Re: Wire gauge calc?

To: tesla-at-pupman-dot-com
Subject: Re: Wire gauge calc?
From: Jim Lux <jimlux-at-jpl.nasa.gov> (by way of Terry Fritz <twf-at-verinet-dot-com>)
Date: Thu, 07 Jan 1999 12:50:37 -0700
Approved: twf-at-verinet-dot-com

Tesla List wrote:
> > I used to have a calculation to convert AWG<->Inches, but
> > I lost it, anyone know what it is?
> >
> > My wire is 11 mils in diameter, including insulation, but the
> > spreadsheet wants it in gauge.
> >
> > Thanks!
> > DK

Actually, all the excess precision in those calculations is just that,
excess. The tolerance for diameter is 1% or .1 mil, which ever is
bigger. For more precise numbers, you need to talk to the manufacturer
of the specific wire you are using. And, remember that changing the
tension changes the diameter, and the resistance. Interestingly, for
very fine wire, one way the machine that makes it is controlled is by
measuring the resistance per unit length.

For run of the mill calculations remember the following:

AWG #10 is ABOUT .1 inches in diameter (actually, a bit bigger: .1019)
AWG #10 is ABOUT 1 ohm per 1000 ft 
AWG #10 breaks at ABOUT 400 pounds tension

A difference of 20 gauges is a difference of a factor of 10 in diameter
(that is, AWG #30 is about .01 inches in diameter).

A corollary: 10 gauges is a factor of 10 in area, and, because
resistance goes as area, every 10 gauges increases the resistance by a
factor of 10: AWG 20 is ABOUT 10 ohms/1000 ft. breaking strength also
goes roughly as area, so AWG 20 breaks about about 40 pounds (actually,
thin wires have higher breaking strengths in a psi sense).

Insulation thickness varies quite a bit, so take the turns per inch
numbers for closewound coils with a big grain of salt.

In fact, excess calculation precision in building a tesla coil is
pointless effort, because the system itself isn't that well
characterized. One exception would be in mechanical tolerances for parts
that have to be balanced (e.g. rotary spark gaps) or mechanically rigid
(i.e. structural members on big coils).
-- 
Jim Lux                               Jet Propulsion Laboratory
ofc: 818/354-2075     114-B16         Mail Stop 161-213
lab: 818/354-2954     161-110         4800 Oak Grove Drive
fax: 818/393-6875                     Pasadena CA 91109