Chapter 9 tables 8 and 9

[mshields]

If I were to do a voltage drop calculation I would use Table 8 and plug in the resistance values listed under the column for Copper/Coated/ ohm/kFT. Do you agree that this is the correct source? Or should I be using Table 9?

Assuming I've been using the right table all these years, I believe that voltage drop is a function of resistance only and has nothing to do with reactance. Is that true.

Anyway, what would one use Table 9 for?

Thanks,

Mike

 

[Dennis Alwon]

I have never used Table 9 but I guess you could. Most just use the online calculator or use the formula with Table 8.

 

[jim dungar]

I have always used a VD method that, at a minimum, took power factor into account. Honestly I can remember using the values from NEC tables only a few times in my entire career.

 

[steve66]

I use an excel spreadsheet to calculate voltage drop, and I think the values for resistance and reactance that it uses comes from Table 9.

Table 8 is a DC resistance, and it looks like the value for #12 is slightly lower than the AC resistance given in Table 9.

Also, Table 9 (and my spreadsheet) allows you to consider the type of conduit.

So I would use table 9. And I'd put it in a spreadsheet to automate the process, or use one of the online calculators like Dennis suggested.

 

[kingpb]

According to Southwire's Power Cable Manual - when calculating VD for ac circuits, you must consider resistance and reactance as well as power factor of the load.

 

[charlie b]

. . . I believe that voltage drop is a function of resistance only and has nothing to do with reactance. Is that true?

Here I must disagree. I believe that VD through a wire will be equal to the current flowing through it times the overall impedance (resistance and reactance combined). I usually use the "effective Z" column in Table 9 for manual VD calculations (which I try to avoid having to do :happyyes: ).

 

[steve66]

Here I must disagree. I believe that VD through a wire will be equal to the current flowing through it times the overall impedance (resistance and reactance combined). I usually use the "effective Z" column in Table 9 for manual VD calculations (which I try to avoid having to do :happyyes: ).

I agree completely.

Not that I would ever include this in a voltage drop calculation, but a capacitive load could cancel out the inductive reactance of the line. This would be similar to a series resonant circuit. It is even theroetically possible to have more voltage at the load than at the source.