Voltage Drop

[acrwc10]

When calculating voltage drop on a branch circuit, say #12-2 @120volt that runs 100', is it calculated at 100' or 200'. Because the run goes 100' out, then 100' returning on the neutral?

 

[tallgirl]

When calculating voltage drop on a branch circuit, say #12-2 @120volt that runs 100', is it calculated at 100' or 200'. Because the run goes 100' out, then 100' returning on the neutral?

How many wires have a voltage drop on them?

Answer that and you answer your own question.

 

[dcspector]

VD = 2 x Kfactor X distance "one way" X load divided by wire size. Or VD = load x resistance table 8 per 1000'......K factor cu 12.9

 

[acrwc10]

the reason I asked this is I was reading an article in EC mag. and they used the length on the hot and the neutral in an example for a run of 14-2 romex serving a 120volt load.

 

[dcspector]

Is that neutral or grounded conductor?

 

[crankshaft]

the distance one way is used. the 2 in the equation accounts for the return distance and the square root of 3 is used for 3 phase. as someone else said k/cma or ohms/ft can be used

 

[radiopet]

or you can visit www.electrician2.com and view a online VD calculator and make it easier on yourself....

 

[George Stolz]

I agree with Crankshaft. The 2 at the beginning of the equation accounts for both wires.

I use the formula Vd = 2 x R x I x D .

I never liked the K constant, I can't remember it very well, and I think it changes based on wire size involved, doesn't it?

 

[GeorgeB]

George, I like to remember as little as I must ... my voltage drop rule (amps in 3phase are the same as amps in single phase for voltage drop) is to remember 2 things in addition to ohms law, E=IR.

first, #10 copper is 1 ohm/1000 ft

second, every 3 sizes halves or doubles the area and thus the resistance. (#13 has 2 ohms/1000, #16 4 ohms, #7 0.5 ohms, #4 0.25 ohms, for example)

You can look at tables to get closer, but this serves quite well in my experience.

 

[dcspector]

Yep george! I know it was on My exam years ago....well not that long ago....K factor for copper 12.9 and al 21.2

 

[acrwc10]

If a 120v 12a load is on a #14 circuit 3% VD would limit the run to about 56ft..In residential work that is not a limit I see many stick to. The run around a single room is often that far.

 

[jeff126]

Vd = I x 2L x R (R = Values given in NEC chapter 9, table 8; divided by 1000)

for 3 phase:
Multiply answer by .866

 

[RB1]

I prefer the K factor method because I can arrive at the required wire size in the first calculation. Using the gauge resistance method requires an approximation that may result in having to plug in another number.

By the way, the K factor can be found in Table 8 by multiplying the area of the conductor by the resistance per thousand feet then dividing by one thousand.