What size fuses?

[BrentMcGhee]

I am getting ready to install a complete sytem in my new car and i had a question about what size fuses to use. What wattage spec should i be looking at when deciding on fuse size for a particular amp. For instance one of my amps is the C300.2. It's 4ohm wattage (wich i will be using) is stated at 150watts per channel so a total of 300 watts. At that rating i would want about a 25amp fuse.

But what is the other wattage spec? Total Dynamic Power (all ch. driven) 850 watts

At that rating i would need a wopping 70 amp fuse for one amp!

Now this is my first system i am setting up in a car, i am a home audio person. So those numbers may sound normal for you but a 70amp fuse for one amp sounded excessive to me, but i could be way off. ANy help would be great.

[deepinya]

This is a good question as Im gonna be in the same boat as you. The prepackaged AMP kits come with fuses but I think they base that on the gauge wire they are supplying.

[neomagus00]

'total dynamic power' is - as far as i can tell - basically, peak power, and therefore irrelevant. the normal rule for finding total amperage is to simply add up the all the fuse ratings of all your amps, then round to the nearest available fuse.

[1996blackmax]

That is what I have always done.

[BrentMcGhee]

Cool, that is what was asuming but i figured i would ask the guys that have probably done this hundreds of times

[PoweredByDodge]

300 / 0.7 efficiency = 428.5

428.5 / 14.4 V = 30A draw.

get a 40A fuse.

[doxquzme]

Actually, I would aim low on the fuse, not high. Fuses are a lot cheaper than amps! If it calulates out to 30A then get a 30 amp fuse. If you are talking about the fuse next to the battery, try getting a cicuit breaker instead.

[Vestax]

This is a good question as Im gonna be in the same boat as you. The prepackaged AMP kits come with fuses but I think they base that on the gauge wire they are supplying.

I agree, fuse it to the size of the wire. If you fuse a 40 amp on a 16awg, I will guarantee you'll melt the wire before you blow the fuse. In the case that your amp doesn't have onboard fuses, then that you'd want to fuse with the amp's amperage draw rating. You'll be fine up to 150a on a 4 awg wire on the inline fuse at the battery. If you use a fused d-block near the amps, then you should be closer to the amp's fuse ratings.

[PoweredByDodge]

you shouldn't be pulling more than 100 A through 4 gauge. If you are, you're overloading the line. Bump up to 2 gauge.

sucking 150A through 4 gauge over a 2 foot run from alternator to battery is one thing... through a 20 foot run to the trunk --- that's another... you'll dump almost 2 volts.

[Vestax]

You won't overload the line if you properly fuse it. The inline fuse at the battery is a short run, and unless you did the big 3, the alternator wire normally runs through a stock fuse box. If anybody needs a chart on maximum amperage current on the size of wire, here's one right here:

http://www.the12volt.com/info/recwirsz.asp

I doubt a .05 ohm resistance on a 20 feet run vs a 2 feet run will cause you to lose 2 volts..... and if you lost 2 volts, it's because you didn't ground your component/amplifier/appliance properly.

[PoweredByDodge]

Those charts are inaccurate...

I utilize the 12 volt for various things, but not for that chart...

"current draw by power" -- they are assuming an amplifier efficiency of less than 50% ... otherwise, in most cases you should be getting 160 watts or more if you're drawing 16 amps at 14.4 V ... even if its 16 amps @ 12.5 volts, you should be producing around 150 watts rms. that chart is stating that a 100 watt rms amplifier will draw 16 amps... that's atrocious unless you're using a true class A amp, in which case it would be even more inefficient.

"power and ground cable specs" -- charts of this type are usually made up based upon an average cable length of 20 feet, plus a 1 foot ground... so ballpark to 20 feet.

When utilizing a lot of current, the 'layman's goal' is to have no more than a 0.5 to 1 volt drop at the end of the line. ideally, you would seek an even lower drop, but since we can't all use 3/0 (000) gauge, aiming for a 0.5 to 1 volt drop is reasonable.

at 350 amps, over 20 feet, 0 gauge stranded copper will yield a 0.707 volt drop ... good. 2 gauge at 225 amps = 0.723 V ... 4 gauge @ 150 amps yields a 0.766 drop ... now - the worst is 8 gauge at 100 amps -- that's insane, it yields a 1.3 volt drop over 20 feet... unacceptable. but at 60 amps, 8 gauge is right about at 0.78 volts...

i've always had a rule of thumb --- 10 gauge up to 30 amps, 8 gauge up to 50 amps, 4 gauge up to 100 amps, 2 gauge up to 150 - 200 amps, and 0 gauge up to 300 amps. my rule of thumb is much more in line with the realistic performance of the wire.

i'm not even going to get into the "power and distance" chart because i believe i've made my point with the above explanation.

now - onto the more important topic -- resistance of the line...

4 gauge stranded copper is rated at about 0.00025 ohms per foot --- not 0.05 ohms per foot.

over 20 feet -- 4 gauge yields a resistance of 0.005 ohms ...

150 amps through 0.005 ohms yields 0.75 volts --- there's your drop.

over 2 feet... it's only 0.1 volts.

that's a 0.65 volt difference, and it is the reason why you can use 4 gauge to suck 150 amps over a foot or two and not even blink and eye... but if you want true voltage -- as close to that ideal 14.4 as you can be -- then you will not skimp on wire size when you're dumping gobs and gobs of money into capacitors and extra batteries and a high output alternator. why bother stiffening 0.5 volts when you are wasting over a volt in the line...

as far as stock fuse boxes go -- there is usually a large 150A fuse on the alternator input line, that then goes right to the battery... so you're looking at 2 feet to the box... 6 inches to the battery. that's not the end of the world consdiering most stock alternators are anywhere between 70 and 130 amps... if you're going high amperage and you want to do the job right, then you've already replaced the 4 gauge (or 8 gauge) factory line with 2 gauge or 0 gauge or whatever you will be using to go to your distribution block or amplifier... ideally running the thicker wire from alt to battery, adding a ground strap from battery to engine block of that same heavy gauge, deleting the factory 'alt to fuse box' line, and then running 4 gauge from the battery to the fuse box -- you have to take the total length into account... alternator to battery + battery to distro block + distro block to amp -- and then use that to figure out your gauge wire.

as far as properly fusing a line... fuse 5 amps above what you expect to be your peak draw --- and buy theright size wire or larger... if you had 0 gauge laying around, but only needed 2 gauge, but the 0 gauge is free because it's kicking around... then you dont want to put a 350 amp fuse on it if you're only going to draw around 200... if you're amps draw more, then its because they're cooking themselves... so you want the power to kick out before they are destroyed beyond repair. get the idea?

[Vestax]

Great explanation but no offense, but it got way off tangent, I could've pulled out my Physics book on Electromagnetism, that sh!t kinda hurts your head after a while. I'll just stick to installing remote start alarms and overheads. LOL :D

Trying to stay on topic with inline fuses at the battery.... I threw an arbitrary number on the impedance since I knew the number was so little, I forget the actual value. Correct me if I'm wrong on this one but the only reason why I neglected anything at the end of the wire is because the fuse is to limit current to max out at specific rating.... and to protect: to keep the wire from burning up or catch on fire. The wire length and thickness represents a capacity. Overload that capacity, then you have trouble... the amp efficiency and input voltage shouldn't matter, only the amount amperage/current flowing the wire. Kind of like water in a hose. ;)

[PolkThug]

I've got a 60amp fuse on 4awg running back to my amps. Then it splits into two 8awg wires with a 30amp fuse on each, going to the Polk 300.2 and Polk 400.4. Polk 300.2 is in bridged mode.

[PoweredByDodge]

Correct me if I'm wrong on this one but the only reason why I neglected anything at the end of the wire is because the fuse is to limit current to max out at specific rating.... and to protect: to keep the wire from burning up or catch on fire. The wire length and thickness represents a capacity. Overload that capacity, then you have trouble... the amp efficiency and input voltage shouldn't matter, only the amount amperage/current flowing the wire.

the current - regardless of voltage, is what causes the voltage drop in the cable -- so 'yes' to that.

as far as 'maxing' and 'protecting' -- the fuse serves two purposes... as you said... 1) to protect against shorts [like if you put a screw through the power line by accident or something - thus preventing fire or who knows], and 2) to regulate the max current availale to your amplifiers so that if they go bad on you, the power will cut off (hopefully) before they're trashed completely.