[ausev] sizing wires based on current

[Christopher Robison]

I find it helps to consider wiring like plumbing. The copper conductor,
like the internal diameter of a pipe, is sized depending on intended
current flow. The thickness of the insulation, like the thickness of the
pipe wall, is determined by intended voltage (pressure).

Size the pipe or conductor too small, and you'll have voltage/pressure
loss.  Size the insulation or pipe wall too small, and bad things happen
-- the pipe will burst, or current will leak through the insulation. 

Also, for a given amount of intended current, the longer the pipe or
wire, the larger you'll want the conductor or pipe diameter to be, to
reduce loss over a long distance.

In reality though, the analogy doesn't completely fit in all aspects. In
plumbing you generally size for maximum typical current flow. In wiring,
you can be a little more flexible if your maximum current is only
periodic and short-term. If you're drawing 40A for only brief moments,
and most of the time it's more like 10A or so, then you could use a much
smaller wire to save weight or cost. The result will be some voltage
drop and wire heating at high currents, but in many cases not enough to
care about.

For actual values, charts like this are a good place to start:

http://www.powerstream.com/Wire_Size.htm


As an example I'll have 4/0 wiring in my truck, at least in the motor
loop. My controller will be able to deliver up to 2000 amps to the
motor, so by the chart linked above the wiring would be WAY undersized.
However, I'm not delivering 2000A continuously but rather only in short
bursts of a few seconds at a time, most of the time it would be a couple
hundred amps for steady state driving. So, 4/0 should be sufficient,
especially if the motor loop is kept short.

  --chris




On Sun, 2006-03-12 at 07:49 -0600, MLAB wrote:
> I assume that one should choose the thickness of one's wires based off 
> of the amount of current going through them, and that voltage does not 
> make as much difference.
> 
> Is there any rule of thumb suggesting what gauge of wire should be used 
> for some expected amp draw?  For example, if I am expecting draws up to 
> 40A, what gauge of wire is recommended?
> 
>