Okay, so I'm running one amp, JBL gto75.4II w/ 8awg power and ground wire. Powering one sub, Alpine 10, and front speakers, sony 4-way.

The amp has 2x30A fuses.

Question is.... What size of fuse should I throw into my fuse holder under the hood. 50A? 80A? 100A?

 

80A would be fine

 

Sweet, thanks! Do you think there would be any problem running 100A?

 

You really should be more concerned with running that big of an amp with only 8ga. power/ground.

Why would you want to run a 100a fuse instead of 80a? Is there a problem running 100a fuse? In day to day operation, no. But if something happens and you short out your power wire, but the "short" isn't a perfect ground, then you could feasibly be drawing less than 100a, but enough current to heat up 8ga wiring to the point you may melt/burn carpet, other wiring next to it, etc.

 

There is a simple answer to this:

30 + 30 = 60. Generally you want to run at 80% of maximum. 60A is about 80% of 80A.

The maximum recommended amperage of an 8GA wire is 100A. You would never want to exceed 80% of that. Therefore 80A is the fuse you want to use.

The key is to get the fuse to blow before the wire gets too hot and starts a fire. If you have a 100A fuse, and you are running solid 99A for a good period of time, the wire will eventually get too hot, and could melt the casing, causing a short and whatever else....

 

Not arguing, but where did you get 100a for 8 ga cable?

 

I couldn't tell you where those numbers came from. These are just numbers I've always been familiar with. (been building stereos as a hobby since I was in high school) And they are also extreme maximums. What the cable itself can handle without actually melting the cable metal. 100A on an 8GA wire, sustained for a period of time, will melt the shielding. One thing to add to this is that this maximum means the cable is ACTUALLY 8GA. Not 8.x like some of that ghetto cheap *** wire you can buy.

I have just always used these numbers:
max 60A for 10GA
max 100A for 8GA
max 150A for 4GA

Of course bigger is better. I've never personally had to deal with any systems with more power than that. You would still never want to run a consistent amperage over a wire that is reaching 80% of its capacity.

 

Ok. That seems more reasonable. I was thinking more about how much voltage would be dropped using such a small cable with that big of an amp. If that amp pulls 40a he'll be dropping a volt according to my handy little wire calculator and 18 feet of cable. That was my point.

 

yup, it's even worse with 18feet of cable... also, those numbers are based on ~12ft...

Truly, it's best to go 60A max on 8ga and 100A max on 4ga, @18ft.

 

Well, I had an 80A in there, but the Amp goes into protect mode when I turn it up to my desired volume level. So I thought i'd try a 100A, and it gets louder.

If I upgrade to 4ga, will I be able to run 100A and be safe?

The reason why I have this amp with this wiring, I had a smaller amp and upgraded (my friend gave it to me) and I have yet to get 4ga.

 

So putting in 100a keeps it from going into protect? Have you tried a different 80a fuse? There's no way a different sized fuse should affect amp performance. Its possible there's something wrong with the original 80a that was in there. If putting 100a fuse keeps the amp from shutting down then it should blow the 80 and not shut down. If you have a multimeter, measure resistance across the 80a fuse.

 

I'm with him... try a diff 80a fuse

 

yeah, like everyone else says use the rule of adding up all fuses on your amp(s) 2x30=60. You dont want to go too big because the whole point of a fuse is to make sure that if something, somehow gets backfed, it blowes the fuse on the powerwire, and doesnt get to any other electronics in your car. A little bigger fuse is ok, just dont go for a monsterous sized fuse.

 

The info I found on your amp shows a max current draw of 85a so a 100a fuse would be fine, but an 80 would be safer cuz if you hit 85a with your setup, than something's wrong. I highly recommend upgrading your wiring to 4ga also.

 

Thanks for the help guys, I am upgrading to 4ga soon, and I will use a 100A fuse once I have that, but until I do that, i'm putting a different 80A in.