How do you calculate voltage drop in a cable?

The answer to this question is simply ohms law. For most cables the resistance of the cable per meter will be defined by the manufacturer, so firstly multiply this by the length of the cable you are using.

Then apply ohms law V=IR. This will give you the voltage drop across the cable.

In most cases for domestic use, this value of R is so small the voltage drop is negligble, though using a cable with the lowest value R will give you the lowest voltage drop for any application.

If you're referring to a voltage drop in an Electrical Network the calculation becomes abit more complex and involves solving some implicit and explicit equations (assuming you know the voltage and power factor at the sending end and the real and complex powers consumed at the receiving end). If you wish to know more about this calculation let me know.

THIS CONSIDERATIONS SHOULD BE STRICKLY FOLLOWED: SOUND ENGINEERING AND ACCEPTABLE ECONOMICS;

1.DEFINE DEMAND LOAD - AFTER APPLYNG POWER FACTOR AND EFFECIENCY FOR MOTOR LOADS AND ACTUAL SYSTEM VOLTAGE AND PHASE.

2. ONE WAY LENGTH OF WIRE OR CABLE - FT. (AWG) OR MTR. (SQ.MM) SELECTION

3. MULTIPLY BY 125% TO THE CALCULATED LINE CURRENT TO GET AMPACITY REFERENCE

4. LOOK OR SELECT THAT NEAREST RESULT TO ANY LOCAL CABLE SUPPLIER CATALOGUE OR NEC TABLES AND MATCH IT TO CORRESPONDING CABLE SIZE.

5. APPLY DERATING FACTORS - ALSO AVAILABLE TO NEC OR CABLE SUPPLIERS AND DEFINE THE FF:

A. MEDIUM OF INSTALLATION - PVC OR RSC CONDUIT, CABLE TRAY OR CABLE LADDER OR TRUNKING, THROUGH, ETC.

B. AMBIENT TEMPERATURE - INDOOR OR OUTDOOR?

C. GROUP FACTOR - CONSIST OF CABLE SPACING AND DISTANCE TO WALL OR INSIDE CONDUIT INSTALLED IN TRAYS OR UNDERGROUND

6. APPLY DERATION IN WHICH RESULT IS LESS THAN ONE (1) TO THE PRE-SELECTED CABLE - THIS IS NOW YOUR ACTUAL FOR SELECTED CABLE

7. DEFINE REQUIRED OR ACCEPTED VOLTAGE DROP IN PERCENT VALUE

8. NOW USE THIS FORMULA:

VDapp(APPLICABLE VOLTAGE DROP) = %Voltage Drop x Actual Voltage x 1000 divided by Derated Current x One Way Length x 100

9. THE RESULT WILL HAVE A UNIT OF -- mV/A.Mtr OR MILLIVOLT PER AMPERE-METER

10. NOW COMPARE IT TO AVAILABLE TABLES FOR VOLTAGE DROPS OF CABLES - IF IT IS LOWER THEN SELECT THE NEXT SIZE OR WHICH EVER MEET THE REQUIREMENT.
you'll need to know the resistance of the conductor. copper has a certain amount of resistance per length. there are charts available on this