Ron,

A grounded conductor is still a conductor as was pointed out by Stan. You have to count the three conductors? Wouldn't you considered the 120 to be a derivative of the 240?

The 120 VAC represents a line to Neutral, the 240 is the Line to Line connection.

What is your formula for Ed?

Don,

Formula method:
2KId/CM
2=Two lenghts of wire to the load
K=12.9 for copper
I=Amperage
D=Distance
CM=Circular Mil

You can also use Ohm's law to get VD.
Yet, in this case you would need the conductors resistance.

The formula would be:
I(R)=VD

Voltage drop is only recommended by the NEC, but is not required. What's recommended:
-Max combined volatge drop for both feeder and branch circuit should not exceed 5%
-Max on the feeder or branch circuit should not exceed 3%

So this means. The conductor volatge (feeder only) drop on a 120V source would be: 120(.03)=3.6V then 120-3.6=116v recommended for the feeder.
If 240, then: 240(.03)=7.2V then 240-7.2=233V recommended

I use Lectri-Calc for voltage drop.

Much as I like math, the math to figure VD it too tedious for me. :D

A lot of simplifications come from 2 basic equations:

R = pL/A; Resistance = Resistivity*Length*(Cross sectional area) and
V= IR

p=16.8 nΩ-m; yep nano-Ohms-meter

Don:

This should give you an intuitive idea as to why you would not consider the conductors as 3. I made the math EASY by saying that each conductor has 1 ohm of resistance.
Note that when a 240 volt load is connected you are dissipating 800 W and when a single 120 V load is connected your also dissipating 800 W. And when you allocate between all three conductors only 350 watts are dissipated.

So, you can hopefully see that you must use 3% of 120 V because of case 2 which is the worst 120 V load case. In case 1, you would take 3% of 240, Case 2 and 3, 3% of 120.
In case #1, assume the neutral wire isn't run.

.... Amp...0hms......Power (I^2*R)
L1... 20... 1... 400
L2... 20... 1... 400
N... 0... 1... 0
Ptotal......................800 W Case 1

L1... 20... 1... 400
L2... 0... 1... 0
N... 20... 1... 400
Ptotal......................800 W Case 2

L1... 15... 1... 225
L2... 5... 1... 25
N... 10... 1... 100
Ptotal .......................350 W Case 3

What this says is that with a 20 A protection device on a dual breaker, you cannot exceed 800 W.

Remember used 1 ohm to make the math very EASY.