Eisermann, Phil wrote:
> 
> > At 01:54 PM 12/30/98 -0500, Eisermann, Phil wrote...
> > >
> > >        Then multiply Irms*500 to get the number of circular mils.
> > >
> > >        For example, assume Irms = 40mA, you'd need
> > >        0.04 * 500 = 20 circular mils. so you need a wire
> > >        with a diameter of sqrt(20) = 4.5 circular mils.
> > >        That's between #37AWG and #36AWG. use #36AWG.
> > >
> > >        (#36AWG has a diameter of 5 mils. That's an
> > >        area of 25 circular mils, or 19.63E-6 sq. in)
> >
> > Because there is a potential error here of > 10%, I thought I would
> > point it out:
> >
> > Circular area is PI*r^2, so #36 is 3.1416 * 2.5 mils ^2 = 19.63
> > mils^2.
> >
>         i will agree that the area of a circle 5 mils in
>         diameter is 19.63 square microinches.
> 
> > For 20 mils^2, the diameter would be 2 * sqrt(20 mils^2 /PI) = 5.05
> > mils
> >
>         no, because the area *IS*NOT* 20 square mils,
>         but 20 circular mils. There is a difference
>         between area in square inches and circular mils.
>         I figured out the required diameter to get 20
>         circular mils, then calculated the area in
>         square inches.
> 
>         if you use a diameter of 5.05 mils, you would
>         get 25.5 circular mils. That's equal to
>         ~20 square microinches, as you pointed out.
>         In the previous example, a diameter of 5 mils
>         gives you 25 circular mils, which is equal to
>         ~19.63 square microinches.
> 
>         is anyone confused yet?

I am.
According to my limited knowledge, current only flows on 
the skin of a conductor because a potential difference 
cannot exist inside a conductor (like a Faraday cage).
Because we work with less-than-perfect conductors, some 
current will also flow a little deeper into the conductor, 
but the heavy current flows on the surface.

This is why a ribbon can conduct more than a rod. Following 
this potentially flawed reasoning, I would expect the 
circumference of a conductor to be of more significance than 
the diameter.

-- 
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