What is the faster way to calculate the drop voltage?

please re-ask the question this is design or actual measuring?

first, if ti is a design use cable manufacturer catalogue exactly to determine the maximum voltage drop.

second, if ti is an existing project use voltmeter to determine the actual voltage drop.

Refer to the cable catalogue and multiply the specific resistance given for the specific cable with the length of the cable to get the voltage drop

check the approved catalog for cables & approved branch loads for all the systems by knowning the Routing method, V.D cab be calculated very easily & in a Quick manner. Use ECODIAL by Schienider or DOCWIN by ABB for Advance calculation.

"ohms to neutral per1,000 ft" is1.1 ohms. To complete the numerator, multiply as follows: (2 x0.866) x200 ft x1.1 ohms x20A =7620.8 Dividing7621 by1000 ft gives a voltage drop of7.7V. This drop is acceptable for our480V circuit. A No.12 would drop11.8V. Boost the length to500 ft, and that No.10 drops18V; the No.12 drops29V.

check the technical catalog of the cable manufacturer, they give you  the voltage drop per length unit, just multiply it with your actual cable length 

reading the cable manufactorer catalogue/data sheet 

Use Ecodial Free Software from Schneider

There is a formula you can apply to determine this

 

A current source has its voltage defined by its load. V=IZ.A voltage source has its current defined by its load. I=V/Z.

 

one of cable characteristic is voltage drop as mV/A.m, obtained from cable datasheet, multiplying this value in the actual length and in demand current ( net current after taking all design factors into account).

go through this link

http://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=8.286&voltage=24&phase=dc&noofconductor=2&distance=50&distanceunit=meters&amperes=10&x=38&y=10