After consulting copper wire tables and typical connection resistances,
I can present the following analysis for a 20 amp load:
Often hams power their 100 watt transceivers through DC distribution
devices such as a RigRunner which will contribute to the voltage drop.
Also in-line fuses will contribute to voltage drop.
Connection resistance contributes .05 volt loss under a 20 amp load for
each contact point, you have 6 contact points in the path for a fused
RigRunner plus two in the APP connector at the transceiver, plus the
connection to the power supply for a total of 10 contact points. You
have to consider the negative path as well as the positive. So that is
a total of 0.5 volts of the total voltage drop.
By contrast, a 5 foot length under a 20 amp load of #12 wire has 0.1588
ohms (in both conductors) for a voltage drop of 0.3176 volts, and #10
wire a drop of 0.1998 volts.
So conclusion is that while increasing the wire size can reduce the
voltage drop, the main contributor is in the power distribution system.
A path direct from the power supply terminals can be expected to have a
0.5 volt drop with 5 feet of #12 wire to the inside of the K3. and #10
wire can have a 0.4 volt drop - the difference is only slightly significant.
Conclusion - in a 5 foot power cable run, the difference between #10 and
#12 wire is only 0.1 volts (0.05 volts in each of the positive and
negative leads) - the major contributor is the number of contact points.
So for those who choose to measure the voltage drop from the power
supply terminals to the APP connector on the outside of the K3, you
should measure about 0.2 volts in each conductor with #12 wire and 0.15
volts with #10 wire. If it is much more than that, check your power
supply connection tightness and your crimp connections.
Run the transceiver direct from a power supply using ring terminals
instead of routing through a power distribution accessory.
73,
Don W3FPR
On 6/12/2019 6:29 PM, Bob McGraw K4TAX wrote:
Regarding "heavy gauge wire", I suggest one measure the voltage drop
using a DVM connected direct between the power supply Pos terminal and
the radio Pos terminal. Likewise do the same for the Neg DC line. Put
the radio in CW mode and close the key for rated output. If one finds
more than 0.25 volts drop in either the Pos or Neg line, I'd say that
attention to the power cable and connectors would be in order. Also
measure between radio ground and power supply ground. Again a voltage
value greater than 0.25 volts indicates attention to the power
distribution system and station equipment grounding is needed.
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