Question about power chart

Printed From: the12volt.com
Forum Name: Car Audio
Forum Discription: Car Stereos, Amplifiers, Crossovers, Processors, Speakers, Subwoofers, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=59707
Printed Date: May 10, 2024 at 2:01 PM

Topic: Question about power chart

Posted By: Poormanq45
Subject: Question about power chart
Date Posted: July 19, 2005 at 12:15 PM

I was looking over the power chart and noticed an error.

It says that at 200w RMS at 20ft you should use a 4guage wire.

Well, 200@12v = ~16amps.

WEll, this is in the range of a 12gauge wire. So why does it call for a 4gauge wire?

-------------

Replies:

Posted By: wirewise
Date Posted: July 19, 2005 at 12:26 PM

No error at all. You would be correct if your amplifier was 100% efficient and distance from power source was not a factor, however this is never the case.

~wirewise~

Posted By: Poormanq45
Date Posted: July 19, 2005 at 12:50 PM

Ok, so lets assume 60% efficiency. THis would mean it'd take about 335w RMS. WHich would mean it'd draw ~27amps.

This is still within the range of a 12gauge wire.

Note, I've wired a few houses for and we made runs over 50ft with 12awg romex, and that's NEC rated for up to 20 amps, and can easily handle 30~40amps without getting hot.

So why does it call for a 4guage wire? At the MOST it should be 8 or 10 guage which are rated to 40 and 30amps, respectively.

-------------

Posted By: the12volt
Date Posted: July 19, 2005 at 2:33 PM

Hi Poormanq45, using your numbers and a voltage drop calculator... 27 amps of current draw from a 12 volt battery on a 12 gauge copper wire 20 feet long, there would be a voltage drop of 2.1 volts at the amplifier or roughly 9.9 volts. Using a 4 gauge wire with the same numbers, you would only have a voltage drop of 0.3 volts or 11.7 volts at the amplifier.

I should probably write a voltage drop calculator for this site sometime soon, but you can find a lot of them on line already that will give you comparable results. BTW, my figures in the power wire chart are based on 50% efficiency.

-------------
posted_image

the12volt • Support the12volt.com

Posted By: Francious70
Date Posted: July 19, 2005 at 2:45 PM

Don't forget to calculate for resistance. That's why they call for 4 gauge and not 10 gauge.

Paul

Posted By: Poormanq45
Date Posted: July 19, 2005 at 2:46 PM

Ah, I forgot about the voltage drop posted_image

Question: Does the voltage drop only occur in DC currents? The reason I ask is that on an AC circuit you can measure the voltage at one point, and then measure it 50ft away(extension cord) and it's the same.

-------------

Posted By: the12volt
Date Posted: July 19, 2005 at 2:57 PM

No, it occurs in AC circuits as well. When you are measuring, do you have any load (current draw) on it? If not, then that's why it looks the same.

-------------
posted_image

the12volt • Support the12volt.com

Posted By: JAydawg21
Date Posted: July 19, 2005 at 3:00 PM

No, voltge drop is current dependant, if you had 2 Ohms of resistance with only 1 amp of current, you would only drop 2 volts across the conductor. If 10 amps of current are flowing, there will be a drop of 20 volt across your conductor, or resistor, or whatever. Car stereos (or at least the ones we all have) draw ALOT more current that a typical house under normal circumstances. A house has manyseperate circuits (as you know), most of which carry no more than 10 amps on average. Plus, 120 V compared to 12. The more voltage you have, the less current you need to do the same amount of work (same power). Which is why power companies transmitt at very high voltages, so the can reduce current, and consequently, reduce voltage drop and power loss.

-------------
'97 Jeep Cherokee sport
subs: JL 12w3v2 : RF 500a2 :
RF 1F capacitor
doors: 6" Diamond D361i -> RF 200a4