Thread: Noob seeks mockery for his misunderstanding of Ohms Law in hopes of preventing fire

Good man with the "ridicule and mockery" thing...
It's a virtue to be able to laugh at one's self, and not get too inflated. There's a tendency to do that as the head fills with knowledge and experience.

"If a man thinks himself wise, then surely he is a fool" - Proverbs

You know, I never really gave a lot of thought to wire guages and current carrying capacity. How it went with me was that I built my car audio system first, before I even knew people installed pc's in cars.

I use http://partsexpress.com (PE)
for pretty much all my audio needs, connectors, wiring...

So, I bought an overpriced Street Wires car audio amp install kit from PE, that included a length of 4 gauge multi-strand cable, a power distribution block, fuse units, and some 8 gauge cable and also connectors, cable ties, and such.

The 4 ga is connected right to the battery, with a 100 amp fuse right there. SHORTS anywhere along the line? The 100A fuse protects.

Rather than go into all the boring detail, pictures are worth 1000 long words, so you might check PE for the cable kit. Maybe it IS worth the price - less running around finding stuff...also you could see my pictures and look at forum posts for other's installs.

They have all the wiring install kits at Best Buy, Radio Shack, etc, etc. One thing about big wires and distrib. blocks is that when/if you go with amplifiers and such - you will have a powerful hub to draw whatever current you may need.

Try not to worry. It's not rocket science. Just have a lot of fun.

Yes, difinitely keep on 'going to school" and educating self, and reading forums posts...

You've got your calcs right on that psu problem:

156 Watts = 12 volts x 13 amps

Now, that 160 watt psu meand the total wattage output available. If you could put ammeters in series with the various voltage outputs, you'd find that there are different currents flowing through the 5 volt, 12 volt, 3.3 volt, etc, all adding up to the TOTAL current draw. That 160 watt psu is rated at 160 watts, but your carpc would draw less than that, if you had selected the right psu - gives some overhead to add more cards and other usb devices, etc.

I always kind of wanted to have various ammeters in series with my car's voltages, but especially on the main +12 volt cable from my battery to the rear of my car, where amplifiers and all computer loads are. Might not only be interesting, might help to decide what to change, optimize...


I try to help, but there's a LOT of guys out there who reeeally know their stuff on pc's in general. You'll meet 'em if you stick around.

As far as I understand things, the 160W rating is a total possible output for the PSU. Assuming a 12V input (though it varies), and 100% efficiency, that means a possible 160/12=13.3Amps. Since we know nothing's 100% efficient, a 10% loss in efficiency would add another amp or so onto the input. Also, if your voltage dips (like while during a crank), the current could spike to keep up.

All that being said, I've never had an issue with my M2-ATX pulling any more than what a 15A fuse can handle. As such, I'm running wire heavy enough to be well protected by that 15A fuse. I'm running 10ga, but only because it's what was available as a spool from Autozone cheap. 14ga would be the smallest I went on that line, as described by that first table you posted (good resource) and it's corresponding max fuse.

Personally, I never use anything smaller than 16ga or 18ga in a car environment, and the small wires are usually only for signal or <2A devices such as radar detectors and such. You can run the calculations until you're blue in the face, but in practice, just go with a general figure, add in some safety margin and grab what's available that will work *safely. Engineering at its finest. Good luck with the install!

cyan,

welcome to carPC'ing

don't worry, there is no reason for anybody to "mock" or "ridicule" you. your questions are perfectly legitimate. sometimes new members get harsh responses/answers because they don't bother to search or read anything first, but you have already been reading the FAQs so you're off to a good start

there's nothing wrong with keeping a head unit. there are just as many arguments for it as there are against it. there are pros and cons for both setups, neither is right or wrong, it all comes down to personal preference. personaly, I use a head unit (via AUX-in). for me the carputer is a novelty, I don't need or want it on every time I drive. I use it for GPS, internet, Video, DVDs, cable TV (slingplayer), and MP3s. 90% of the time I'm in my car I don't even turn my carPC on (short trips to the store, friend's house, or commuting and don't need anything more than basic FM/XM radio from my head unit). I ran a poll on this topic a few months ago and it was pretty much 50/50, meaning half of us chose to keep a head unit vs. running audio directly into an amplifier. don't worry, you're not alone

your calculations are fine, as well as your understanding of basic ohm's law. but remember, it doesn't matter what your PSU's output wattage is, it will always draw [in] more power than it puts out. with power wires, you should always over-estimate the current draw. it's never a bad idea to go bigger when it comes to wire gauge. some people go way overboard with PC power wires, and others don't go big enough. while neither is ideal, its always better to go bigger than necessary. if you calculated that you'll need 14/16AWG wire, then I would go the next step up (10/12AWG) so you know you'll be safe. use the smallest fuse possible. fuses are cheap, don't worry if you blow a few of them in determining what size to use. start with a fuse rated for less than you think the PSU will draw, if it blows then replace it with a higher rated fuse, but don't ever exceed the amperage rating of the wire itself.

good luck

Yea, I forgot to mention that I run everything thru a head unit as well. The HU is just a lot simpler for things like CD's and radio - better tuner, easier than mounting a slot-CD on the dash, and I don't have to lose my on-the-fly playlist to play them. Plus when the PC is down (for repair or upgrade or whatever else - because it *will* be down at some point), I still have a fully-functional sound system.

Oh and I see you've got an EBY701 - I just picked one of those up myself. PM me with any questions you can't find answers to on the forums (its sorta new so info is sparse) such as the hidden menu and physical dimensions in and out of the case (I've got lots of pics just haven't uploaded yet).

Yeah, I concur with ComputerGeek. If you calculated that you need 14/16, I'd go with 10/12 and you'll be fine. Going with a larger gauge wire helps reduce the resistance and thus the voltage drop over the 10 feet you are going to run it.

This calculator shows that at 10 feet and 14 amps, you'll get a .14 volt drop with 10 gauge and a .22 drop for 12 gauge.

Hey FordNoMore,

That link to http://www.bcae1.com/

...is about the best that I've seen. It's going to take me away from these forums some, but there's a lot of great info there...