Mark,
You said; "If we tried to use the same cable at 10 times the current we
would have 10 times the
voltage drop. To avoid that we use a larger diameter conductor."
Yes!
People interpret statements differently. There is a problem with brevity
on a reflector such as this.
My opening statement of my original post would have been better as this;
Our house wiring codes in my area say that #12 wire can be used for 20 Amp
max (with certain conditions) for 120 Volt circuits.
I said 'typically ~117 volts'. (There are other factors for this rating.)
When I think of 20 amps of current in a 13.8 volt circuit, I first think 12
gauge wire, but if voltage drop is a concern, then it may
be necessary to go with a larger diameter conductor. I mentioned short lead
lengths often save us (from voltage drop).
The real point of my post was how to measure the voltage drop in each
conductor at key-down (load). It seems to have been missed.
You can do this with one DMM lead at the power supply end, and the other
lead at the radio end* for each conductor.
You will measure the actual voltage drop in each conductor, one at a time.
The total drop will be the sum of the two conductor drops.
*For this measurement, the 12V out jack will not show an accurate voltage
drop on each conductor. However, the 12V out jack can be
used for a quick and convenient fault check of the power cable and power
pole connector continuity by measuring the voltage during
key-down at the 12V out jack. I caution folks to wire up a plug for making
fault-free connections to this jack.
My K3/100 at the 12V out jack reads approx. 1.4 volts less than my power
supply voltage with adequate cabling at 100 watts key-down.
Sorry for so much band-width, but it is necessary to avoid
mis-understanding.
Dick, n0ce
-----Original Message-----
From: Mark Bayern
Sent: Monday, October 07, 2013 10:57 AM
To: Richard Fjeld
Cc: j...@audiosystemsgroup.com ; Elecraft Reflector
Subject: Re: [Elecraft] K3 Suddenly died
If a conductor (wire) is rated for 20 amps at 120 volts for a given length
(due to it's R),
1 -- The voltage rating of a conductor has to do with its insulation.
It has nothing to do with the potential current flow or what you're
calling the wire's "R".
2 -- The applied voltage has no effect on the voltage drop of a
conductor. Look at Ohm's Law again. The voltage drop is due to the
resistance of the conductor (wire) and the current flowing through
that conductor. A conductor with a 2 ohm resistance will have 2 volt
drop when 1 amp flows. That 2 volts is the same for any applied
voltage as long as there is 1 amp flowing. It can be a 12 volt system,
a 120volt system or a 1200 volt system. One amp flowing through a
conductor that has a 2 ohm resistance will have a 2 volt drop.
... 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.
Wrong. If the conductor is good for 20amps it is good for 20amps
_at_any_voltage_. This is probably what activated Jim's "Nonsense
Alert!" beacon.
So ... why use larger conductors at lower voltages? Because for most
uses we want to transfer power. To keep the power the same, we need
more current at lower voltages. Example: A cable that supplies 120
watts at 120Volts needs to conduct 1 amp of current flow. (Power =
current times voltage. P = I * E ). To get the same 120watts from a
12V system you will need 10 amps of current flow. If we tried to use
the same cable at 10 times the current we would have 10 times the
voltage drop. To avoid that we use a larger diameter conductor.
Mark AD5SS
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