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# Wiring Fundamentals: Sizing the Fuses

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## Sizing the Fuses

### 01.51

 00:00 - Much like we determined how many relays our EFI power supply system required by breaking the design down into sections defined by when they were to be powered, we determine the number of fuses the power supply system requires by further breaking down these sections into what we refer to as circuits. 00:16 Each circuit requires a fuse which is put in place to act as a safeguard in case something goes wrong with that particular circuit. 00:23 As the fuses are only there in case of a problem, they must be sized to not interfere with the EFI system's normal operation. 00:31 I size fuses so they will fail at around 150% of the largest current we would expect to see in the circuit they're protecting. 00:38 This might sound slightly dangerous as it's possible we've sized our wiring to handle a current below this level. 00:45 However in the instance that something goes wrong, it's takes appreciable time for the wire to heat up to a point where it would be dangerous and the fuse will have failed long before this happens. 00:55 A good example of fusing can be illustrated using fuel injectors. 00:58 Our enable power relay will supply power to both the injectors and the ignition coils. 01:04 We would break the supply out to two individual fuses, one for the injector supply circuit and one for the ignition coil supply circuit. 01:11 If we look at an engine with four injectors, each drawing two amps, we could expect the injector circuit to draw a maximum total current of eight amps. 01:20 The optimal fuse size for this circuit would be 1.5 times this current draw, giving us a fuse size of 12 amps. 01:27 The next largest size of commonly available automotive blade fuse is 15 amps. 01:32 So that would be our choice to protect the injector supply circuit. 01:35 Breaking the power supply from the relay down into individual circuits like this is also very useful when it comes time to troubleshoot a problem.