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

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.

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.

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 )

I used a 'install your amplifier' wire set from wallmart...It was 8 guage.

I guess you are running one wire from the battery to power your amps, headunit, and carputer?

Or just running a wire to the carputer?

Doesn't the M2-ATX have a fuse onboard? What size is it? Use that to determine what wire to use...and put another fuse of that size at or near the battery connection.

Originally Posted by Xargon321

i dont know if anyone has already said this or not but, you psu will not make 160w all the time that is the max it will produce, the actual draw will vary due to how much stuff you have and how hard its working.

yes, that is true. but it's irrelevant when it comes to wiring...

when you run power wires for a component (like a PSU), you MUST use a wire gauge that is capable of carrying the MAXIMUM current that the component will draw. for example, if you think your system will only draw 50% of the PSU's rated power, you still need to wire it as if it draws 100% all the time.