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Automotive Fuse Amp rating selection is really just based on current, the doctrine is the fuse is there to protect the wiring not the load, but normally you will find manufactures size fuses based on the current the circuit actually draws. The tricky part in choosing the correct fuse to allow for the extra in-rush current at switch on. A motor or lamp, solenoid or lamp bulb will draw a much higher current at initial turn on than when running continuously. To some extent fuses manuafacturers allow for this by using a "slow blow" fuse design but you will find that say fuel pump circuit is designed and fused for a20 or 25amp current but normally runs at only 6 amps after start up.
Automotive fuses ae rated for up to 48v , fuses designed for mains voltages only differ in design for safety reasons they have to blow cleanly at higher voltages without arcing or exploding, this why 230v mains cartridge fuses are filled with sand.
" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)
Last edit: 8 years 8 months ago by Andy.MacFadyen.
Thanks for your reply Andy.
I understand that, how does one get through the "tricky" part?
Im new to wiring and circuits and I want to learn more about it..
For example, if I have to rewire a car from the 80s, most of them dont have relays for headlamps which I think is the common cause of the headlamp switch burning out/melting..?
So if I want to add a relay to that circuit (headlamps), how do I choose the correct size (amperage) relay and fuses?
Also, how does one know how much load a certain gauge of wire can handle before melting/burning?
A normal (non-uprated) head light bulb draws between 4 and 5 amps so 10 amps per continuous current for a pair of lamps .
Non-highway legal bulbs might draw 6 to 10 amps continuous each.
Standard automotive "Bosch" style 4 pin relays are fairly cheap and you won't pay much if anything more for a 30 or 40 amp relay than a 20 amp so go for the higher current rating . You will find 30 amp relays on the market that have built in blade fuse holders which makes wiring up simpler and neater.
" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)
Last edit: 8 years 8 months ago by Andy.MacFadyen.
Good quality relays have a built in diodes with these it is important the power +ve is connected to terminnal 30 and th load connected terminnal 87. The diode is there to quench the arcing at the contact points especially on inductive loads like fuel pumps and solenoids.
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