Re: Wiring an amp

In rec.audio.car on Fri, 19 Aug 2005 01:19:45 -0500 "Sean"
<anonymous@xxxxxxxxxxx> posted:

>On
>>I have a Kenwood KFCW-300S Sub that I am connecting to a SPLX SPLA350 Amp
>>using an Autoleads PC4-20 wiring kit.
>>
>>The wiring kit comes with a 15, 20 and 30 Amp Fuse, should I use the 30, 20
>>or 10 Amp fuse for the amp live from the car battery?
>>
>>
>>
>
>Look up the current draw the amp pulls from the battery (it should be in the
>manual), and put in a fuse that's closest, but not above, that amount.

I think you mean closest but not *below* that amount.

If the amp draws 17 amps and he puts in a 15 amp fuse, it will blow
every time.

Another thing you could do, Chris, is start with the smallest fuse,
prepared to see it blow. If it does, move up to the next bigger.
Etc. You can do this when the wires are connected but not in place,
or after the wires are in place unless you think you messed up already
:) (I don't like to waste things, but I don't think there are any
low-supply materials used in fuses. ??)

I don't know details about amps, but it seems they must draw more
current when one is playing something loud. So either test with
something loud, or keep the next size bigger fuse around for the day
you go real loud and blow this one. (It's a car. I carry a bunch of
fuses of various physical and electrical sizes so that I'll always
have a spare for any circuit. The fuse box of my car even came with a
place for storing spare fuses. Actually, I don't have the big fuses
that are used under the hood, but none has ever blown.)

Maybe one shouldn't, but temporarily I would use a bigger or even much
bigger fuse than I should. It won't make a difference until there is
a short in that circuit. Even then, a fuse that is 5 or 10 amps too
big will probably blow before there is added damage to the circuit.
If the fuse is 20 amps too big****, what I think will happen is that
from the short (inside the amp, for example, or where the wire to the
amp rubs or cuts against a metal part of the car) back towards the
fuse until it reaches wire that is heavy enough to carry the full load
of that fuse (say 30 amp wire if you used a 30 amp fuse) the wire
itself acts as a fuse, and might blow in one spot, but more likely
will heat the insulation which can drip off the wire. You may be
stuck replacing that entire length of wire, from the short back to the
heavy duty wire.

Other people here know a lot better than I how extensive the damage
can be, and how much trouble it can be to replace it.

****but there was no real short that caused the first fuse too blow.
Either the amp was too big for the fuse, or perhaps you stuck a
screwdriver in a light socket and let the shaft touch a ground.
That's a temporary short, but when you remove the screwdriver it's
gone.

Another thing you could do is put in a big enough fuse and run the
current through an ammeter to see what it really is. The ammeters I
have are part of multimeters, and I think the highest they go is 10
amps. Sounds like "not enough" and you will or may damage the ammeter
I guess if you go higher.

(I suppose you could connect two or three in parallel and add the
readings, but how would you make sure the current spread out so that
no meter got more than 10 amps. A bad connection anywhere would mess
this up, and even if all the connections were good, how would one know
what would happen? Don't try this.)

I built a shunt a long time ago, when someone gave me some low but
measurable resistance wire. So I could run the current through that,
and measure the voltage drop across it. (That's what an ammeter
vasically is, except the low-resistance wire is part of the meter, and
the scale is already calibrated in amps, and not volts) But I've never
noticed this wire for sale since then. And I've only used the thing 2
or 3 times in all these years. My first suggestion is probably better
than my others, or Sean's with the modification.

Meirman
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