- Today we're gonna talk about power cables
and how you can pick a wire size for your cables
that maximizes performance and minimizes risks.
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A question I frequently receive is related to wire.
That is, what size wire is proper
for a particular application.
Selecting the proper size or gauge of wire is important,
as too thin of a wire can be a fire risk
and too thick of wire is cumbersome and not cost effective.
Which leads me into this question I received from Ken.
Ken's building a go kit
and isn't certain about what kinda wire to buy
to make up for his assorted power cables.
This question came at the perfect time
as I was in the process of building new power cables
for my FT-891.
With the recent modification on my rig
to the powerpole connectors,
I needed to make new power cables to mate with it.
So let's jump into this topic.
Picking wire gauge can be a challenge.
12 volt DC power can drop greatly over distance.
So choosing the right wire gauge makes a big difference.
Plus, the greater the current we draw from the wire,
the bigger the resistance in voltage drop.
The general rule for DC power
is to keep the voltage drop under three percent
for the length of your cable.
If the drop is greater than three percent,
then choose a larger wire.
This will properly power your equipment
and keep the current draw
within the specifications of the wire.
Overloading the wire is inefficient at best
and dangerous at worst.
So how do we know what size wire to use?
Well, simply stated,
voltage drop is current multiplied by the total resistance
of the circuit conductors.
You know, Ohms law.
In order to make this calculation,
you'll need to know the total resistance of the circuit,
which is made up of factors such as the wire size
or gauge, total length of the conductors,
and the resistivity of the wire material itself.
Now the easiest method to determine voltage drop
without having to measure the resistance with a meter
or looking up values in various tables,
is to use one of the online calculators
to do the math for you.
One of my favorites can be found over at rapidtables.com.
I'll put links to these calculators
in the video description below.
Basically, in how to use the calculator,
you'll select your wire type, size, cable length,
current type, voltage, and the maximum amperage.
The calculator will tell you the voltage drop,
the percentage of drop and the wire's resistance.
Say for example,
I want a six foot power cable for my transceiver.
If I select 12 gauge wire, enter the length of six feet,
select DC current, 13 volts,
which is gonna be the nominal voltage of my battery
and 20 amps of current draw,
which is my maximum current draw.
I'll get a voltage drop of 0.38 volts or 2.93%.
That's just under the three percent target.
So a 12 gauge wire would be the proper size
for my application.
If I would change the wire size to 14 gauge,
the voltage drop percentage climbs up to 4.7%,
which is outside the recommendation.
I'd either have to shorten the cable
or use it in a lower amperage situation.
Now for a couple of real life examples.
In my camping trailer,
I'm using a 12 gauge wire from my solar panel
to my charge controller.
The cable run is about 14 feet long
and the 200 watt panel puts out close to 12 amps.
At a 17 volt open circuit,
this is slightly over the three percent power drop,
and I honestly should be using 10 gauge wire.
But it's close enough to be within the safety tolerances.
I also have a 100 watt portable solar panel,
and with that I use about 20 feet of 14 gauge wire
to run from the panel to the charge controller.
Since that panel outputs between five to six amps
of current at 17 volts,
that length of 14 gauge is
within the three percent tolerance.
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Now, for powering equipment, the amperage,
or current draw is much higher, so I'll use a larger cable.
My power connection cable from the battery
to the transceivers are all 12 gauge.
This handles 23 amps at 13 volts,
which is the maximum current draw of my FT-891
at 100 watts in transmit.
While I usually transmit at a lower power,
I did the calculations for the max power I would generate,
just to be on the safe side.
Now on my vehicle,
I ran 10 gauge cable from the battery
to the 50 watt mobile transceiver.
I used 12 feet of cable, but at that length,
it's rated for 40 amps of current draw,
which gives me the capability for two transceivers
without having to add a second cable.
For the battery to radio connections,
it's okay to be a little oversized.
Just make sure that you're properly fused
for the current load of your cable.
Now it should be noted
that in commercial photovoltaic solar systems,
installers use a slightly different metric
and will strive for no more than a two percent drop
in voltage in their DC circuits.
This has been a hard and fast rule for a very long time
and can be traced back to the days
when solar cells were expensive and copper cheap.
So spending a bit more money on copper wiring
was a relatively inexpensive way
to increase system efficiency.
Now that solar cells are much cheaper,
studies have shown that modern systems
can handle a greater voltage drop
without a correspondingly greater loss in efficiency.
So what does that mean for the average ham
wanting to use some solar to charge their go-box battery?
Well, the two percent voltage drop goal
is a good one to meet in order to optimize performance.
But in reality,
the three percent limit that we commonly use is easier
and cheaper to achieve
and will reduce resistive heating losses.
So to recap.
Voltage drop in a wire is due to the resistive heating
from the wire size, the length, and the material.
DC power cables should be sized
to carry the expected current draw of their load.
Longer cables will require a thicker conductor.
And in order to minimize resistive heating,
you'll want a voltage drop of no more than three percent.
I hope this helps you in building your box
and properly sizing the power cables for your transceivers.
The power cables that come with most radios
are woefully undersized.
So a proper cable will help make
your radio system more efficient.
Questions or comments, leave them down below.
Thanks for watching, I'm Michael, KB9VBR.
Have a great day and 73.
And hey Ken, thanks a lot for sending me your question.
And I hope that this expanded reply
is a little bit more in depth
than what we conversed about in the email.
If any of you have questions about amateur radio, power,
solar, things like that, drop me an email,
and I'd love to feature them in a future video.