Understanding wire thicknesses

I just want to have a discussion about wire thicknesses. Not being really knowledgeable about this subject, i wanted to get some thoughts on it.

I was running about 8 servos and delivering power through spare servo wire from some AA batteries. The signal wires were taken from an old usb cable, it having some really thin wires inside it. All was well.

I added extra servos to my project to give 13 and the servo's seemed to loose power. I couldnt understand why. I replaced the main wire that came from the battery with a much thicker one, and this seemed ok.

My question therefor is, is a thick wire always better? What are pro's and cons. What if a thick wires was used for something like the signal wire or to deliver a low current @ 3.3v?

Think of wire like a water hose. If you only need a drip at a time, then a very small hose will do. If you need to fight a fire, then you need a lot of water so you would use a large hose.

A small hose offers a lot of resistance to the flow through it while a large hose offers minimal resistance.

In your case current is like the water, when you need only a little bit if current, thin is ok. As you need more current you need to have a larger wire to carry it without excess resistance reducing the flow.


Sounds like it's the AA batteries that are unable to deliver the current needed. Peak is about 1A and that will result in a lower output voltage and pretty short life.

Wire is just another resistor, so you do V=IR calculations if necessary. The resistance
of copper wire is about 1.7x10^-8 * area / length (all in metres).

With area in sq mm and length in metres that becomes 17 milliohms * area / length
(in other words a 1 sq mm wire 1m long has a resistance of 17 milliohms.

You have 8 servos, lets say they pull 1A each at 6V, so the combined load is 750 miiliohms
(6V/8A), and a wire of 17 milliohms won’t be a problem. If the wire was 300 milliohms, say,
then you’d be losing a lot of power in the wires and have problems. Keep the wiring
resistance < 0.1 times the load resistance (or better).

Note that connectors can have significant resistance too.

So the lessons are:

thickness and length matter - if in doubt calculate.
Each grade of wire has a maximum current it can take without overheating, check this too.