Voltage Drop Calculation - Table 8 vs table 9

[Kishore25]

For voltage drop calculation, do we take resistance value from Table 8 or from Table 9.

Online southwire uses, resistance value from table 8 and reactance value from table 9 of the NEC to calculate the voltage drop

 

[JoeStillman]

Table 8 is DC resistance. Table 9 is AC. I think they are different because of skin effect. AC current density is higher as you get closer to the outer surface of a wire.

Does Southwire distinguish between AC and DC?

 

[Kishore25]

it does differentiate. One of the dropdown selection is for AC current or DC current

 

[wwhitney]

Note that the Southwire voltage drop calculator currently has a bug when used for power factors other than 0.9. The coefficient of the resistance that goes into the "effective impedance" is properly updated based on the power factor entry, but the coefficient of the reactance is not. You can see this most easily by choosing power factor 1 and power factor 0 and comparing the results. They should be in the ratio of resistance to reactance, but they are not.

Cheers, Wayne

 

[Kishore25]

Thanks Wayne.
So then, for change in resistance due to temperature, i am using the formula from note2 in table 8. Is there a formula to calculate the reactance change due to the temperature (from 75 C to 90 C).

I am doing voltage drop calculation for 90 deg C

 

[wwhitney]

So then, for change in resistance due to temperature, i am using the formula from note2 in table 8. Is there a formula to calculate the reactance change due to the temperature (from 75 C to 90 C).

My understanding is that the reactance is not temperature dependent, at least to any significance. But I don't have the qualifications to say that definitively.

I am doing voltage drop calculation for 90 deg C

Are your conductors actually going to be operating above 75C? For example, if your conductors are limited to their 75C table ampacity by terminations, and there is no ampacity adjustment or correction required, you can safely say they will be operating at 75C or less. Whereas if the 90C table ampacity is the limiting factor in conductor selection, due to the necessary ampacity adjustment and correction, then there is no guarantee the conductors won't be operating above 75C, so correcting the resistance would be conservative.

Cheers, Wayne

 

[Kishore25]

My understanding is that the reactance is not temperature dependent, at least to any significance. But I don't have the qualifications to say that definitively.

Are your conductors actually going to be operating above 75C? For example, if your conductors are limited to their 75C table ampacity by terminations, and there is no ampacity adjustment or correction required, you can safely say they will be operating at 75C or less. Whereas if the 90C table ampacity is the limiting factor in conductor selection, due to the necessary ampacity adjustment and correction, then there is no guarantee the conductors won't be operating above 75C, so correcting the resistance would be conservative.

Cheers, Wayne

yes, my understanding about the reactance is the same. believe the reactance is not temperature dependent.
The requirement for the project that i am working on is to use XHHW-2 90C conductors and i am using the ampacity table under 90C in 310.15. so, i am calculating the change in resistance value at 90 C using the value at 75C from table 9.

 

[wwhitney]

The requirement for the project that i am working on is to use XHHW-2 90C conductors and i am using the ampacity table under 90C in 310.15. so, i am calculating the change in resistance value at 90 C using the value at 75C from table 9.

OK, say you are using 1/0 Al, which has a 75C ampacity of 120A and a 90C ampacity of 135A.

If you are using it with at most 3 CCCs in a conduit or cable, at an ambient temperature of 30C, and you have a continuous 120A load, then the operating temperature may be as high as 75C, and using the 75C resistance is appropriate. This is an application where you would have been fine with a 75C rated insulation temperature, so obviously it won't be getting above 75C.

Whereas as if you have the above conditions but in the controlling case for voltage drop you know the current will only be 100A, now you know that the conductor operating temperature will be at most 30C + (100/120)^2 * (75C - 30C) = 61C, so if anything you could adjust the resistance down from the 75C resistance.

Conversely, if you are using the conductor with say 6 CCCs in a conduit or cable, and the ambient temperature is elevated so you have a temperature correction factor of 0.9, now your ampacity is limited to 135 * 0.8 * 0.9 = 97A. If it is really carrying 97A, now the conductor temperature could be as high as 90C, and correcting the resistance to a 90C value would be appropriate.

Cheers, Wayne

 

[jim dungar]

For voltage drop calculation, do we take resistance value from Table 8 or from Table 9.

Online southwire uses, resistance value from table 8 and reactance value from table 9 of the NEC to calculate the voltage drop

I don't know where the NEC values came from, I just know they are difficult to back into.
I have usually used other sources to perform my voltage drop and short circuit analyses, such as software from SKM or E-Tap, though some of them may have been based on these NEC tables. My favorite manual method was from Shawmut fuses and was based on IEEE values.

 

[wwhitney]

I don't know where the NEC values came from, I just know they are difficult to back into.

One thing that confused me initially for the DC resistance values in Table 8 is why the resistance depends on the number of strands. And the answer is that with 7 strands, the outer 6 strands are a bit longer than than the center strand, because those outer strands twist around the center strand. So even though the copper cross-sectional area is the same for 1 strand as for 7 strands, the 7 strand resistance is slightly higher.

Cheers, Wayne

 

[Julius Right]

Table 8 the resistance shown it is for 75oC d.c. indeed.
Table 9 the resistance shown it is for 75oC a.c. -that means skin effect and proximate effect -for 3 conductors single core in cradle formation and 60 Hz- was considered.
Calculations are according Neher and Mc Grath -or IEEE 835/1994.