonding is per 250.104.

I understand that, my question now is where do you get the size of the conductors referenced in 240.68. I take the part of the title "Grounding Electrode Conductor" to refer to T250.66, not 250.122.

 

[George Stolz]

Yes, but how do you make the leap to fault clearing from those references?

 

[jumper]

I understand that, my question now is where do you get the size of the conductors referenced in 240.68. I take the part of the title "Grounding Electrode Conductor" to refer to T250.66, not 250.122.

GECs have nothing to do with clearing faults.

As George pointed out 250.104 is the section you should look at.

Some of the sizes of bonding jumpers in that section use T250.66 for size, but not purpose.

 

[iwire]

Of course it is the insulation but do we have evidence that a few days in the high temperature will affect it. That's what I was getting at. The copper is fine it is almost always the insulation we worry about with ambient temp.

And I am asking do we have evidence that running a 12 AWG at 35 amps will effect it?

I am failing to see a difference.

 

[macmikeman]

Besides Alaska, and the Canadian Providences , guess which state has the next lowest avg summer roof temperature values according to the chart George provided....

 

[iwire]

It might be interesting to compare NEC ampacities with other standards. I have no idea how they would compare. Is the NEC more conservative or less than other major wiring standards. :?

 

[ActionDave]

Here's another nugget from a fortune cookie: If the NEC has always been conservative, then doesn't a 50% additional level of conservatism raise it from a wink and a nod to an outcry?

No doubt. It seems to me that once something becomes accepted as standard then what is considered better or safer has to move up the scale also.

 

[Dennis Alwon]

And I am asking do we have evidence that running a 12 AWG at 35 amps will effect it?

I am failing to see a difference.

Now you are asking to change the ampacity chart. I am sure the chart is based on human error to some degree. I bet all conductors with their insulations can handle more than the table allows. Given the table the question I have always had is what does ambient mean. There has never been help from the NEC on this. Ambient does not mean average as you know.

 

[iwire]

Now you are asking to change the ampacity chart.

I not asking for that at all.

I am comparing overheating a wire only by current as opposed overheating a wire by current and ambient.

 

[don_resqcapt19]

I agree with Bob. The issue with both higher currents and high ambients is damage to the conductor insulation. If the high ambient won't damage the insulation, then the same conductor temperature caused by high current will not damge the insulation.

 

[K8MHZ]

And I am asking do we have evidence that running a 12 AWG at 35 amps will effect it?

I am failing to see a difference.

Off the top of my head I would think that 35 A would generate some heat, and heat, over time, accelerates the decomposition of plastic.

For short periods, I am sure that 12 AWG could handle much more than 35 A without melting the insulation. So I think that the ratings the NEC uses aren't based upon the melt point of the insulation as much as the temp over the life of the conductor.

 

[iwire]

So I think that the ratings the NEC uses aren't based upon the melt point of the insulation as much as the temp over the life of the conductor.

I absolutely agree with that.

I am just pointing out that insulation degradation by heat is the same regardless if it is caused by current flow or ambient temperatures.

So considering we treat ampacity ratings as firm numbers not to be exceeded I feel ambient temperature figures should be based on max temp not the 'average' or 'normal' ambient temperatures.

 

[K8MHZ]

I absolutely agree with that.

I am just pointing out that insulation degradation by heat is the same regardless if it is caused by current flow or ambient temperatures.

So considering we treat ampacity ratings as firm numbers not to be exceeded I feel ambient temperature figures should be based on max temp not the 'average' or 'normal' ambient temperatures.

The difference is that in addition to accelerating decomposition, temperature is also responsible for delaying it. The same can't be said about current. Having an amount of current 50 percent below the rating for one hour will not offset the effects of having it 50 percent above the rating for an hour.

 

[Dennis Alwon]

So considering we treat ampacity ratings as firm numbers not to be exceeded I feel ambient temperature figures should be based on max temp not the 'average' or 'normal' ambient temperatures.

Again my complaint is with the term ambient. Ambient means surrounding temperature- totally meaningless since each day is different. I believe the tables are all based on average high days or something like that which I understand is your issue with the table. I still question whether a few days of high temp. is enough to affect the insulation.

For example the max temp here is say 102?, but every 5 years or so the temp. may hit 105? for part of a day. This changes the ampacity of my conductor. Do I use the 105????

I don't think this is studied enough or the info is certainly not readily available thru the NEC, IMO

 

[don_resqcapt19]

The difference is that in addition to accelerating decomposition, temperature is also responsible for delaying it. The same can't be said about current. Having an amount of current 50 percent below the rating for one hour will not offset the effects of having it 50 percent above the rating for an hour.

I don't think that having a lower temperature reverses the damage caused by a previous higher temperature. I don't see any difference in the effect on the insulation from a high ambient temperature or from the same temperature caused by the I?R heating of the conductor.

 

[iwire]

Having an amount of current 50 percent below the rating for one hour will not offset the effects of having it 50 percent above the rating for an hour.

And having 360 days of 50F ambient does not offset the effect of 5 days of 150F ambient.

 

[iwire]

Again my complaint is with the term ambient. Ambient means surrounding temperature- totally meaningless since each day is different. I believe the tables are all based on average high days or something like that which I understand is your issue with the table.

I don't think we are communicating well.

I have no issue with the tables.

What I have an issue with is saying 'a couple of extra hot days' can be ignored which is what I think George and perhaps yourself are saying.

For example the max temp here is say 102?, but every 5 years or so the temp. may hit 105? for part of a day. This changes the ampacity of my conductor. Do I use the 105????

Yes, you use 105. (IMO)

I don't think this is studied enough or the info is certainly not readily available thru the NEC, IMO

This is where I get lost, few people can say they understand how the ampacity tables are arrived at yet they will treat them as rock solid numbers.

OTH when we get to ambient temps suddenly we bend things to our advantage. At least that is what it seems like to me.

 

[K8MHZ]

I don't think we are communicating well.

I have no issue with the tables.

What I have an issue with is saying 'a couple of extra hot days' can be ignored which is what I think George and perhaps yourself are saying.



Yes, you use 105. (IMO)



This is where I get lost, few people can say they understand how the ampacity tables are arrived at yet they will treat them as rock solid numbers.

OTH when we get to ambient temps suddenly we bend things to our advantage. At least that is what it seems like to me.

If it weren't for OCPDs, I am sure we would bend ampacity to our advantage, too. OTOH, if there was something that went click and shut a circuit down for excess temperature I'll bet we would treat temperature ratings as rock solid numbers just like we do ampacity.

 

[rbalex]

Insulation thermal damage is both aggregate and cumulative. Aggregate in that the cause the over-temperature in itself is somewhat irrelevant. Both ambient and overcurrent, either separately or collectively, will cause about the same damage for the same over-temperature held for the same duration. Overcurrent has the minimal advantage of possibly being cleared.

Damage is also cumulative; that is, if over-temperature deterioration occurred, no matter what caused it, returning the conductor to a lower ambientor to below rated ampacity (or both), even for a prolonged period of time, will neither repair nor restore the conductor – even if it doesn’t fail immediately. At best, it only curtails further damage; but, depending on the nature of the damage, it may only slow it.

 

[charlie b]

Physics is one thing. What the code has to say is another.

The physics is addressed by the Arrhenius Equation. What it tells us is that the speed at which the insulation will degrade is a function of the temperature, and the relationship is not linear. Spending one hour at a temperature of 10 degrees above 30C will not be offset by spending an hour at 10 degrees below 30C. Some of you have already made that point. You can look at it in this way: If you drive your car at 40 MPH, you will use up gasoline at some rate, and if you stay at that speed you will be able to drive some distance. If you instead drive at 50 MPH, you will use up your gas faster, and you might not be able to drive as far. If you slow down to 30 MPH when you gas tank is half full, you will burn gas at a lower rate, but you will not be getting back any of the gas that you have already burned. The damage done to conductor insulation by high temperatures works in a similar way.

All that said, the NEC does not give us the ability to calculate the life expectancy of a conductor?s insulation system on the basis of a time versus temperature profile. So all we have to go by is a vague statement about ambient temperature. I would like to see that clearly defined or explained within the NEC, but I doubt it will ever happen.