What AMP fuse to use?

phatchink · 01-13-2005, 11:25 AM

I'm in the final stages of buildign the carputer, I have one question.

So i'm connect the battery directly to the inverter then to the ocmputer, inorder to protect my circuit i'll have to put a fuse in between the inverter and battery???
if the above is true, then how do i know what amperage to use? Someone provide me with a formula?

DallasCamaro · 01-13-2005, 11:55 AM

Well, Hopefully the inverter is fused itself, see if it has a fuse in it. If it does, it's still a good idea to put one right by the battery post, to shutoff that whole wire in case it were to short to ground. In that case, just use the same amperage fuse that the inverter already has in it. If it doesn't, then see if the inverter has a rating on it, look for Max. Input Wattage or Max Input Amperage. If no input levels are given, then there should at least be an output wattage on the inverter (i.e. 200 watt inverter). I would add 10% to compensate for the power being consumed in the inverter (turned into heat -- this would give us 220 watts in my example) Now that you have a gross input wattage, simply divide by 12 to find amperage. So for a 200 watt inverter, you should assume that input current will be just under 20A. Thus, a 20 amp fuse should suffice. I would personally use a 25A fuse, it will leave a little room on the circuit if you tap something in later on, but still will protect in case of a short. By the way, remember that you will have about 20 amps running through this wire, be sure to use a proper size wire for the job (at least 8 gauge, and in any run over 6 or 8 feet i would use 4 gauge)

n8scstm · 01-13-2005, 12:10 PM

My 1500 watt inverter recomends a circuit braker rather than a fuse. You may want to consider that. Make sure you compensate for surge power. For example, my inverter is 1500 watts but the surge is 3000 watts. There is a %10 power loss also. So It's possible for mine to draw 3,300 watts which converts to 275 amps. So I would use a 300 amp fuse or circuit breaker. I used the cheap one but should have gotten the water proof one.
Fuse holder
Fuses

phatchink · 01-15-2005, 12:09 AM

thanks for the excellent advice guys!

so what if i use a resister with a value too high, say i put a 30 amp fuse in instead of a 20 amp fuse? what' would happen then?

I'm guessing there'll be less current passing through the wire?

and fuses comes in a whole variety of shapes and sizes, are there any difference between them?

DallasCamaro · 01-15-2005, 12:13 AM

30 amp fuse instead of a 20 will make no difference in the circuit....the fuses have no noticable resistance. You are only trying to use as small a fuse as possible, so that is something goes wrong <short circuit>, the fuse will blow before wires melt or something catches fire. If the entire circuit is run with good wire, there's nothing wrong with a 30 or even 50 or 70 amp fuse. As long as all the wires can support that amperage without catching fire. A 30 amp fuse will be fine for your application, just remember the 8 gauge wire, and a good matching ground wire.

fantomas · 01-15-2005, 12:24 AM

how many watts is your inverter? do essentially what dallas said to find out the amperage that you're pulling, and use this chart:
http://www.crutchfield.com/S-uSUSbPv...ble_gauge.html
to figure out what size wire to use.

put the fuse within 18" of the battery. the closer, the better.

by resister do you mean fuse? if you use a larger fuse, more amperage will be able to pass through the wire before it pops. so, for the purpose of this discussion, you put a 150 amp fuse on a 10 foot piece of 12 gauge wire. the 12 gauge wire will only be able to handle up to (using the above chart) 20 amps. if there is a short in the wire, as in it touches the car chassis without mcuh resistance, the wire will heat up to a point where the plastic will start to melt, and eventually start a fire. if you had a 20 amp fuse on there, it probably would have popped within 5 seconds, thus saving your life and preventing a car fire.

point is, use a big enough fuse for the inverter, but dont use a fuse thats too big for the wire. if the inverter needs a larger fuse than the wire can handle, use a thicker gauge wire.

n8scstm · 01-16-2005, 01:52 AM

Nice chart but damn, they never go up high enough for my 3000 watt inverter and amps that im running on the same line. I hope my 1/0 wire is good enough. Im only running it about 5 feet. Anyone know of another chart?

fantomas · 01-16-2005, 02:42 AM

3000 watts, max, right? you realise the usual house circuit is only 15-20 amps....which is at most 2000 watts. this is why space heaters only go up to 1500 watts....

DRWeside · 01-16-2005, 11:11 AM

Quote: Originally Posted by n8scstm

Nice chart but damn, they never go up high enough for my 3000 watt inverter and amps that im running on the same line. I hope my 1/0 wire is good enough. Im only running it about 5 feet. Anyone know of another chart?

How many alternators do you have in your truck? That inverter will suck up to 250 amps.

nsmoller · 01-16-2005, 01:25 PM

by this, if i get a 400w inverter my fuse should be 40amps or less?

i'm running wire in a 4runner (SUV) to front seat area, approx 6' would do...

i could use 10g, but if i use 8 or less, is there a problem then?

i figure just incase i get a 750w inverter in the future

DallasCamaro · 01-16-2005, 01:33 PM

Thanks fantomas. U laid out what I was trying to say. Big enough to handle the load, but small enough to protect the wiring. By the way....ALL wires attached to that circuit should technically be sized to handle the fuse. Thus, don't tap in a 20 gauge wire just to run to one little light bulb somewhere. If that lil wire shorts out, it could catch fire before the fuse blows. So, as Fantomas said, the fuse should be large enough to handle your loads, but small enough to protect every wire in the circuit. And, of course, as close to the battery terminal as possible.

n8scstm · 01-17-2005, 12:39 PM

Quote: Originally Posted by DallasCamaro

Thanks fantomas. U laid out what I was trying to say. Big enough to handle the load, but small enough to protect the wiring. By the way....ALL wires attached to that circuit should technically be sized to handle the fuse. Thus, don't tap in a 20 gauge wire just to run to one little light bulb somewhere. If that lil wire shorts out, it could catch fire before the fuse blows. So, as Fantomas said, the fuse should be large enough to handle your loads, but small enough to protect every wire in the circuit. And, of course, as close to the battery terminal as possible.

You would have to put an additioal in-line fuse on the small bulb wire to protect it. I have a main fuse at the battery. That 1/0 wire goes to a fused distribution block with smaller fuses.

n8scstm · 01-17-2005, 12:41 PM

Quote: Originally Posted by nsmoller

by this, if i get a 400w inverter my fuse should be 40amps or less?

i'm running wire in a 4runner (SUV) to front seat area, approx 6' would do...

i could use 10g, but if i use 8 or less, is there a problem then?

i figure just incase i get a 750w inverter in the future

A 40 amp fuse would work fine. If you plan on using a 750 watt inverter in the future, I would use 6 gauge wire. There will be no problem in using a larger wire. It's just more $$$.

n8scstm · 01-17-2005, 12:45 PM

Quote: Originally Posted by fantomas

3000 watts, max, right? you realise the usual house circuit is only 15-20 amps....which is at most 2000 watts. this is why space heaters only go up to 1500 watts....

Yes that is 3000 surge, but for sizing wire and fuses you need to make sure you can accomidate that. Its 1500 watts normal. 20 amps * 120 volts is 2400 watts. But besides that, my inverter has 2 outlets.

n8scstm · 01-17-2005, 12:48 PM

Quote: Originally Posted by DRWeside

How many alternators do you have in your truck? That inverter will suck up to 250 amps.

I have one 130 amp alternator. I dont plan on using that many watts though. I'm only running the computer, stereo and some screens. But I got a good deal on the inverter and it's nice to know that if you ever need it, its there, at least until you kill your battery which would be pretty quick at 275 amps.

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