Before we start wiring up our projects, let’s have a look at and try to gets to grips with, a basic understanding of electricity.
The first thing we need to understand is that there’s no such thing as “volts” of electricity.
Voltage is like the pressure in a water pipe -when the tap is turned off no water flows = “no electricity”.
You can feel the pressure if you stick your thumb over the end of the tap and turn it on -it tries to push your thumb away, but if you press hard enough, no water comes out -that is, you’re “resisting” the flow of water, which is the same as a “high resistance” in electrical terms, stopping an electric “current” from flowing.
If the tap is turned off, we have lots of pressure, (voltage), but no water flow, (current), in the pipe feeding the tap.
When you turn the tap on, the water starts to flow in the pipe -and this is the same as electrical “current” flowing in a wire.
Yeah, I know this is about electricity and not plumbing, so let’s use these ideas with electricity.
Take a car battery for example.
The “pressure” is the 12 volts of electrical “pressure” that exists across the battery terminals, but if nothing is connected to the battery, no electrical current can flow because although the “pressure” or voltage is there, there is nowhere for the “flow ” to go, unless you make a path for it across the terminals. O.K, so what if we make that path by attaching a 21w lamp or bulb to the battery, with two wires, one on the + side and one on the - side.
Now we have 12volts, (pressure), supplying 1.75 amps, (current flow), to the 21 watt, (power), lamp.     
So to sumerise, we use Volts, (pressure), Amps, (flow), and Watts (power), when talking about electricity.
If you’ve read this far, I guess it means I haven’t bored you to death and you managed to make sense of my explanation. You may even be asking "how do you know it’s supplying 1.75 Amps?" For that, I can thank a clever man who lived a long time ago and came up with a mathmatical equasion to help me out, and that is: Amps=Watts/Volts.
So you can work out the amp rating of anything as long as you know the wattage and voltage. Of course, if I was using 2, 21w lamps, the Amperage would double).
This becomes useful when you start your wiring project because it allows you to select the correct cable and fuse sizes too.
Now you know how to work out Amps, you can select your cable. The cable needs to be able to carry the correct Amperage for your consumer unit, (the thing you’re wiring up). Never use a cable that’s not at least the same Amp rating as the consumer. Always go to the next size above what’s needed rather than smaller. Even if you fit the correct rated fuse to a smaller cable, the cable will get hot, the insulation will melt and if you’re lucky the fuse will blow. If you’re unlucky, you could end up with your pride and joy bursting into flames.
Staying with my example of a 21w lamp, I could quite happily use the smallest cable, 0.65mm2, as that has a rating of 5.75Amps, and a single lamp will draw 1.75Amps, (A= W/V).
If however I were wiring a twin headlamp system using a 60w lamp in each, I would need cable capable of handling 10A, (60w/12v=5a x2 lamps = 10A). For this I’d have to use the 1.5mm2 cable, which can take up to 12.75A, (always go bigger).
Of course, both these circuits would have an appropriate fuse fitted too.

-Tbone