How big should my fuse be?
One of the biggest questions we see are relating to how big a fuse should I use for my stereo or fog lamps?
You will need 2 values to find this out. The wattage rating of your lights or stereo and the voltage it runs at.
Converting Watts to Amps is very simple and makes a safe installation
We’ll do this a little backwards to get to the good stuff first.
Were using 12 volts as a safety buffer, true running voltage of a modern automotive system is 13.8 volts, you will see below.
The conversion of Watts to Amps is governed by the equation Amps = Watts/Volts
For example
@12 V
12 watts/12 volts = 1 amp
300 watts/12 volts = 25 amps
@13.8 V
12 watts/13.8 volts = .869 amp
300 watts/13.8 volts = 21 amps
Wiring theory
You cannot convert watts to amps, since watts are power and amps are coulombs per second (like converting gallons to miles). HOWEVER, if you have at least least two of the following three: amps, volts and watts then the missing one can be calculated. Since watts are amps multiplied by volts, there is a simple relationship between them.
However, In some engineering disciplines the volts are more or less fixed, for example in house wiring, automotive wiring, or telephone wiring. In these limited fields technicians often have charts that relate amps to watts and this has caused some confusion. What these charts should be titled is “conversion of amps to watts at a fixed voltage of 110 volts” or “conversion of watts to amps at 13.8 volts,” etc.
Explanation
Amps are how many electrons flow past a certain point per second. Volts is a measure of how much force that each electron is under. Think of water in a hose. A gallon a minute (think amps) just dribbles out if it is under low pressure (think low voltage). But if you restrict the end of the hose, letting the pressure build up, the water can have more power (like watts), even though it is still only one gallon a minute. In fact the power can grow enormous as the pressure builds, to the point that a water knife can cut a sheet of glass. In the same manner as the voltage is increased a small amount of current can turn into a lot of watts.
