I better say 'twice the current carrying capacity'.
-----Original Message-----
From: Richard Fjeld
Sent: Monday, October 07, 2013 1:44 PM
To: Brad Blasing ; elecraft@mailman.qth.net
Subject: Re: [Elecraft] Elecraft Digest, Vol 114, Issue 11
I didn't check your figures, but I agree with your understanding except at
240 volts, that same wire gauge would have twice the capacity.
Dick, n0ce
-----Original Message-----
From: Brad Blasing
Sent: Monday, October 07, 2013 1:22 PM
To: elecraft@mailman.qth.net
Subject: Re: [Elecraft] Elecraft Digest, Vol 114, Issue 11
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
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