Re: Electrical wiring/stereo question

[Bob Hazelwood <woodyh@xxxxxxxxxxxxx>]

On the assumption that there is some general interest here since the concept also
applies to things close to Digester's hearts like Blue Lights and hair dryers,
I'll post this instead of E-mailing.

> -----------------------------
>
> Date: Fri, 15 Jan 1999 10:36:18 -0600
> From: Michael_Bird@domain.elided
> Subject: Electrical wiring/stereo question
>
> <snip> 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.   <snip>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.

You understand better than the salesman. I suspect that the salesman fully
understands the increased commission which comes with more expensive high-margin
accessories. This is how "Ohm's Law" can instantly become "Ohm's Opinion". Wire
does not contribute to current draw. Wire simply passes the current, it is the
load which determines how much current flows in the circuit. In this case the amp
is the only load. The only thing the wire can do is reduce current flow as the
result of it's resistance. If the amp is not drawing power, there is no current.
If there is no current what is supposed to blow the fuse? Nothing. Nada. Zilch.
Null. Of course if you short this wire to ground directly there will be plenty of
current, but not for long! Keep a fire extinguisher handy.

> He also was adamant that nothing
> less than 4 AWG would do the job.  I think that's overkill.

Maybe. It all depends on the amp you end up with. There is no such thing as a wire
which is too big. It just means that there is less voltage drop across a larger
wire than there is across a smaller one at any given current level. The question
is: "when is this drop too much?" The answer is that it depends on the amp. I
would recommend the unthinkable: Take the advice of the manufacturer of the
amplifier! It's probably written somewhere in the owner's manual. I know this
advice will mark me as a radical, but so be it. The '60's (at least what I can
still remember) were good to me so "radical" is fine. Most amp designers assume a
practical loss across the power wiring and design their amps to work perfectly
well with rational power wiring.

In the absence of a manufacturer's recommendation, with an amplifier capable of
400W total RMS output, AWG #8 wire should be fine, as long as you are not going
hundreds of feet with it. The fuse value only needs to be equivalent, or slightly
higher than, whatever fuse comes in the amp. If the amp, for example has a 40A
fuse, all you need is 40 in the wire at the battery end. The purpose of this other
fuse is to protect the wire in case the wire shorts to ground. The lower you can
keep this value, the more protection you have. If you go too low, the fuse will
blow unnecessarily on loud music. If the amp manufacturer puts a fuse of a given
value it means they have determined that the amp does not need more than that
value, then the same value fuse at the battery end of the wire will not blow under
normal use either.

Now if I run AWG#00 to the computer can I get 1000HP from my M50tu? Any comments
Jim C.? ;-)

> 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?

With AWG#4, the 80A fuse will still blow first before the wire burns or the
battery explodes. And yes, you can get 80+ amps from the battery, although not for
long. There is a lot of energy in a car battery. Don't ask how I discovered that
you can weld a wrench to the car very firmly with this energy.  BTW, Anybody want
to buy a 1/2" box wrench? Call before midnight tonight and I'll throw in a 1968
Austin America wheel well welded to the other end. Dial 1-800-DUM-MOVE.

> But he
> was young, and maybe he knows something I don't...

Like the lyrics to all the songs by Green Day? Maybe.

> <snip>
>
> 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?

Special tool. 5-sided. Bavarian Autosport sells them, as do most of the other
parts sources and the dealer, or you can do like I do. Use the "handle" end of a
small Craftsman miniature rat-tail file. The one I have is 5-sided and fits
perfectly. It's about 10 years old now so I don't know if current models are the
same. Also, turn the screws clockwise for release, not counterclockwise as you may
intuitively think.

Best regards,
Bob Hazelwood
VP Product management, a/d/s/
'93 525i 5-sp, Sharked, BL/ss'd, etc. etc.

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