How much voltage does a wire consume?

 Normally, we can ignore the resistance in electric wires, such as the little leads of wire that stick out of resistors, because it’s trivial. However, if you try to force large amounts of current through long lengths of thin wire, the resistance of the wire can become important.

How important? we can use Ohm’s Law to find out.
Suppose that a very long piece of wire has a resistance of 0.2Ω. And we want to run 15 amps through it. How much voltage will the wire steal from the circuit, because of its resistance?
 Once again, you begin by writing down what you know:
R = 0.2
I = 15

 We want to know V, the potential difference, for the wire, so we use the version of Ohm’s Law that places V on the left side:

V = I × R

 Now plug in the values: V = 15 × 0.2 = 3 volts Three volts is not a big deal if you have a high-voltage power supply, but if you are using a 12-volt car battery, this length of wire will take one-quarter of the available voltage. Now you know why the wiring in automobiles is relatively thick—to reduce its resistance well below 0.2Ω.