Seeking a sane explanation for NEC 110.14(C)

[eeprom]

All,
I want to first point out that I understand the rules about not using conductors that are higher rated than the terminal lugs or other circuit devices. My question is not about applying the rule. It is in regards to why this rule exists as it does.

It would be logical to say that if I have a breaker rated at 75C that I cannot use a 60C conductor on that breaker because the conductor is rated lower than the breaker. But that is not what the rule says. The rule says that I cannot use a 90C rated conductor on a 75C breaker terminal, even though that conductor is rated at a higher temperature than the breaker.

It seems backwards. Can anyone explain why it's there and what's being protected?

 

[waross]

Does it say that you cannot use the conductor or that you cannot use the 90 degree ampacity of the conductor?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[eeprom]

Why does that rule exist? It's rated higher than the breaker but I have to derste the conductor to use it?

What's being protected? The conductor or the breaker?

 

[davidbeach]

You can use the 90C conductor, you just can't exceed the 75C rating because that's what the terminals are good for.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[eeprom]

I'm not asking about how to use the rule.

Am I doing this to protect the conductor or the breaker?

If I have a 75c conductor rated for 100A, why would it matter if I installed it in a 100A breaker rate at 60C? It's rated for 100A.

If I have a breaker rated for 100A, why would the conductor I use have any effect on the breaker?

 

[davidbeach]

If the breaker is rated for 100A and the conductor is rated for at least 100A at the temperature rating of the breaker terminals, and you don't push more than 100A through it then you're good; conductor insulation temperature rating doesn't matter. But what you can't do is use a conductor physically smaller than that which has the right ampacity at the terminal rated temperature. The terminal rating includes the ability of the conductor to transmit heat away from the device. Physically smaller conductors don't conduct the heat away as well as larger conductors.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[eeprom]

I understand the rule, just not the reason behind it.

It sounds like you are saying this rule protects the conductor'not the breaker.

Why would the rating of the breaker terminal have any effect on the temperature ofc the conductor?

 

[davidbeach]

It doesn't, it protects the breaker. If applied at its 90C rating the conductor could run hotter than the breaker is capable of handling.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[eeprom]

Let me ask another way. For the same example, if I use a 90C conductor in a 75C breaker, that's a code violation. But if I use a 60C conductor, it not.

Doesn't this seem backwards?

 

[eeprom]

So you are saying the conductor will/can operate at 90C?

That's very hot, and probably unlikely. But it does make sense.

So the conductor rating determines to some degree how hot the conductor "can" get at full load.

It's amazing how many times I've encountered that rule without really understanding it.

Thank you.

 

[che12345]

"...#1. I want to first point out that I understand the rules about not using conductors that are higher rated than the terminal lugs or other circuit devices. My question is not about applying the rule. It is in regards to why this rule exists as it does...#2. It would be logical to say that if I have a breaker rated at 75C that I cannot use a 60C conductor on that breaker because the conductor is rated lower than the breaker. But that is not what the rule says. The rule says that I cannot use a 90C rated conductor on a 75C breaker terminal, even though that conductor is rated at a higher temperature than the breaker"
I have the following opinion for your consideration
Ref [NEC Art 110.14(C) Temperature Limitations. The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both...]
1. The key word here is "The temperature rating associated with the ampacity of a conductor ..."
2. The key word here is "Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for..."
Che Kuan Yau (Singapore)

 

[eeprom]

Thank you to davidbeach for the sane explanation. This might be obvious to some, but not all. I've been a PE for 20 years and I've never questioned this rule. I asked two friends yesterday, and both told me to just follow the rule. Neither even tried to explain it. I am guessing that my confusion is not all that uncommon.

What this rating is about is treating the conductor as a heat source that under certain conditions can get hot enough to cause damage to other parts of the circuit. The ratings in the table correspond to how how the conductors can get at rated amps in some ideal laboratory conditions. This does not mean how hot it will get.

The temperature ratings of the conductors are based on their surface areas. The greater the surface area, the more it can dissipate heat. The amount of heat a conductor will generate will be based on the resistance of the conductor itself.

I used 1000' lengths of copper gauges #14 to #2 and calculated their I^2R losses at rated amps based on resistances given for 25C. Then I calculated the expected steady state temperature of 1000ft of copper based on an ambient temperature of 30C, and a convective heat loss coefficient in free air of 10 W/(m2K). This is the low end of free air cooling, which is probably what NEC or IEEE would use. I used no conductive dissipation.

The individual calculated conductor temperatures loosely corresponded to 60, 75, and 90. For the smaller gauges, the calculated temperature was too high; for the larger gauges the calculated temperature was too low. But the averages per temperature category followed closely. The results were 61.1 C for the 60C column, 76.9C for the 75C column, and 93.5C for the 90C column.

I know these numbers are just calculated estimates, but they follow the pattern I was hoping to find. This makes sense. Thank you again davidbeach for pointing me in the right direction.

 

[moon161]

An electrician friend explained that when capacities of conductors, terminals and so on are mixed one can form a wrong idea about the circuit based on incomplete inspection. If incomplete inspection leads you to unsafe ideas about the wiring, then that's the problem.

 

[che12345]

I have the following opinion for your consideration.
1. A breaker tested successfully passed the test with (Lower insulation temperature) conductor say X mm[sup]2[/sup]. If a (Higher insulation temperature) conductor is used, it requires only Y mm[sup]2[/sup] for the same current. Where X > Y
Opinion: The (Higher insulation temperature) conductor of Y mm[sup]2[/sup] should NOT be used.
Reason: 1) The breaker was tested with X mm[sup]2[/sup] conductor,
2) Conductor Y has a lower physical cooling surface area.
1.1 The relationship is a Higher insulation temperature conductor requires a lower area mm[sup]2[/sup], for the same amperecity.
2. A (Higher insulation temperature) conductor of X mm[sup]2[/sup] is permitted.
2.1 The relationship is between the insulation temperature class, where the physical cooling surface area is the same. Therefore, a (Higher temperature class with X mm2 [ not Y mm2 ] is permitted.
Che Kuan Yau (Singapore)

 

[waross]

The higher the current the hotter the conductor.
The extra heat when a cable is loaded to the 90 degree limit will conduct to the breaker internals and may affect the calibration.
The breaker has not been tested nor approved with cables running at 90 degrees.
The connectors are part of the breaker approval.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!