electrical experts check my calculations

[talon2nr7588 10+ year member]

ok let me know if i got this right.

power= voltage x current right?

so current= power/ voltage right?

ok so i plan on running 2 bxi2006d for my 15L7s. each at 1 ohm to each sub.

with the formula above each brutus would be drawing 142.8 amps. but thats only max current draw correct? i'ma buy one to hook up to the subs at 2 ohms, it will be temporary until i get another one. i have a 115amp alt, will one bxi2006d be good with my alt and big 3 in 0 gauge?

so with both amps it will be 285.6 amps of max current draw. but again that is only max current draw so in order to find the constant current draw i would have to know the efficiency of the amp right? but whatever it is will i be okay with a 240 amp alty?

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[squeak9798 5,000+ posts]

Your understanding of ohms law is correct.

But it sound like you are a little confused on how amplifiers draw current.

In order to find the maximum current draw, you would need to know the efficiency of the amplifier.

If the amplifier is 70% efficient at full power and outputs 2kw @ 14.4V, then the maximum current draw would be:

2000/.7 = 2857 (total wattage draw of the amplifier)

2857/14.4 = ~198.4A (total current draw of the amplifier)

Not sure what you mean by "constant" draw....an amplifier only draws current based on the output. If the output is zero, the draw is essentially zero (very miniscule).

If the output starts at 0, peaks up to 2kw then drops back down to 0 all within a matter of 3 seconds (such as an SPL burp), then the current draw would be zero, jump to 198.4A during those 3 seconds (assuming voltage and efficiency stayed constant), then drops back down to 0.

For music, there is really no good way to judge the "average" current draw as it depends on way too many variables. Settings, type of music, level of music, etc etc. About the best "guesstimate" would be to expect an "average" output of about 30% of the amplifier's total output (this is just a very rough estimate, don't use it as fact). But then, efficiency is not constant, so at 30% output the amplifier's efficiency will be different than at full output.

But the current draw will be jumping all over the board constantly...

 

[talon2nr7588 10+ year member]

Your understanding of ohms law is correct.
But it sound like you are a little confused on how amplifiers draw current.

In order to find the maximum current draw, you would need to know the efficiency of the amplifier.

If the amplifier is 70% efficient at full power and outputs 2kw @ 14.4V, then the maximum current draw would be:

2000/.7 = 2857 (total wattage draw of the amplifier)

2857/14.4 = ~198.4A (total current draw of the amplifier)

Not sure what you mean by "constant" draw....an amplifier only draws current based on the output. If the output is zero, the draw is essentially zero (very miniscule).

If the output starts at 0, peaks up to 2kw then drops back down to 0 all within a matter of 3 seconds (such as an SPL burp), then the current draw would be zero, jump to 198.4A during those 3 seconds (assuming voltage and efficiency stayed constant), then drops back down to 0.

For music, there is really no good way to judge the "average" current draw as it depends on way too many variables. Settings, type of music, level of music, etc etc. About the best "guesstimate" would be to expect an "average" output of about 30% of the amplifier's total output (this is just a very rough estimate, don't use it as fact). But then, efficiency is not constant, so at 30% output the amplifier's efficiency will be different than at full output.

But the current draw will be jumping all over the board constantly...

how did you get 2857? i thought power was voltage x current?

 

[squeak9798 5,000+ posts]

how did you get 2857? i thought power was voltage x current?

You must take into account efficiency.

If the amplifier outputs 2000w, and is 70% efficient (i.e. the power output is 70% of the power input), then the amplifier is going to have a total wattage draw from the electrical system of 2857w

2857w * 70% = 2000w [rounded]

Or, using elementary mathmatics to work backwards since we know the power output of 2000w and our theoretical efficiency of 70%;

2000/.7 = 2857w

Thus when calculating current draw, you need to calculate it using the total power consumed by the amplifier (i.e. the power input), not what the amplifier outputs.

So, to output 2000w at 70% efficiency at 14.4V, we divide the total power consumed by the amplifier (found above) by the voltage to find the current draw;

2857/14.4 = 198.4A

So, the full picture looks like this;

14.4V * 198.4A = 2857w power input (rounded)

2857w input * 70% efficiency = 2000w output (rounded)

 

[thch 10+ year member]

short explaination:

if you want to know _total_ current into the amplifier, you need to know the _total_ power use of the amplifier.

we know some power goes to the speaker -- it makes sound after all!

but we also know that some power goes into running the amplifier -- it gets hot after all!

so the total power would be the sum of these two uses.

 

[talon2nr7588 10+ year member]

You must take into account efficiency.
If the amplifier outputs 2000w, and is 70% efficient (i.e. the power output is 70% of the power input), then the amplifier is going to have a total wattage draw from the electrical system of 2857w

2857w * 70% = 2000w [rounded]

Or, using elementary mathmatics to work backwards since we know the power output of 2000w and our theoretical efficiency of 70%;

2000/.7 = 2857w

Thus when calculating current draw, you need to calculate it using the total power consumed by the amplifier (i.e. the power input), not what the amplifier outputs.

So, to output 2000w at 70% efficiency at 14.4V, we divide the total power consumed by the amplifier (found above) by the voltage to find the current draw;

2857/14.4 = 198.4A

So, the full picture looks like this;

14.4V * 198.4A = 2857w power input (rounded)

2857w input * 70% efficiency = 2000w output (rounded)

okay so you have to find the total power input because out of all the power input not all of it is being used becaue the amp is only 70% efficiency.

short explaination:if you want to know _total_ current into the amplifier, you need to know the _total_ power use of the amplifier.

we know some power goes to the speaker -- it makes sound after all!

but we also know that some power goes into running the amplifier -- it gets hot after all!

so the total power would be the sum of these two uses.

yep i got it now

2000watts(power output) * 70% efficiency(.7) = the 2857 watts going in to the amp, and 70% of the 2857 is 2000 right?

 

[smd4life 5,000+ posts]

yessir