Chapter 9 tables 8 and 9

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mshields

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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
 

jim dungar

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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

Senior Member
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.
 

charlie b

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. . . 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

Senior Member
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.
 
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