Cable R adjustment for changing Ambient Temperatures

[MyCleveland]

Discussion on how mathematically to adjust a given cable R value at a known cable operating temperature of 60C or 75C or 90C and current ambient temperature of say 30C >>>>>>>>> and adjust that R value to another ambient temperature either up or down.

NEC Chapter 9 tables 8 & 9 give us Rdc & Rac values at 75C and the formulas to move these R values to a new operating temperature either up or down.

These tables do not state anything in regards to the ambient temperature where these R values are valid.

My assumption here is the ambient is 30C since the 600V wire ampacity tables are all developed at 30C.

I have reached out to three wire manufacturers and after 2 months I still have no answer.

SKM users have a pulldown window where they can adjust the ambient for either a single circuit or the entire one-line but they do not address how they are making the adjustment internally in the program.

Southwire’s “Power Cable Manual” provides Rdc values at 25C and more generic formulas for moving from say 25 to 60, 75, or 90C.

AL : R2 = R1( (228 + T2) / (228 + T1) )
CU : R2 = R1( (234 + T2) / (234 + T1) )
Again, there is no discussion on how to address ambient temperature.

Back to the NEC, which has some generic multipliers for ampacity adjustment at different ambient temps, but I cannot find anything relating to an R adjustment for changing ambient temperatures.

Any help is appreciated.

 

[david luchini]

Cable R is directly related to operating temperature, not ambient temperature. There is no adjustment to be made for ambient.

A 3/0 conductor operating at 75C in a 30C ambient would have the same resistance as a 3/0 conductor operating at 75C in a 40C ambient.

 

[MyCleveland]

David
If that is true, how do you explain the NEC adjustment on ampacity for a changing ambient or that SKM makes adjustments for ambient temperature ?

 

[david luchini]

The ambient adjustment is for ampacity, not for resistance.

If you had a 200A load on the 3/0 conductors in a 30C ambient, you would expect the conductor operating temperature to be approx. 75C.

If you had a 175A load on the 3/0 conductors in a 40C ambient, you would expect the conductor operating temperature to be approx. 75C.

The conductor resistance would be the same in both cases, as the operating temperature of the conductor is the same in both cases.

 

[MyCleveland]

I agree with your cable ampacity example, or at least the end result with your 3/0 comparison.
Is not the cables ampacity being derated because in fact the cable R value increased ? The ampacity was reduced because the end result would be the terminal temperature would still be around 75C.

Add this to the mix. Think in terms of a SCC calc at the end of the 3/0 cable run.
Calc in a 30C ambient would use a Z --- with R taken from Chap 9 T9 yielding a value.
Calc at 40C ambient would be a different Z would it not, because the cable R at 40C has changed.

 

[david luchini]

I agree with your cable ampacity example, or at least the end result with your 3/0 comparison.
Is not the cables ampacity being derated because in fact the cable R value increased ? The ampacity was reduced because the end result would be the terminal temperature would still be around 75C.

Add this to the mix. Think in terms of a SCC calc at the end of the 3/0 cable run.
Calc in a 30C ambient would use a Z --- with R taken from Chap 9 T9 yielding a value.
Calc at 40C ambient would be a different Z would it not, because the cable R at 40C has changed.

The ampacity was derated so that the conductor wouldn't operate at a temperature greater than it's rating (or the rating of the devices/terminations on the circuit.) The conductors in both ambients are operating at 75C...the resistance hasn't changed in the 40C ambient.

If both conductors have the same resistance because the are operating at the same temperature, then the Z for the SCC calc is going to be the same on both circuits.

It is the Conductor Operating Temperature that affects the R of the conductor, not the ambient temperature.


To extend my example, imagine the 200A load on the 3/0 conductors in the 30C ambient, and a 150A load on the 3/0 conductors in the 40C ambient.
The resistance of the conductors in the 40C ambient will be lower than the resistance of the conductors in the 30C ambient...because the conductor operating temperature of the conductors in the 40C ambient will be lower.

 

[MyCleveland]

To extend my example, imagine the 200A load on the 3/0 conductors in the 30C ambient, and a 150A load on the 3/0 conductors in the 40C ambient.
The resistance of the conductors in the 40C ambient will be lower than the resistance of the conductors in the 30C ambient...because the conductor operating temperature of the conductors in the 40C ambient will be lower.

David
Per the bold text of your reply, you just stated the R changes...if it changes then so does the SCC calc result.

 

[david luchini]

David
Per the bold text of your reply, you just stated the R changes...if it changes then so does the SCC calc result.

Read the entire answer...

If both conductors have the same resistance because the are operating at the same temperature, then the Z for the SCC calc is going to be the same on both circuits.

I'm not sure what you are not getting about this....It is the operating temperature of the conductor that changes the conductor resistance, not the ambient temperature.

Circuits of the same conductor size in different ambients can have the same operating temperature and therefore the same resistance.
Circuits of the same conductor size in the same ambient can have different operating temperatures and there different resistances.
Circuits of the same conductor size in different ambients can have different operating temperatures and there different resistances.
Circuits of the same conductor size in the same ambient can have the same operating temperature and therefore the same resistance.

 

[charlie b]

Is not the cables ampacity being derated because in fact the cable R value increased?

No. The resistance of a wire will change with a change in its temperature. But that is a relatively minor influence.

I agree with what David is saying. But I will take it from another angle. The cable's ampacity is being derated in order to protect its insulation system from damage. A 75C insulation system (e.g., THWN) can sustain a higher temperature than a 60C insulation system (e.g., TW). Insulation will degrade over time. A rule of thumb is that if you operate a wire at 10 degrees higher for its entire life, you will lose about 50% of its design lifetime. For example, a #6 wire of type THWN (75C insulation) can handle 65 amps indefinitely in an environment that has an ambient temperature of 30C, and will remain in good condition at the end of its design lifetime. It the ambient temperature is higher than 30C, or if the wire is getting heat not only from the current running through it but also from the heat generated by too many other conductors in the same conduit, then you can't continue to run 65 amps through it without reducing its lifetime, or even causing a short circuit when the insulation starts to melt.

 

[MyCleveland]

David
Bad mistake on my part to use the BOLD text as a highlight. It comes across as raising our temperature.

Your prior post, second from last line. Same conductor size in different ambients will have different R values, are we in agreement here ?

 

[MyCleveland]

Charlie
You stated No, and went on to state that R changes.

 

[charlie b]

Your prior post, second from last line. Same conductor size in different ambients will have different R values, are we in agreement here ?

I agree. But as I said earlier, the change is insignificant.

 

[charlie b]

Charlie You stated No, and went on to state that R changes.

I said "no" to your statement that a change in R is the reason that we derate a cable's ampacity.

 

[MyCleveland]

Can i appeal to anyone that has access to SKM run a simple SCC calc of two points....service to a main with 3/0 wire length of your choice and run with an ambient at 20C and rerun with an ambient at 40C.

 

[david luchini]

Can i appeal to anyone that has access to SKM run a simple SCC calc of two points....service to a main with 3/0 wire length of your choice and run with an ambient at 20C and rerun with an ambient at 40C.

What is the operating temperature of the conductor at the 20C ambient, and what is the operating temperature of the conductor at 40C ambient?

 

[MyCleveland]

Lets set it at 75C.

 

[david luchini]

Lets set it at 75C.

You can't "set it" at 75C. The conductor operating temperature for a similar circuit (load and conductor size) in the two different ambient temperatures will have a different conductor operating temperature.

Maybe you are confusing the operating temperature of the conductor with the insulation rating of the conductor.

 

[MyCleveland]

David
Sorry, I shut down Friday after last post and did not see your last response until now.

Started a kitchen remodel and head is full of brick and drywall dust....exhausted.
You are probably right on the last comment. I don't know where to go with any further questions.
Thanks for the responses.

 

[david luchini]

You are probably right on the last comment. I don't know where to go with any further questions.
Thanks for the responses.

The resistance of the conductor depends on the operating temperature of the conductor...the actual temperature of the conductor for the conditions in which it is operating, not the insulation rating of the conductor.

The operating temperature of the conductor depends on several factors, including the load current on the conductor, the number of current carrying conductors in the raceway, the ambient temperature in which the conductor is operating, etc.

So in the formula in your original post...

AL : R2 = R1( (228 + T2) / (228 + T1) )
CU : R2 = R1( (234 + T2) / (234 + T1) )

T2 would indicate the operating temperature of the conductor for the revised operating conditions, ie, if ambient temperature was increased or if the load current increased, then the operating temperature of the conductor would increase, changing the resistance of the conductor.

Again, there is no discussion on how to address ambient temperature.

There is not a direct correlation to ambient temperature because ambient temperature is not the only factor affecting the operating temperature of the conductor.

 

[MyCleveland]

David
Will try and restate a few items to see if we are on the page.

Conductor Insulation Temp : whether 60, 75, etc is the temperature of the cable IF operating at its rated full ampacity.

Conductor Temp and Termination Temp : is temperature of cable based on value of ampacity on circuit.
This will = 60, 75, etc if conductor ampacity is operating at its max ampacity in tables.