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

cyan · 03-05-2008, 06:14 PM

After reading many of the posts here and searching the FAQs, I am still having trouble getting my head around the calculations involved with determining wire gauge. I wonder if I'm not making it more complicated than it is.

First, am I correct that once I determine which PSU will be enough for my system, I can simply use the max wattage for my chosen PSU to do the calculations for wire gauge?

For example, my setup (listed below) uses an M2-ATX PSU. I ran the calculations to determine peak wattage and to make sure I didn't exceed the power available on any given rail and everything seems fine. In fact, I've been running the system from my workbench without a problem.

So now I turn to the install phase and I want to make sure I understand wire gauge and fusing so that I don't start a fire and know what to change if I expand later.

First, I am hooking the sound into the Aux jack on my HU. I've read all the arguments for using an amp but I want to take things one step at a time and make sure I understand everything before getting into a more complicated install. Well, complicated for me, anyway.

So, with all that said, now I try to figure out the proper wire gauge and fuse size. I have a 160w PSU that draws 8amps and is hooked up to a 12volt source. My cable run will be no more than 10'. So I look at this chart here and now I get confused. Is my current demand just the 8amps that the PSU draws? If so, I just need 16awg for a 10' run of cable, correct?

Or, do I look at this calculator here and enter 160w and 12volts to get 13amps? This in turn would tell me that I should use 14awg wire for a 10' run.

The two wire gauges would require different fuses. If I look at the fuse chart here, I find that at 14awg, I should use a 15amp fuse while at 16awg I should use a 7.5amp fuse.

I guess it's safe to say that I can plug numbers into these formulas: V(volts) = I(amps) x R(ohms) and P(watts) = I(amps) x V(volts) but I am not completely sure which numbers I should be using.

Thanks for any help, I know this is probably extremely basic so I'll happily accept ridicule and mockery along with any help anyone can spare.

----------------------------------
My setup:

Jetway J7F4K1G5D-PB Motherboard
1.5ghz VIA processor
1gb RAM
40gb 2.5" sata hard drive
7" Lilliput EBY701
Wireless antenna
Slim DVD-ROM drive
4 port USB 2.0 hub
Mini-keyboard
Optical mouse (will probably remove this for the car install)
160w M2ATX power supply

brucedog · 03-06-2008, 10:16 AM

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.

cyan · 03-06-2008, 12:56 PM

Brucedog,

Have seen your posts and you are great help for noobs like me, thanks for the suggestion. Yeah, I have looked at just going with overkill on wire gauge and not worrying about it as long as I know I won't cause any irreparable harm. But it just gnaws at me that I don't understand such a simple concept.

Anyway, I may go the route you suggest and just keep going to school on the issue until it penetrates my thick skull. All the while, enjoying my car pc.

Thanks again!

Cyan

brucedog · 03-06-2008, 04:53 PM

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.

cMags · 03-06-2008, 06:35 PM

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!

FusionFanatic · 03-06-2008, 09:57 PM

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

cMags · 03-06-2008, 11:03 PM

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).

Bugbyte · 03-07-2008, 12:06 AM

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.

FordNoMore · 03-07-2008, 12:21 AM

Dude, that title made me laugh. I think you actually have a better grasp of this stuff than you're giving yourself credit for, though.

My question is where you gathered that your PSU doesn't pull more than 8 amps. Ohm's law (and the P=IV law) don't lie: if the input is 12 volts and the power output is 160 watts the current HAS to be at least 13.3 amps. (And actually more, as was mentioned, because no power supply is 100% efficient.) I think the 8 amp assumption is what sent you astray.

Another general comment: There is no hard-and-fast rule for which size to use, so various sources will quote slightly different numbers depending on their assumptions. Some will assume it's okay for the wires to heat up a bit, some may assume you want no more than (say) a 2% power loss, some will assume the wire has thick insulation so it'll get hot faster, etc. The smaller the wire, the more power you'll lose and the hotter it'll get, but there's no real exact answer to which size is "right". So in case you find slightly different answers from different sources, don't panic.

Also, I'll leave everyone with another great link for information on Basic Car Electronics. On the sidebar, check out 8 and 12 for more on Ohm's Law, and 16 for some interesting car wiring calculators.

Years of advanced calculus and engineering classes and several years more of professional experience have taught me: When you can look up a table or calculator and avoid the equations, it's a lot easier.

Oh yeah, and: Always err on the side of safety. That may apply here too...

brucedog · 03-07-2008, 10:07 AM

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...

cyan · 03-07-2008, 03:10 PM

Thanks to everyone who has replied, this is incredibly helpful! FordNoMore, you are right, I think I got confused when I looked at the ratings for the different rails on the M2-ATX.

According to the FAQ here, the M2-ATX has the following ratings for each rail:

12v rail = 8A (96w)
3.3v rail = 8A (26.4w)
5v rail = 8A (40w)

I looked at this and thought, okay, all the rails pull 8amps so that must be the max. What I think I was missing is that the whole point of a PSU is to take in unregulated power from the battery and regulate it to each rail so that you don't fry a component.

So if I'm starting to understand Ohms' Law better, my two constants are the maximum possible wattage (162.4w = 96w + 26.4w + 40w) and the max voltage (12v) coming from my car battery.

Sooooo, 162.4w/12v = 13.5amps max draw (with a little extra to account for less than perfect efficiency. Now everything starts to make a little more sense. I look up my handy-dandy wire gauge chart and decide that even though 13.5amps suggests 14awg for a 10' run, I might as well cover myself a little and go with 10awg. Then I look at my fuse chart and choose a 30amp fuse. Although if I'm understanding ComputerGeek correctly, I should go with a lower amp rating on the fuse since I probably won't be pulling the max with this system. So a I might start with a 20amp or 15amp fuse.

Okay, so I realize the above is basically what everyone has been saying (just trying to summarize it for myself and make sure I'm not misunderstanding something). Thanks to everyone for getting me this far.

The true test of my understanding comes if I want to install a more complicated set up. If you're still with me, and I haven't overstayed my welcome, let me ask this question:

Let's say I want to add a 500watt amp. My 12v from the battery is still one of my two constants. So to get my other constant, can I just add the 162.4w from my current set up and my 500w from the amp to get 662.4w and then divide by 12v? That gives me 55.2amps max and would suggest 8awg (or even 6awg to be more conservative) and a 50amp fuse (80amp fuse if I go with the 6awg).

Did I get that right? If so, I have crossed my first hurdle: understanding the science of it enough to keep from starting a fire and being able to run the numbers to improve the set up later.

Again, thanks to everyone for all your incredible help. It's pretty exciting to feel like I'm starting to see the light enough to feel ready to move to the next step: installation!

brucedog · 03-07-2008, 06:31 PM

Cyan,

Your estimates / calcs for current draw from amp and psu look right.

That makes it even more clear why those "amp install kits" come with 4ga cable, so that there is decent amount of overhead for the average install ----

But, I have to put on he brakes right there - there IS NO AVERAGE INSTAL!
Two examples:
Mine: 200 w main amp + 70 w aux amp + 400 w sub amp
A Co-worker: 800 w main amp + 1200 w sub amp
..and so on...

That's what makes your wire gauge estimator so handy.

Keep comin back.

FusionFanatic · 03-08-2008, 10:23 AM

cyan, you still seem to be using power output for your input current draw calculations. unless you know the efficiency of the component, then the output rating is useless in determining how much current the component will draw. also, keep in mind that the manufacturer's specs can be misleading, they often will not tell you the true efficiency (they will list an efficiency rating at a partial load only).

the efficiency is not something to be overlooked. never assume a PSU or amplifier's efficiency is simply "less than perfect"... many times the efficiency is quite low, and input draw can be significantly higher than the output rating.

for example a typical home computer's AC PSU is only 70%-75% efficient (higher quality PSUs are up to 85% efficient). this means that a 70% efficient [500W rated] PSU will draw over 700W to output the 500W it's rated for. if you were to base your calculations on the output power, you would determine that [at 115VAC] a 500W PSU would draw a maximum of 4.3A, but when you takes it's efficiency into account you realize that would actually draw up to 6.2A. that's a pretty significant difference. if you had ran a wire rated at up to 5A, then you would risk burning up that wire and starting a fire.

in general, DC-DC power supplies (or regulators) are more efficient than AC-DC power supplies, but if you don't know, then you shouldn't assume anything either. the M2-ATX spec sheet states; >94% efficiency, all rails combined, at 50% load. this assumes your carputer is drawing exactly 50% capacity from each rail (+12V@4A, +5V@4A, and +3.3V@4A). in reality, your carputer will draw more than that, and in different combinations.. since we don't know it's true efficiency at higher [more realistic] loads, we can assume it's significantly less than 94%. if we were to guess 80% efficiency, then it would draw ~16.6A @12VDC to output 160W.

with audio amplifiers, efficiency can vary quite a bit depending on design and quality. again, don't assume it's simply "less than perfect", the difference is often more than that. if you want to power both a 500W audio amplifier and your carputer off a single cable, then use 4AWG or higher.

cyan · 03-08-2008, 09:31 PM

ComputerGeek,

Brilliant, thanks! That fills in one of the issues I was getting lazy about, component efficiency. I'd seen it mentioned but kept telling myself I didn't need to worry about it. But I guess it's one of those things you either learn about now or learn about the first time you try to debug why your system keeps dying. But either way, you're going to learn it. Thanks for helping me with this, your explanation was very clear.

Just to summarize (for myself) again:

Let's say my carputer's PSU is rated for 160W. I either look up, or estimate as best I can, that it is 80% efficient at full load. Then my first calculation is to determine the actual (or closer to actual, anyway) power draw as follows:

Actual Wattage x 80% = 160w
Actual Wattage = 200w

Now I add in a 500w amp with a 70% efficiency rating at full load:

Actual Wattage x 70% = 500w
Actual Wattage = 714w

Now I'm ready to look at a more realistic draw of 714w + 200w = 914w

The calculation is now:

914w/12v = 76amps (whereas leaving efficiency ratings out of the calculations would give 55amps)

My more accurate set up would involve 6awg for a 10' run (the less accurate calculation suggests 8awg). And, of course, the fuse rating would be different as well.

So I guess most of us avoid disaster by not really understanding this issue but being so conservative in our wire guage and fuse ratings that we end up covering our butts without knowing exactly why. It's nice to understand at least a little more of why.

Thanks again ComputerGeek and everyone who has contributed to this post, you have been invaluable!

FusionFanatic · 03-09-2008, 11:20 AM

glad I could help.

again, since we don't know the actual efficiency of either component, we don't know if those calculations are accurate. but you've got the idea

4AWG wire is recommended for amplifiers up to 1000W, which will provide plenty of power for your audio amplifier, and still have plenty of headroom to power your carputer as well.

bottom line is; when it comes to wire gauge, always go bigger than you think you need to. take it from me, my first car caught on fire (and burnt to a crisp) due to my inadequate amp wiring. (some lessons are learned the hard way

). you're doing the right thing by asking questions now before wiring it all up (if only I had done that back when I was 17, my '84 Cavalier wouldn't have met such a horrible death

)

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