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Using Q in the voltage drop calc

Merry Christmas

Mark Stone

Member
Location
Sioux Falls, SD
Occupation
Trainer
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.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
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

Member
Location
Sioux Falls, SD
Occupation
Trainer
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.
 
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