Feeder cable size

[James L]

That is an interesting question. I think it has been suggested that an equipment termination rated at 75C accepting a maximum wire size of, say, #3 Cu must be rated for 100A. Not sure about that.

Regardless, the point of the text in 110.14(C)(1) is that the termination ampacity check is a direct table lookup--you don't apply correction or adjustment factors. So at the termination you can't take credit for a lower than typical ambient temperature.

Cheers, Wayne

No, termination ampacity is not strictly a table look up. It is a reference to the ampacity of the conductor. And the ampacity of a conductor has two table lookups.

One table tells you the ampacity at 30 degrees Celsius

The other table gives you an adjustment factor, plus or minus, depending on ambient temperature.

And the adjustment of a 500 kcmil conductor in an ambient temperature below 78 degrees Fahrenheit is +5%

380 + 5% = 399 amps

A 500 kcmil lug rated at 75 degrees Celsius is good for 399 amps by default

 

[wwhitney]

No, termination ampacity is not strictly a table look up.

I'm sure you are mistaken, but don't know what else to say beyond the references I've given. [And I agree that the code requirements don't seem to match the physics, as far as temperature correction at connections.] Hopefully others will chime in.

Cheers, Wayne

 

[DrSparks]

I'm sure you are mistaken, but don't know what else to say beyond the references I've given. [And I agree that the code requirements don't seem to match the physics, as far as temperature correction at connections.] Hopefully others will chime in.

Cheers, Wayne

Again, I believe these are engineering precepts that can be applied under engineering supervision. Not for a dumb electrician to determine.

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[electrofelon]

Again, I believe these are engineering precepts that can be applied under engineering supervision. Not for a dumb electrician to determine.

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Yes engineering supervision is allowed to calculate ampacity in 310.15(C). However that doesnt have anything to do with the termination question we are talking about.

Like Wayne, I have always understood that the table value as is without modification is to be used for the termination. That said, it doesn't seem to specifically say that we cant use the note to modify the value if we have a different temperature.

 

[DrSparks]

Yes engineering supervision is allowed to calculate ampacity in 310.15(C). However that doesnt have anything to do with the termination question we are talking about.

Like Wayne, I have always understood that the table value as is without modification is to be used for the termination. That said, it doesn't seem to specifically say that we cant use the note to modify the value if we have a different temperature.

Interesting. I'm going to look into this because it could be very useful. Idk about convincing our city inspectors though haha.

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[hhsting]

A lot of times we don’t know the physics or reasons behind code or code is code like once I was asked why service conductors inside building routed need to be concrete encased to be considered outside. I said its in code. Then I was asked whats the physics and reason behind concrete encased and magically its considered outside. I said I don’t know its in code.

So termination stuff of ampacity makes sense physics wise but its not in code. If it was that important I am sure CMP would have cited it in 110.14 as Wayne mentioned few posts ago.



As plan reviewer I have to look at enforce code and Not the physics behind it.

More interesting question though lets says I have 60C cables, 75C termination, ambient temp can range high in summer 101F and in winter 50F and demand load calculation comes to 390A. What would be the minimum awg require factoring into all adjustment, corrections, terminations taking into account the code way not the physics way?

 

[James L]

Interesting. I'm going to look into this because it could be very useful. Idk about convincing our city inspectors though haha.

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It's very useful when you're only talking about 10 making that much difference.

390 load
400 breaker
500 and terminals are only 380 amps
vs. 500 and terminals are 399 amps

 

[James L]

Yes engineering supervision is allowed to calculate ampacity in 310.15(C). However that doesnt have anything to do with the termination question we are talking about.

Like Wayne, I have always understood that the table value as is without modification is to be used for the termination. That said, it doesn't seem to specifically say that we cant use the note to modify the value if we have a different temperature.

Here's the basic conundrum...
Is the temperature of the terminal derived from:

a) the amperage flowing through it, or

b) the measured temperature of the conductor in it?

amperage by itself doesn't equate to heat. Otherwise there's no way possible for a #2 lug and a 650kcmil lug to both have a temperature rating of 75 degrees.

The 75 degrees (max) is going to come from the conductor. It's not physically possible any other way.

If the temperature of the conductor is lowered by the ambient temperature, how can it not affect the lug?

 

[romex jockey]

Not for a dumb electrician to determine.

well thx Doc, glad someone gave me an out here......~RJ~

 

[DrSparks]

Here's the basic conundrum...
Is the temperature of the terminal derived from:

a) the amperage flowing through it, or

b) the measured temperature of the conductor in it?

amperage by itself doesn't equate to heat. Otherwise there's no way possible for a #2 lug and a 650kcmil lug to both have a temperature rating of 75 degrees.

The 75 degrees (max) is going to come from the conductor. It's not physically possible any other way.

If the temperature of the conductor is lowered by the ambient temperature, how can it not affect the lug?

The conductor actually acts as a heat sink for the lug.

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[DrSparks]

well thx Doc, glad someone gave me an out here......~RJ~

Yup. I keep that card up my sleeve

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[electrofelon]

Here's the basic conundrum...
Is the temperature of the terminal derived from:

a) the amperage flowing through it, or

b) the measured temperature of the conductor in it?

amperage by itself doesn't equate to heat. Otherwise there's no way possible for a #2 lug and a 650kcmil lug to both have a temperature rating of 75 degrees.

The 75 degrees (max) is going to come from the conductor. It's not physically possible any other way.

If the temperature of the conductor is lowered by the ambient temperature, how can it not affect the lug?

The thing is that 110.14(C) directs us to ONLY one table. It seems that they want us to use THAT table regardless of if we can use another table for the conductors. For example you could have a termination in free air but it doesnt say you can use one of the free air tables if it applies.

 

[augie47]

Worked with dozens of inspectors for five decades and can just about guarantee that everyone will look at 110.14 without physics.. 500 kcmil =380 amp termination.
James rhetoric would be classified as smoke and mirrors.

 

[DrSparks]

Worked with dozens of inspectors for five decades and can just about guarantee that everyone will look at 110.14 without physics.. 500 kcmil =380 amp termination.
James rhetoric would be classified as smoke and mirrors.

Right. That's what I've been saying. I know my inspectors would say get an engineer to say it's okay.

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[James L]

The thing is that 110.14(C) directs us to ONLY one table. It seems that they want us to use THAT table regardless of if we can use another table for the conductors. For example you could have a termination in free air but it doesnt say you can use one of the free air tables if it applies.

Sure. 110.14(c) directs us to only one table, which is 310.16

But at the bottom of that table, it directs us to 310.15(b) to make corrections for ambient temperature.

Surely you see that, don't you?

 

[James L]

Worked with dozens of inspectors for five decades and can just about guarantee that everyone will look at 110.14 without physics.. 500 kcmil =380 amp termination.
James rhetoric would be classified as smoke and mirrors.

It's not my fault if people don't know how to read

 

[hhsting]

Sure. 110.14(c) directs us to only one table, which is 310.16

But at the bottom of that table, it directs us to 310.15(b) to make corrections for ambient temperature.

Surely you see that, don't you?

Oh dear . I will just wait for people expert ones to respond to above cause I am lost and confused. However additional question that table also references more then 3 current carrying conductors how does that apply to terminations?

 

[James L]

Look here. These people * who sell lugs * understand the ambient temperature adjustment.

They have both charts on the same page.
I think I may call them in the morning to see how they understand this issue.

Wire Current Ampacities NEC Table 310-16

 

[DrSparks]

Sure. 110.14(c) directs us to only one table, which is 310.16

But at the bottom of that table, it directs us to 310.15(b) to make corrections for ambient temperature.

Surely you see that, don't you?

I understand your argument, but I interpret 310.16 to be a absolute maximum ampacity allowed for conductors based on temperature rating. You're right that it does not specifically state that temperature correction could not potentially increase the ampacity, but I think the intent was for the correction factors to derate conductors under higher than normal operating temperatures.

I could be wrong.. I would put in a request for clar on this in the next code cycle.

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[James L]

That table also references more then 3 current carrying conductors how does that apply to terminations?

Terminations are rated by temperature, and are directly determined by conductor temperature relative to ampacity, actual current, and ambient temperature

If your conductor is ok at 75 degrees Celsius, then your 75 degree lugs are good - it's that simple