RE>Electrical wiring/stereo question

["Chao, Harvey" <harvey.chao@xxxxxxxx>]

Now my question:   I was at a local electronics store last night.  For sake
of example, let's call it Circuit City.   I was looking at getting a new
stereo for my E28, and was talking with the installers about the
requirements for an amplifier.  This amp would be around 400 watts (peak),
and he recommended a 4 AWG wire with an 80 AMP fuse. 

If all you are going to run on this wire is the amp, probably overkill.
Because - [Assumption - you will listen to music, and are not going for
distorted whump whump "bass" that can be heard blocks away]
400 watts (Peak)  even if it is 400 watts, and let's further take a worse
case that it is only 50% efficient, i.e. takes 600 watts of power to
generate 400 peak watts of audio, 600/14.4=41.7 amps.
BUT - not only is this peak, it is also instantaneous/ not sustained.  If
you look at audio on an oscilloscope, you will see that the peaks are
typically both non-sustained, separated, and infrequent.  A 400 watt peak
might last a millisecond(?) and the capacitors in the power supply that step
the 14.4 car voltage up to something higher will provide the extra power for
that brief burst.  The most telling thing is how big a fuse does the
manufacturer of the amp install in the amp or it's power line?  That should
be sized to carry the necessary current for start up surge and maximum
sustained output.  Using that as a guide - size your power line accordingly.
Consider that at 120VAC (i.e. in you house) a 12 gauge wire is good for 20
amps, and a 10 gauge wire I think is good for 30 amps.  I would not be
surprised if the manufacturer's fuse for the amp is between 20 or 30 amps
max.  A conservative approach is to add the amperage of all the fuses of all
the equipment you will run on this line and size the fuse for the line at
the battery accordingly.  Note that a lot of low power items tends to be
overfused.

  The last time I did
something like this (back in the eighties, when BMW owners were yuppies,
not the sophisticates that we are now) I ran two large amps and a crossover
off of a single 30 (fused) Amp circuit using 10 AWG wire, with no problems.
The young man at CC was trying to tell me that with 4 AWG that the wire is
so large, even if the amp is not on (not drawing power), a 30 amp fuse
would blow the minute you plug it in.  

That's [BULL PUCKY]e +8 and then some!
If he is literal, that the amp is not on in any way and not drawing any
power, it is electrically impossible and in contravention to the applicable
laws of physics that hooking one end of a wire to the battery, no matter how
big the wire, that the surge will blow the fuse.  Unless the other end of
the wire goes to ground, either via a short circuit, or via some appliance
with a huge turn on surge there can be no current flow, hence even a 1 micro
amp fuse will not blow.  Biggest line of BS I have heard in years.   See my
next response below.

 I just don't get it.   That's not
how I understand electricity to work.   He also was adamant that nothing
less than 4 AWG would do the job.  I think that's overkill.   I also told
him that the car would be on fire before an 80 AMP fuse would blow.   I
also wonder if the electrical system would even produce 80 amps?   But he
was young, and maybe he knows something I don't...

What he knows is that he works on commission.  If he sells you Installation
through his shop and the parts for the specialty 4 gauge wire and special 80
amp fuse, that is more commission in his pocket!  

So, do any of you guys/gals have any thoughts?  Am I right?  If not, can
you explain it to me in layman's terms?   I am assuming that there are
enough youngsters and oldsters alike (I fall in-between, thank you very
much) that this would be of general interest, but if I am mistaken, I
apologize (and you may answer privately).

Personally, I conclude he was trying to pad his profit/commission.

Question #2 :  Removing the stock head unit from dash: there appears to be
two allen head bolts holding it in, somewhere between 2 and 2 1/2 mm.
None of my various metric and standard allen wrenches will fit.
Suggestions?   Special tool? - 

Yes - a special 5 sided cross section.  BUT - my experience has been that if
I went to the local dealer at a time when the service writers were not
particularly busy and asked politely, they had one of these tools in their
desk and would walk out to the car and unscrew the bolts for me.

Harvey
That kind of sales guy's self serving BS really lights my fire!

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