NEC Chapter 9 - Table 9

[MyCleveland]

Question(s) revolve around 600V building wire R values.

I hope we agree on the fact that a conductor R varies with the conductor temperature.
Two different factors are involved here, ambient temperature and conductor operating temperature.

Please refer to NFPA (NEC) 2020 Chapter 9, Table 9, page 70-706.
This tables Note 1 states “resistance values are valid only at 75C (167F) and for the parameters as given”.

My assumption is that this is stating the conductor operating temperature is 75C, and that the conductor is in fact under load at its MAXIMUM allowable ampacity as taken from NEC Table 310.16 (page 70-161), 75C columns for either CU or AL conductors.

Ques 1: What is the ambient temperature of the cable when these R values were recorded ?

Ques 2: Is Chapter 9, Table 9, Note 1 stating “valid only at 75C”, in fact is stating the conductors are under load at MAXIMUM allowable ampacity per Table 310.16 ?

Ques 3: If question 2 is YES, then if conductors are under a load of a lessor ampacity an adjustment to R is required ?

Please help me in resolving Questions 1-3.

 

[David Castor]

The resistance is a function of the actual conductor temperature, regardless of the ambient temperature. The ambient temperature isn't a direct factor. The table is simply giving the ac resistance at a conductor temperature of 75 C. If the conductor temperature is not exactly 75 C, the resistance will be slightly different. Under normal operating conditions, the conductor temperature will likely be less than 75 C and the actual resistance will be lower.

The table is not really directly related to Table 310.16. It's simply giving a physical property of a conductor at a particular temperature.

 

[MyCleveland]

The resistance is a function of the actual conductor temperature, regardless of the ambient temperature. The ambient temperature isn't a direct factor. The table is simply giving the ac resistance at a conductor temperature of 75 C. If the conductor temperature is not exactly 75 C, the resistance will be slightly different. Under normal operating conditions, the conductor temperature will likely be less than 75 C and the actual resistance will be lower.

The table is not really directly related to Table 310.16. It's simply giving a physical property of a conductor at a particular temperature.

David
Are not all of the NEC derating items related to heat gain or loss. Specifically ambient temp is directly addressed with a derating table or adjustment table. If you are adjusting for ambient change you have to have a starting value at some known ambient, so the table 9 must have an associated ambient value.
I believe Southwire and General Cable tables R tables state an ambient and the cables operating temp.
If the cable is at max temp, why can I not make the association with table 310.16 ?

 

[David Castor]

Maybe it would help if we knew what you were trying to calculate? Again, this table is simply stating some physical properties of copper and aluminum conductors. These reactance and resistance values are mainly used for manually calculating voltage drop. The ambient temperature correction is normally handled by the table in 310. 15.

The change in resistance due to temperature is a function of the conductor temperature. The ambient temperature is a factor only in so far as it would effect the conductor temperature.

 

[MyCleveland]

The change in resistance due to temperature is a function of the conductor temperature. The ambient temperature is a factor only in so far as it would effect the conductor temperature.

I am trying to pin down a method of calculating R for a given ambient and conductor temperature.

Every table of values should reflect these two variables...as I stated the manufacturer tables do.
The SCC programs I have seen allow for user adjustment of ambient, but I do not believe they address conductor operating temp leading me to believe the R values they use are based on a cable assumed to be operating at rated temperature or 100% loading.

Do you not think a change in operating temp of the cable, ex: a 3/0 (75C) running at 100 amps as compared to running at 200A would not result in a different result for SCC at a given run of cable ?

Asking the questions in the interest of accuracy for a calculation.

 

[JoeStillman]

Be careful, you could wind up spending $150 on engineering to save $75 on the wire size.

 

[David Castor]

Changing the conductor loading from 100 A to 200 A will change the conductor temperature and the conductor resistance. The change will be pretty insignificant in a short circuit calculation. But also, if your concern is short circuit calculations, how do you know what the conductor loading and temperature will be when the fault occurs? Regardless, if you account for the actual conductor temperature in adjusting the resistance, the ambient temperature is not a direct factor. Other factors such as conductor spacing and raceway type will also effect the ac resistance if you really want to consider every variable.

 

[MyCleveland]

Be careful, you could wind up spending $150 on engineering to save $75 on the wire size.

Joe
Not sure where you are headed....I am not attempting to do anything related to reducing required wire sizes.

 

[MyCleveland]

Changing the conductor loading from 100 A to 200 A will change the conductor temperature and the conductor resistance. The change will be pretty insignificant in a short circuit calculation. But also, if your concern is short circuit calculations, how do you know what the conductor loading and temperature will be when the fault occurs? Regardless, if you account for the actual conductor temperature in adjusting the resistance, the ambient temperature is not a direct factor. Other factors such as conductor spacing and raceway type will also effect the ac resistance if you really want to consider every variable.

David
calculation. >> I want to see the numbers before i can agree with you.
will be...>> can I not assume the load is what I designed the circuit for ? I can adjust R for this revised loading rather than assuming it is running at 75C.
direct factor. >> It is a direct factor, as stated earlier I believe ALL of the major software vendors allow you to adjust for a different ambient than 30C.

 

[JoeStillman]

Joe
Not sure where you are headed....I am not attempting to do anything related to reducing required wire sizes.

Sorry, I thought you were using the R values in a voltage drop calculation. Is this for a SC study or something?

 

[MyCleveland]

Sorry, I thought you were using the R values in a voltage drop calculation. Is this for a SC study or something?

Yes, I am trying to settle on how to handle R values for a SCC calculation. I am looking for some advice on how to get out of this rabbit hole.

The same questions also apply to VD calcs....if you are using the R values from table 9, these reflect a cable running at its rated temp of 75C or fully loaded to its maximum rated ampacity. Do you think using this value will yield a good result if you are trying to determine an accurate VD calc and you are running your calc at half the cables ampacity ?

 

[jim dungar]

When I was doing power system studies, we would choose a base/ambient temperature of 25°C or 30°C when selecting our wire type and sizes. This was the only temperature adjustment we ever made. We did not even have a hard and fast rule what was used, as long as it was spelled out in our write up.

All conductor properties tables I remember seeing, such as those from Southwire, have been based on 25°C.

None of my short circuit studies ever included actual loads so they never took into account conductor resistance based on current through the conductor. For the most part, bolted short circuit currents do not flow long enough to heat up the conductors beyond their normal operating currents.

My interpretation is that Chapter 9 Table 9 is based on the conductors being at their full load temp of 75°C as that is the typical max termination temperature for listed equipment for the past several decades.

 

[MyCleveland]

When I was doing power system studies, we would choose a base/ambient temperature of 25°C or 30°C when selecting our wire type and sizes. This was the only temperature adjustment we ever made. We did not even have a hard and fast rule what was used, as long as it was spelled out in our write up.

All conductor properties tables I remember seeing, such as those from Southwire, have been based on 25°C.

None of my short circuit studies ever included actual loads so they never took into account conductor resistance based on current through the conductor. For the most part, bolted short circuit currents do not flow long enough to heat up the conductors beyond their normal operating currents.

My interpretation is that Chapter 9 Table 9 is based on the conductors being at their full load temp of 75°C as that is the typical max termination temperature for listed equipment for the past several decades.

Paragraph 1: understood.
Paragraph 2 & 3: agreed.

Paragraph 4: Thanks…would you then say I can also extrapolate that these R values equate to the NEC ampacity values at 75C ?
I can then allow for an ampacity from rated temp all the way down to zero, or cold start in my calculation.

 

[jim dungar]

Paragraph 4: Thanks…would you then say I can also extrapolate that these R values equate to the NEC ampacity values at 75C ?
I can then allow for an ampacity from rated temp all the way down to zero, or cold start in my calculation.

Yes, I think the NEC resistance values are at full NEC ampacity values. The problem becomes deciding what the temperature of the conductor would be at a reduced load. Does a 50% loading equal a 37.5°C conductor temperature?

For voltage drop calculations I would use the higher NEC resistance, while for momentary loads, like a short circuit, I would use 25°C values.

The NEC change in resistance formula, for copper, can be 'reduced' to:
R₂ = R₁ X (234 + T₂)/(234 + T₁)

 

[MyCleveland]

"JD >> The problem becomes deciding what the temperature of the conductor would be at a reduced load. Does a 50% loading equal a 37.5°C conductor temperature?"
There is a (IEEE Buff Book) formula for this I am using in a VD calculator but I cannot put my hands on it at the moment, I will come back to this when I find it.
It basically allows you to calculate a NEW cable operating temp based on max amps, new lower amps, TEMP-amb & TEMP-Operating.
Once this is calculated use.... Rnew = Rstart ( 1 - (MatFactor) * (deltaT) )


"JD >>
For voltage drop calculations I would use the higher NEC resistance, while for momentary loads, like a short circuit, I would use 25°C values."
Q1: for VD you want to be conservative in your result, rather than using actual R at a lower amp value would give you a lower VD ?
Q2: You stated 25C for SCC's....do you mean unloaded cable or cold start values because this will yield the max SCC ?