Re: Correct wire sizing

[Don Hoefer <hoefer@jaxom.eng.pko.dec.com>]

Mike requests:

> Is there a formula or table to choose the correct guage wire for accessories
> like fog lights?

Probably the best place to look is in the National Electrical Code.  They
are conservative and are confusing because they take wire insulation type,
use, and temperature into account.  You can find it in any well stocked
book store.  Automobile manufactures often use the smallest gauge that they
can to save weight and cost.  An alternative is to decide how much voltage
drop you are willing to tolerate and size the wire accordingly for your
current requirements.  Also bear in mind that the most loss will usually
occur at the connections.  That is why appliance plugs often get hot,
while the cord does not.

Here is a quick guide for the most common sizes:

AWG       AMPS       OHMS/1000ft      Vloss = #ft*#amps*(OHMS/1000)/1000

20          2           10.35
18          5           6.510
16         10           4.094
14         15           2.575
12         20           1.619
10         30           1.081
 8         45           .6405
 6         60           .4028
 
There is some religion involved here too.  Some people will use 10 gauge
wire for a few amps while others don't sweat when using 14 gauge for 25
amps.

Is there an SAE specification for this stuff?
 
Most "automotive hookup wire" sold in auto parts stores has the teflon
type of insulation that will tolerate the high temperatures found in a
car.  Don't use the PVC type or other general purpose electronic hookup
wire.  It won't last in the engine compartment and will melt more easily
or burn in the event of a failure.

Happy wiring.

Don Hoefer
'82 Coupe