NEC voltage drop definition over 3 wires (single phase 3 wire)

[lalver1]

Hi,
It's not clear to me from the NEC's definition how voltage drop should be calculated in this case (see diagram).
I can find the drop across the cables serving the 2 loads (branch circuits) using the usual 2*I*R formula for each circuit and dividing by 120 for the first load and by 240 for the second. The sum of both drops should be less than 3%. My problem is when I get to calculating the voltage drop (since it is a long run from the service point to the panel board) across the 3 cables. Should I calculate 3*I*R (and assume I to be the sum of both load currents) and divide that by 240? Or calculate 3*I*R (and assume I to be the sum of both load currents) and divide that by 120? Or is the correct formula different? In the end, the drop due to the cables supplying the loads and the cables supplying the panel should be less than 5%.
EDIT: I ran a quick LTSPICE simulation to get a feel for it, and it seems that to get the voltage drops I need to calculate:
R*(240load current) + R*(120load current) + R*(240load + 120load currents). This will give me the voltage drops for the cables serving the panel, but then, should I divide this sum over 120 or 240 to be consistent with what the NEC requires?

Thanks for your help,

Luis

 

[waross]

Calculate the voltage drop on line 1.
Calculate the voltage drop on line 2,
Calculate the voltage drop on the neutral.
The voltage across the largest load will be the greatest voltage drop plus the neutral voltage drop.
The voltage across the lessor load will be the lessor voltage drop minus the neutral voltage drop.
On long lines with a large difference in loads, the voltage across the lessor load will often be a voltage rise rather than a voltage drop.
Interestingly, this effect may cause metering errors when the voltage drops occur on long service conductors from the transformer to the KWHr meter.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[7anoter4]

If you said "single phase 3 wire" did you mean a.c.? On the second sketch it is only D.C. sources.
It is hard to understand how you calculated the currents per branch.
In my opinion, the second sketch could be transformed as per attached sketch.
The currents I1,I2 and I3 are partial currents and the current per branch it is the sum or difference according to the direction.
You can calculate the voltage supplied at RL1 and RL2-let say VL1 and VL2 and compared with the rated-let say 120 V.
Considering the supply voltage is rated also the voltage drop will be (120-VL1)/120 %.