is there a formula for fuses???

[Rough60]

just wondering if there is a way to really work out what fuse to run?
you could just run a 30A fuse on everything to stop truck burning but,
for example, 1 100W spottie pulles about 8.3A, it's halogen so should a 10A slow burn fuse or 15A normal fuse be used, 85W spottie pulls 7.0somethingA, should the closest fuse (7.5A) be used or double or what?
cheers

[Bartso]

well all fuses are for is exactly that to stop your 4by from burning down

no use running cable that can handle 4amps and have a 10amp fuse on it

so in otherwords put the fuse on that will protect your cable that you have

run

[Rough60]

true bartso, but i don't want to damage my fridge (2.5A) or keep blowing globes in spotties.
I thought a fuse should be the weakest link?

[Bartso]

well its all in contrast really you don't run a cable that can handle 50 amps for a device that only pulls 10 amps then put in a 50 amp fuse so if your spotties pull 7.5 amps run a cable that can handle 10 amps along with a 10 amp fuse allowing a little extra like this won't hurt any device if your fridge pulls well over what it should be something is seriously wrong your fridge should have a fuse on it already but you should put another one on as well

also fuses aren't really designed to protect equipment its there to protect the wiring

hope this has made a little sense

[Top Cat]

Bartso is spot on correct.

The fuse is there to protect the wiring.........not the electrical item.

If the electrical item draws say 10A then it is pointless running cable that can only handle a max 10a as you would need to protect the cable with a 7.5a fuse, which will blow simply because your item is gunna try and drag 10a.

Hence you would need to use a cable that will allow u to use say a 15a fuse.

The fuse is there to protect the wiring only.

Obviously you should also therefore place the fuse at the very start of the run.

[totto]

.........but i don't want to damage my fridge (2.5A) or keep blowing globes in spotties.
I thought a fuse should be the weakest link?

Amperage is the amount of current the item, i.e the fridge of the spotties consumes, or "let's through" the item.
Voltage is what'll kill your fridge and globes if that is too high, but that is limited by the voltage regulator in the generator, not by fuses.
And yes, fuses should be the weakest link in a circuit. It should blow before wires get toasted from overload or shortcut, but should be large enough to handle small surges of power conumption on fridge startup etc.

[murcod]

This has been discussed before in great length- do a search.

Fuses close to the battery are there to protect the main cables from shorting out; then they branch off into smaller cables via more fuses (usually under the dash) that are rated to protect both the cable and the load (or device) hanging off the end of the cable.

If you don't believe me then check under the dash of any vehicle - the fuse ratings directly relate to the device hanging off the end of the wire. Often the wire guages will all be the same, but the fuse value won't.

When selecting fuse values don't forget a number of devices can have a large in rush current when first turned on. So never select a fuse that is exactly the same rating as the rated current drawn by the device, go to the next value. For example a car radio may draw 5 Amps, a 7.5A fuse would be a good choice; two 12v 100W spotties would draw around 8 Amps each, so fit a 20 Amp fuse (or possibly 25Amp.)

You can also run large diameter cables from your battery if you don't want large voltage drops. To be safer it is wise to still use a fuse suited to the total load at the end of the wire- NOT a fuse that is rated at the max current the cable can handle.

[GeneralFubashi]

larger diameter cables less voltage drop, less current required by those huge spotties. With large draw items like spotlights you should choose a fuse with a percentage margin above what you need, not just a few amps above what you think the max draw will be.

ages ago when i was working all this out i think i came up with 6.5amp draw per 100w spotlight and confirmed with a ammeter. I reckon you cant go wrong with 20A fuses for 2x100watt spotties, even if you are running 20A wire, because if its a true rating, its for continuous current, and the surge different from a short will be enough to cook the fuse before the wire.

[RRover85]

To work out fuse ratings.......

the wattage of the 'globe' or 'load' divided by the battery voltage of the vehile ie. 24volts or 12 volts...

so for instance...

2 100watt driving lights = 200 watts there for 200watts divided by 12 volts = 16.6 amps

so then i would use a 20 amp fuse... for instance

[drivesafe]

Hi folks, murcod’s suggestion of running a higher fuse rate than the load is actually a good practice because running a fuse rated at about the same as the load rate of the device is asking for trouble not only for the reasons given by murcod but also because the fuse will get very hot and even melt when the load is to close the the fuses maximum handling capacity.
You are better off first making sure the wire’s load rating is high enough and then use a fuse about 25% high than the maximum expected load. This way the fuse will not heat up but will still blow quickly if there is a short.
Cheers