Re: Alt Reg. Mod (sizing wire)

[James Marriott <marriott@micron.net>]

Huw Powell wrote:
> The only comment I would have it it seems like it says to use 16 GA wire
> for the + output.  Isn't that a little light for 90-120 amps?  Aside from
> the overkill typical in my high current applications, what are the DC
> ratings for the various AWG's?  Anyone?  VAG seems to think 18-20 GA will
> handle just about anything, but we know better.

What do those speakers use, like 40?

>From _Mark's Mech Eng Handbook_:
Ga*  Amp  *American Wire Gauge
 14   15
 12   20
 10   30
  8   40
  6   55
  4   70
  3   80
  2   95
  1  110
  0  125
 00  145
000  165
0000 195

>From memory
 16   11
 18    8
 20    6
 22    4

Issues of concern here are voltage drop (some of your power goes to heat
the wire) and temperature. A larger wire will have less drop, and
therefore deliver more of the juice to the desired point. That is, it
will be more "efficient." Also, while you can get 30 amps through a 14
gauge wire, but it's going to get hot and likely burn the insulation.
Generally, using a larger wire than the table says is OK or even
desirable. Places where I can imagine it might hurt (Igor?) is when the
circuit is depending on a certain resistance from the conductors. The
larger wire throws of this calibration.

One other thing:  to find the current, divide the power by the voltage.
eg, a 100 W headlamp draws 100/12=8 amps. Therefore two bulbs would be
just over the rating for 14 gauge. Since you're doing the wiring because
the stock stuff is substandard, step up to 12 ga or even 10 ga. Also, a
fuse that is rated AT OR _BELOW_ the current of the weakest wire is a
good idea, and place it as close to the source (batt or alt) as
possible. Why the "weakest?" Say you use a 10 gauge to the relay, and
then a 12 to each bulb. A 30 amp fuse (sized for 10 ga) will _not_ save
a 12 guage wire. Congrats, you've just engineered a harness to Audi
standards! Do this one right and use a 20 A fuse.

hth,
James