VD calculations per different publications

[Designer69]

hey guys, different publications (ie, NEC, IEEE) have differrent methods of obtaining voltage drop right? for instance their equations vary.

I just wanted to get confirmation on this. Thank You

 

[dkarst]

hey guys, different publications (ie, NEC, IEEE) have differrent methods of obtaining voltage drop right? for instance their equations vary.

I just wanted to get confirmation on this. Thank You

I guess the shortest possible answer to this is "yes, confirmed" but I'm not sure that's what you're looking for.

Each method/equation or on-line tool usually makes some simplifying assumptions that hopefully are noted so the user verifies they are valid or not... for example, for small conductors the ratio of inductive reactance to resistance may be small and you can ignore the reactance. For other applications where the conductor may be large and the power factor may be poor, you absolutely need to consider the reactance and load power factor using IEEE method 141 or you may have an unpleasant surprise.

IEEE phase-to-neutral voltage drop = I?(R?cos(?) + X?sin(?)) and even this is an approximation with a known error, although usually very small.

I guess something to keep in mind is there is only one "correct" answer... it is up to the user to determine whether the answer they got is "correct enough" and IMHO, a lot of the simplifying assumptions aren't needed these days with spreadsheets/apps etc. that can crank out the exact solution.

 

[steve66]

I guess something to keep in mind is there is only one "correct" answer... it is up to the user to determine whether the answer they got is "correct enough" and IMHO, a lot of the simplifying assumptions aren't needed these days with spreadsheets/apps etc. that can crank out the exact solution.

For any feeder calculations, you have to make assumptions about how much of the load is on at once, and you have to make assumptions aobut what the power factor will be. And there will even be a mix of single and three phase loads which may make the currents different on the line and neutral wires.

So maybe we can be exact on branch circuits, but I don't think there is much point in trying figure an exact voltage drop on feeders.

 

[dkarst]

For any feeder calculations, you have to make assumptions about how much of the load is on at once, and you have to make assumptions aobut what the power factor will be. And there will even be a mix of single and three phase loads which may make the currents different on the line and neutral wires.

So maybe we can be exact on branch circuits, but I don't think there is much point in trying figure an exact voltage drop on feeders.

Of course you're correct that carrying the Vd % to four decimal places when the load varies daily/seasonally/whatever 25% is nonsensical, I was really trying to point out that we should be careful when applying tools/equations that use simplifying assumptions when they are not valid (or sometimes even noted).

 

[Designer69]

ok, so would the voltage drop acceptance criteria then be per the calculation publication used?

 

[dereckbc]

Well after many years of running calcs, and then actually measuring I have found they are all made by copper companies and very liberal with the copper. I have my own in which I use that is more realistic which you will recognize: You will find it saves money and works if you input real data. Real data mainly being the load current.

CM = [22.2 x I x D] / VD

Where:

CM = Circular Mills of copper
I = Load current in Amps
D = Distance 1-way
VD = Voltage Drop in volts.