is there a general guide line of how big an alternator needs to support howmanywatts

[adulbrich 5,000+ posts]

It's fine.Current demand is getting serious at that point.

300A should protect the wire, but with that much current demand there are some bad things that can happen before the fuse opens.

Personally, I'd be concerned about battery damage.

It's the nature of the beast though.

I have a Singer 320 amp alternator and an XS D3100 battery. I was wondering if I'd have probems, because my alternator is over 300 amps.

Edit: What other bad things are you talking about?

 

[bumpasauras 10+ year member]

As a general rule of thumb you want to add up the fuse rating of all the amps you are wanting to power. Shoot for an alt that has an amp rating as close to the sum of your fuse rating as you can afford/fit. Say if yor sub amp has a 100 amp rating and your mid/high amp has a 40 amp rating then you will be shooting for a 140 amp alt. For every 10-15 amps below the 140 amp mark add a battery.. This is a crude rule of thumb but it will get you in the ball park..

I build alts on the side so lemme know what type of vehicle and what equipment you are trying to power and I can tell you what amp rating you will need and about how much you can expect to pay.

Just to be clear anything above your fuse rating is great.. I usually advise my customers to buy the largest alt possible (within reason) to allow for future upgrades

 

[hispls 5,000+ posts]

What is your source on the "music over time is about 15% the power of a sine wave?"

The Zed Audio manual. It's quite a good read.

What if i'm listening to Decaf music, where bass lines can be constant through the entire song? If Decaf stuff and a sine wave are recorded at the same level (say, -6 db), the bass amp would see, and produce similar results to the sine waves. I don't see how that would be anywhere near "15%."

Then you are actually listening to a sine wave. Enjoy. That's not my definition of music. Most folks' definition of music would be an average of stuff you'd hear on the radio.

Could you explain further on why amp efficiency is irrelevant / "********?" It takes an amp X power to create Y power. There's nothing more really to it. I don't see how if one wanted to calculate max load, that they wouldn't consider efficiency a valuable factor.

Because the "efficiency" that manufacturers brag about is a ******** number and will only be seen at 4 ohms or so and just below clipping. It is incredibly unlikely you'll get anything like advertised efficiency real-world (even if your definition of music is just a test tone).

I'm not trying to be a jerk here, i'm here trying to learn as much as the next guy. I would agree that buying more is always better. Even more of a reason to get a rough calculation (like I did above^). It's not about getting as small of an alternator as you can. In fact, quite the opposite. If you take my example into a real-world scenario, you would need no more than 532 amps. That would be a MAXIMUM possible load (For amps + car running, anything else would be too small to even bother calculating in. I simply calculated the two biggest draws on the electrical). You would never see more than that. The rest would just be wiggle room.

In most setups here people running big amps / subs, the amps themselves are obviously going to be the top draw on the electrical. Next up would either be the car itself, or the mid/highs amps if they were powerful enough. If you can satisfy the amps than really anything below that (like the small amps for tweets or the car itself) isn't much to worry about imo.

OK, so now buy a 500A alternator or whatever you just guessed at. First of all, that doesn't exist, second that's still probably under-estimating what your maximum draw could potentially be.... or possibly over-estimating what OP will actually need for his daily use. Pretty much 250-300A is about the biggest real world hot current you'll get out of an alternator, so what's the point of counting how many angels could dance on the head of a pin beyond that?

Another note to OP and anyone else reading:
I find that a good guideline is to look at your monoblock amp FUSE rating. (Combined fuse rating if more than one monoblock) It will tell you what kind of class of alt we're talking about.

My buddy has a 7.5k (Fuse rating 750) and dual 370 alts. (740 amps). He says it runs great. The fuse rating gives you an idea how much power it's going to consume (but not exceed, obviously. Otherwise the fuse(s) will blow).

I can almost guarantee that 750A fusing won't in a million years protect that amp from burning up if something goes wrong. Also, most manufacturers have learned that "clever" people on audio forums try to guess output based on the fuse rating and they over-rate that as well. I can't remember the last time I heard of someone opening fuses on the popular Korean amps.... which tells me they're over-fusing. Generally speaking if it's protecting your amp you'll open a fuse here and there when pushing hard.

 

[Mrtallent]

Any of this **** above is pure guessing. There's far too many variables to say what you will need. Buy the largest alternator that can be properly turned by the belt setup you have in your vehicle, if you have trouble after that, add a second alternator or more batteries.

So your saying instead of doing the math to find out you need a 270 amp you should just spend your whole paycheck on a 425amp and cause more problems.

 

[Firewalker]

@Popwarhomie

 

[Popwarhomie 5,000+ posts]

So your saying instead of doing the math to find out you need a 270 amp you should just spend your whole paycheck on a 425amp and cause more problems.

So you're saying you didn't see this was a 9 year old post?

 

[hispls 5,000+ posts]

So your saying instead of doing the math to find out you need a 270 amp you should just spend your whole paycheck on a 425amp and cause more problems.

First of all this thread is almost 10 years old. Second, good luck getting an alternator that'll make 420A real world to fit in something that isn't a bus, semi, or emergency vehicle, third, even the Delco Remy 55SI coming in at around 2000$ is probably still a decent deal compared to buying two standard sized high output alternators and dual brackets when you consider they come with 3 year unlimited mile warranty.

All that said, no, I wouldn't really suggest buying a 2000$ alternator setup these days for most applications because of how much you could do with 2 grand worth of lithium on a parts store alternator.

Did you have a particular goal now or question or did you just join up to e-argue with a 10 year old thread posted by a user who hasn't been back here since 2016?

@Popwarhomie