A very basic introduction to some simple maths that allow you to work out power, current and resistance.
In some of the guides I have included calculations for working out current, resistance etc. A basic understanding of how to do this will help when fault-finding or deciding on the correct cable size when adding accessories. If you have little or no electrical background it can get confusing, so hopefully this guide will make it clearer.
Lets look at the four key terms:
Voltage – measured in Volts, Symbol ‘V’
Current – measured in Amps, Symbol ‘I’
Power – measured in Watts, Symbol ‘W’
Resistance – measured in Ohms, Symbol Ω (Omega)
So how are these four things related?
You don’t need to know the theory behind the formulas, just remember these two triangles:-
Each triangle shows the three most used formulas that you will need. There are a few more and I’ve included a copy of the “Electric Wheel” at the end. (My old Physics teacher told us an easy way to remember them “The Vatican is Roman Catholic” and “The Pope lives in the Vatican and he’s Catholic” – I meaning Current)
As we usually know two parts of the equation, it’s easy to work out the third.
Lets work out the current for a 12 volt television. The rated power for the television found from the manufacturers data is 65 Watts. We know the voltage is 12 volts so using P / V = I we can now work out the current:
P 65 Watts / V 12 Volts = I 5.41 Amps
If we were to measure the current using our multimeter, we probably wouldn’t get a reading of 5.41 Amps. The reason is it’s highly unlikely that the voltage is exactly 12 volts. If we are starting with a fully charged battery, the voltage might be 13.5 Volts, so working it out again for the corrected voltage we get:
P 65 Watts / V 13.5 Volts = I 4.81 Amps
We can see that the current drawn is less. What’s going on? Well manufacturers usually give a ‘nominal’ voltage rating, in this case 12 volts. That’s what they have based their calculations on. So that’s what we will use. When an Electrical Engineer designed the wiring for your caravan they will have used the ‘nominal’ voltage to choose the correct size cable and fuses for various circuits, with a little safety factor built-in of course.
Lets look at another example. The 12 volt accessory socket (cigarette lighter) has a fuse rated at 10 Amps – What wattage appliance can I plug in?
This time we will use V x I = P so:
V 12 Volts x I 10 Amps = P 120 Watts
A device with a maximum rating of 120 Watts can be used. Again we used the ‘nominal’ voltage for this calculation. If we used the actual fully charged battery voltage we would get 13.5 x 10 = 135 Watts which if we used as the battery voltage dropped would overload the circuit – P / V = I or 135 / 12 = 11.25 Amps so greater than the 10 Amp fuse.
Working anything out for mains appliances is the same. How much current does a 1.2Kw kettle draw? (1.2Kw = 1200 Watts)
P 1200 Watts / V 240 Volts = I 5 Amps
You are on a 10 Amp EHU – How many Watt’s is that?
V 240 Volts x I 10 Amps = P 2400 Watts
Your fridge has stopped working on 12 volts and you want to check the 12 volt heating element is OK by measuring the resistance. The information for the element says it is 12 volts, 170 Watts so we need to calculate the resistance in order to check it. First we need to work out the current:
P 170 Watts / V 12 Volts = I 14.1 Amps
Now we can work out the resistance by using:
V 12 Volts / I 14.1 Amps = R 0.85 Ω Ohms
Now that was long winded and required two calculations. If you look at the Electric Wheel below, you can se we could have used V² / P = R
V² (12 x 12) / P 170 Watts = R 0.85 Ω Ohms
So you can now measure the resistance of the heating element and you have a calculated figure as what to expect.
It’s as easy as that. However a word of caution. When you calculate wattage, resistance or current, these should be guides. In the real world things are not always to tolerance and you can get slight variations, so treat you calculated result as a guide and not an absolute.
There are various adaptations of this available on the internet, but this one shows all the basic formulas clearly…