Using Q in the voltage drop calc

[Mark Stone]

I am preparing a calculations training for several of our electricians. I wanted to go a little deeper into voltage drop and give them background into how it was developed. This is when I came across an article on the EC&M website that states that K needs to be adjusted for cable larger than 1/0. The adjustment is achieved by multiplying K by Q. Q is found by dividing the AC resistance by the DC resistance. I wanted more than one source, before putting this information out in training. I see there are a few posts in the forum that references this adjustment. Can anyone provide me with another reference other than the EC&M website? I have posted the link to the article below.

Code Calculations

Last month, we covered how the NEC addresses voltage drop through a discussion of six Fine Print Notes (FPNs) and several specific rules. This month, we’ll review examples demonstrating...

www.ecmweb.com

 

[wwhitney]

Well, instead of using KQ/CM in the formula, you can just use the resistance value for the wire size from NEC Chapter 9 Table 8 (DC) or Table 9 (AC). You can see for yourself that for large conductors the resistance values differ. And as the article notes, Q would just be the ratio of those two values.

It would be an interesting exercise to take the DC resistance values in NEC Chapter 9 Table 8 and use them to back-calculate a value of K at 75C to confirm that in fact the resistances given are inversely proportional to the cross-sectional area.

Note that in doing this comparison, you only have a chance of getting complete agreement when comparing wires with the same number of strands. You will see in Table 8 that when a given AWG is available with 1 strand or 7 strands, the DC resistance is higher for 7 strands. That's because the total cross-sectional area is the same with 7 strands as with 1 strand, but with 7 strands, the outer strands spiral around the inner strand, and so are all slightly longer than the inner strand, and thus have slightly higher resistance.

Cheers, Wayne

 

[Mark Stone]

That would be neat little exercise. If I ever get a break in the action, I will have to break out Excel and input the numbers for multiple sizes. I still would like to have more than one source or confirm that EC&M is a reliable source before putting the information out in a training.