With electricity we have Voltage and Current which combine to make Power and its power that makes things go. If you want to know how much Power you have simply multiply the Voltage by the Current!

Example: The Voltage is 12V and the Current is 5A  

The Power is 12 x 5 = 60 Watts

Yes-power is measured in Watts - This is what you ask for when you buy a 60Watt lightbulb

Now for another

Example: The Voltage is 240V and the Current is 5A  

The Power is 240 x 5 = 960 Watts

So by increasing the voltage we increase the Power delivered with the same current

You can probably work out that to get 960 Watts with 12V then you need 960/12 Amps or

80 Amps of current.

So whats the problem with lots of current? - well think of it as water flowing in a pipe, with just a small current (or flow of water) you can use a small wire or pipe, if you want a lot of current or flow of water you need a large wire or pipe.

What's actually happening is the wire or pipe has a resistance to the current or flow of water (imagine trying to blow through a thin drinking straw compared to a thick one, the thin one is much harder to blow through because it 'resists' the flow)

All wires have a resistance which is dependent on how long they are and how thick they are. If the wire gets longer its resistance gets bigger and if it gets thicker its resistance gets smaller. If you make it longer and thicker in the right amounts its resistance stays the same!

Right here comes the clever bit, think of the current as a flow of water, now think of Voltage as the pressure forcing the water through the pipe. With me so far? good - now because of the resistance of the pipe we lose some pressure along it so the pressure at the far end is less than the pressure at the start of the pipe depending on how much water is flowing.

So turn it into electricity - because of the resistance of the wire the voltage at the end of the wire is less than the voltage at the start of the wire depending on how much current is flowing.

This term Resistance is measured in Ohms and is signified by the letter R or Omega in the Greek alphabet - I will use R.

We have a very simple formula to calculate the voltage 'lost' along the wire

V (Voltage lost) = I (current flowing) x R (Resistance)

This is known as Ohms Law - and now you have everything you need to know to work out wiring

(except the resistance of copper wire!)

 

Before I give you that last piece of information, we now see why you need thick wires for big currents - thick wires means low resistance which means large currents with little voltage drop.

What's wrong with a voltage drop then? - well if you only start with 12V and you 'lose' 6V along the wire then you only have 6V left at the end - which means your lights are dim, or your fridge won't work - ideally you don't want to lose more than 0.5V in your wiring.

So back to the resistance of wire. Wire tends to be sold with different thicknesses (typically in mm²) - the resistance of 1m of 1mm² wire is 0.0168 Ohms. If its twice as long, its twice the resistance - ten times as long, ten times the resistance. Now if its thicker then its resistance is less, if its twice as thick then it's half the resistance - three times as thick a third the resistance.

Right the final example:

If I have 8 metres of wire, which is 6mm² thick and I want to pass 15A down it, what is the voltage drop.

Well - 8 metres of 1mm² wire would have a resistance of 8 x 0.0168 Ohms

which equals 0.1344 Ohms

but it is 6mm² thick, so the resistance is 0.1344/6 Ohms equals 0.0224 Ohms

The Current (I) is 15A

V (Voltage lost) = I (current flowing) x R (Resistance)

V = 15 x 0.0224 which equals 0.336 Volts

This is an acceptable drop in voltage

 

So, lesson over, we can work out wiring for a narrowboat for a 12V system, so why do we need 240V? - well if you want a microwave for example - typically they are 600W in power for a small one which at 12V would need 600/12 Amps in current = 50A. Our voltage drop in 6mm cable would now be 1.12 Volts which is not acceptable (you must remember that there are two wires to every appliance so the total volt loss is 2 x 1.12 Volts or 2.24 Volts). So we now need thicker cable, either 16mm or 25mm cable which is heavy and very expensive.

Lets do the sum for 240V - current is 600/240 = 2.5 Amps, voltage drop is 0.056Volts out of 240! - we can afford to drop the wire down to 1mm and still only lose the original 0.336 Volts.

So there you go - the reason for 240V

One last point - these volt losses, - you can't just lose them - they are dissipated as heat - Power = current x voltage. So if your losses in the cables are too high, not only will your appliances work poorly but your cables will get hot which is not good - 12V may not give you a shock but it can still start fires - so take care and get it right!!!

 

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