#### Kishore25

##### Member

- Location
- New Jersey, USA

- Occupation
- Electrical Engineer

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

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- Thread starter Kishore25
- Start date

- Location
- New Jersey, USA

- Occupation
- Electrical Engineer

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

- Location
- West Chester, PA

Does Southwire distinguish between AC and DC?

- Location
- New Jersey, USA

- Occupation
- Electrical Engineer

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

- Location
- Berkeley, CA

- Occupation
- Retired

Cheers, Wayne

- Location
- New Jersey, USA

- Occupation
- Electrical Engineer

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

- Location
- Berkeley, CA

- Occupation
- Retired

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.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).

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.I am doing voltage drop calculation for 90 deg C

Cheers, Wayne

- Location
- New Jersey, USA

- Occupation
- Electrical Engineer

yes, my understanding about the reactance is the same. believe the reactance is not temperature dependent.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

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.

- Location
- Berkeley, CA

- Occupation
- Retired

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

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

- Location
- Wisconsin

- Occupation
- PE (Retired) - Power Systems

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

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

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.

- Location
- Berkeley, CA

- Occupation
- Retired

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

Cheers, Wayne

- Occupation
- Electrical Engineer Power Station Physical Design Retired

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.