It says this in the book (for a 2-wire dc circuit, 2-wire ac circuit, or 3-wire ac single-phase, at 100% power factor) where reactance can be neglected:

Vd = (2 x L x R x I)/1000

Vd = voltage drop
L = length (one way) of circuit
R = resistance in ohms
I = load current

How do you derive that simply from V=IR or V=IZ (for ac current)?

Also,
1. What is a power factor?
2. How do the 2-wire and 3-wires factor in the equation?
3. Where did the 1000 come from?
4. Does anybody know how material type and phase affect the equation?

 

The R is not resistance, it is ohms per thousand feet.
The length is in feet.

So (L/1000ft) gives the number of thousands of feet. Multiplying that by R gives the number of ohms going one way. The 2 is to get the round trip total resistance.