[Elecraft] Elecraft Digest, Vol 114, Issue 11

[Brad Blasing]

The math looks correct, but the analysis is wrong.  The first statement
"rated for 20 amps at 120 volts" is wrong.  A more correct statement would
be that a given wire size (12 gauge for example) is rated for 20 amps.
Voltage has nothing to do with it.  The amp rating of wire is based on gauge
only and how much heat it is generating per unit length.  Heat is watts, or
I^2*R.

An example:  100' of 12 gauge wire has a resistance of about 0.159 ohms.
At 20 amps, this 100' of wire is generating 20*20*0.159 watts of heat (63.7
watts).
This is what is considered "safe" for that wire gauge.

Having said it has nothing to do with voltage, there is one thing to be
considered.
Over that 100' of wire with 20 amp going through it, there is a voltage
drop of
E=IR or E=20*0.159 or 3.18 volts.  Now, in a 120 volt circuit, a drop of
3.18 volts
may not be very significant.  But in a 13.8 volt circuit, you're down to
10.6 volts
which is quite significant.  So, you may want to up the wire size to cut
down on
voltage drop.

-Brad NN0R

Date: Mon, 7 Oct 2013 09:51:56 -0500
> From: "Richard Fjeld" <rpfjeld at embarqmail.com>
> To: <jim at audiosystemsgroup.com>,        <elecraft at mailman.qth.net>
> Subject: Re: [Elecraft] K3 Suddenly died
> Message-ID: <40B6D4509C21413B80B1A7B30D30E99F at HPCompaq>
> Content-Type: text/plain; format=flowed; charset="iso-8859-1";
>         reply-type=response
>
> To those who replied on, and off line, I don't mind factual disagreement,
> but aren't we above insults? It doesn't get much simpler than this.
>
> R is the resistance of the conductor (wire).
> I is the current flow.
> E is the Electromotive Force, or voltage.
>
> Ohms law is E=IR, and with simple algebra is I=E/R
>
> If a conductor (wire) is rated for 20 amps at 120 volts for a given length
> (due to it's R),
> and we want to use that same wire for 20 amps at 13.8 volts,
> we must reduce R by either increasing the wire size, decreasing the length,
> or a combination of both.
>
> Dick, n0ce
>
>