I made my head hurt,,,

[RichB]

OK-- base line stuff prior to the question--
I understand Voltage drop and the calculations, or thought I did until I was asked by a student, "What happened to using 10.7 for k?"

After really digging into it I haven't found a good answer, But what I have found is the following:

1. 12.9 seems to be for conductors at 90c
2. 10.7 seems to be for conductors at 65C
A. This is from a source I found on line--older--I think it was 50's or 60's--can't remember and can't find it again
3. I also realize that for AC we need to take Z into account--the reference I found said only for wires of #3 and above as it was negligible for wire sizes under that
4. Formulas I used
A. Vd=2(k)L(I) divided by CM
B. Vd=2(I)(Ohm/KFeet)(L/1000)
C. Where k-12.9 or 10.7
I=10
L=1000
CM=16510(#8)
Ohm/Kfeet=.778 (from Chap 9 table 8 for uncoated #8CU)


Here's the answers
15.63 Volt drop when k=12.9
12.96 when k=10.7
Using equation in B=15.56

It appears that using 12.9 is a quick way of finding Vd without knowing the ohms/Kft number-and gives a "Close enough working number"--does it also take into account the value for Z?

Since most of our (in my area and applications) conductors never reach 65C should we be using 10.7 or is 12.9 used like the second ground rod--do worst case and call it good?

I even found an old Simplex calculator on eBay and had to buy to see if I could find a date--BYW--It uses 10.7 according to the info printed on it I could read!!:ashamed:So at least I have a cool new circular calculator for my slide rule collection!!

I didn't post the calcs as this was getting long but if you want to see them I will

Thanks All!!!

 

[jumper]

OK-- base line stuff prior to the question--
I understand Voltage drop and the calculations, or thought I did until I was asked by a student, "What happened to using 10.7 for k?"

After really digging into it I haven't found a good answer, But what I have found is the following:

1. 12.9 seems to be for conductors at 90c
2. 10.7 seems to be for conductors at 65C
A. This is from a source I found on line--older--I think it was 50's or 60's--can't remember and can't find it again
3. I also realize that for AC we need to take Z into account--the reference I found said only for wires of #3 and above as it was negligible for wire sizes under that
4. Formulas I used
A. Vd=2(k)L(I) divided by CM
B. Vd=2(I)(Ohm/KFeet)(L/1000)
C. Where k-12.9 or 10.7
I=10
L=1000
CM=16510(#8)
Ohm/Kfeet=.778 (from Chap 9 table 8 for uncoated #8CU)


Here's the answers
15.63 Volt drop when k=12.9
12.96 when k=10.7
Using equation in B=15.56

It appears that using 12.9 is a quick way of finding Vd without knowing the ohms/Kft number-and gives a "Close enough working number"--does it also take into account the value for Z?

Since most of our (in my area and applications) conductors never reach 65C should we be using 10.7 or is 12.9 used like the second ground rod--do worst case and call it good?

I even found an old Simplex calculator on eBay and had to buy to see if I could find a date--BYW--It uses 10.7 according to the info printed on it I could read!!:ashamed:So at least I have a cool new circular calculator for my slide rule collection!!

I didn't post the calcs as this was getting long but if you want to see them I will

Thanks All!!!

10.8 is for 60C CU, 90C CU is 12.9

 

[bphgravity]

If K is the resistance of a 1 mil conductor, 1-foot in length or a 1,000 mil conductor, 1000-feet in length, then true K = R x cm / 1000 using the value of R from Chapter 9, Table 8 or Table 9 as applicable. Copper and Aluminum will have a different K.

 

[RichB]

10.8 is for 60C CU, 90C CU is 12.9

ok--next question--how do you determine which one to use?

And looking at Ch9 Table 8--the DC resistance is at 75C:? LOL I am getting my self all turned around--It can't be as complicated as I am making it!!--I think I need a 2 by twice smack right betwixt the running lights!!!

 

[RichB]

If K is the resistance of a 1 mil conductor, 1-foot in length or a 1,000 mil conductor, 1000-feet in length, then true K = R x cm / 1000 using the value of R from Chapter 9, Table 8 or Table 9 as applicable. Copper and Aluminum will have a different K.

OK--got that, makes sense--that gives the 12.9 approx.--12.84 for #8CU--close enough--where did the 10.7 come from and why don't we use it anymore??

 

[ggunn]

This thread makes the case for understanding and using the real equations instead of relying on dimensionless shortcuts.

 

[RichB]

This thread makes the case for understanding and using the real equations instead of relying on dimensionless shortcuts.

Absolutely agree!!! I learned it using the numbers from the NEC--but then was taught the 2KLI version using 12.9
because it was easier to use in the field--no need for the NEC to look up specific conductors.

Never really gave it a thought because it always worked--until the other day when that question came up.

I have to teach the 2KLI form as that is what we use for our testing---but will start to use the real equation to show where it derived from and the actual differences.

Oh and BTW--that simplex calculator I ordered came in--Based on the 1949 NEC.

Thanks for the input folks, as always--GREATLY appreciated--and if anyone can explain when and why we switch from 10.7 to 12.9 (I know--one is 60 and the other is 70C) but at what temp would we switch--75C or 80C??--also when did we just start teaching 12.9 and not even bothering with 10.7?

 

[RichB]

ok found where the 10.7 comes from-- a copper conductor 10.7cm one foot long at 60C has one ohm resistance

 

[Smart $]

The thing about using 12.9 in voltage drop calculations is that most 90°C-rated conductors are only configured for a maximum of 75°C operating temperature.

Both Chapter 9, Table 8 and 9 values are based on 75°C operating temperature. K = R × kcmil...

And Note 2 to Table 8 shows us the formula for adjusting resistance to a different operating temperature.

 

[RichB]

The thing about using 12.9 in voltage drop calculations is that most 90°C-rated conductors are only configured for a maximum of 75°C operating temperature.

Both Chapter 9, Table 8 and 9 values are based on 75°C operating temperature. K = R × kcmil...

And Note 2 to Table 8 shows us the formula for adjusting resistance to a different operating temperature.

:slapheadNote to self--remember to read the notes!!!)

Thanks Smart $--NOW it makes sense--saw the 75C but didn't correlate it in my head to what I was asking--and Note 2 answers the rest--OK My head doesn't hurt anymore and I can answer his question!!

12.9 is a short cut to the RID method and THAT is a shortcut to using the actual resistance at the operating temperature!

Thanks folks for all the info everyone!!!