Re: Electrical wiring/stereo question

[Luis Marques <marques@xxxxxx>]

Michael_Bird@domain.elided wrote:
> 
> 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.  
> 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.   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...

I'm not a car audio expert, but sounds like this guy is trying to sell
you a ridiculously oversized "installation kit" for $100 more than what
you need.

First let me say that the "peak power output" spec is pretty much
meaningless.  It just tells you how much power the amplifier can deliver
for a millisecond assuming some load at some frequency, etc., etc.  A
much more meaningful spec is the RMS power output, or how much power can
the unit deliver continuously.  That's how a good 35w RMS amplifier can
sound a lot better than a cheap 200w peak unit that begins to distort
the music the moment you crank up the volume.

Now, let's talk about current consumption.  The 400w peak amplifier only
delivers 200w RMS (or "average" power).  Assuming it's a class A
circuitry, the best efficiency you can expect is 50%, so it will be
sucking at least 400w from the cars electrical system (when you have the
volume at max).  Depending on the system voltage, that's between 29-34
amps.  If you won't be listening to it very loudly, you could get away
with a 30 amp fuse.  If you will be cranking up the volume, you will
want to give yourself some margin and use a 40 amp fuse.  Of course, you
will want to use a wire size that can handle this type of currents.  A
10 AWG wire will handle the 30 amps.  A 8 AWG wire will handle 40 amps. 
At least the salesperson was right in that to handle 80 amps you will
need a 3 or 4 AWG wire.

Granted, when there is a sudden demand like a silence an then an abrupt
loud sound, the internal capacitors may not be able to handle it
completely and the current consumption will increase momentarily, but it
will be so brief that the wires won't heat up and the fuse won't blow. 
Now, if you have a really crappy amplifier, this may be the norm rather
than the exception, but by then the sound will be distorted and you will
probably be turning down the volume soon.

Finally, fuses blow as a function of the current flowing thru them, not
the size of the wire.  So even if you have a 4 AWG wire, unless the
current goes above 30 amps for a few minutes, it won't blow.  Fuses
(unless they are "fast acting") are rated to open only after the rated
current is exceeded continuously for a few minutes.  They only blow
instantly when there is a short circuit and the current could be 1000
amps for a fraction of a second while the fuse blows.

Now, if you were actually looking at a 400W *RMS* amplifier (and that's
a monster of an amplifier), then yes, you need the 80 amp fuse and 8 AWG
wire.

HTH

Luis Marques
'97 328i

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