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John Drake Services, Inc.
1427 E. 68th Street
Long Beach, CA 90805

voice line
(562) 423-4879

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Voltage Drop - Wire Loss, What are they?
I am not going to go into complex mathematics to demonstrate the importance of using the proper wire
size (gauge) in a low voltage d.c. system.

In all candor, I am no math whiz.

The short story is, given a specific length of wire, the more amps (quantity) that pass through it the more the
voltage (pressure) will drop.

This is much more important when putting together a low voltage d.c. wiring circuit than with a 120 volt a.c. circuit.

For those of you who own a 120 volt a.c. circular saw, or any other high amp draw device,
there is usually a guide somewhere in the instruction booklet.
It will show the gauge size of the extension cord based on the length of the cord.
The longer the cord, the heavier the wire size.

When I had a welding truck, all of my extension cords were 10 gauge.
Using a 14" abrasive chop saw or plasma arc cutter one-hundred feet from the
truck requires heavy cord to keep the 120 volt a.c. from dropping too low at your tools
and making them over-amp (smoke).

It is even more important in the systems we are working on.

A not so quick example.

We use biodegradeable laundry detergent and fabric softener.
This allows us to reuse the laundry water in our garden and on our landscaping.

The wash water gravity flows from the washer to a sump where it is pumped up into
two plastic 55 gallon drums with a 120 v.a.c. sump pump.

We have two pumps, one to push water to the front yard to water the lawn and plants.

The other pumps water through a heavy hose for all of the plants (somewhere
around a zillion) in the back yard.

These pumps run off of two Trojan T-105 six volt batteries in series for 12 volts.
They are charged with a solar array.

The cables from the fuse block to the batteries is 2 gauge welding cable.
The wires from the fuse block to the front yard pump is 12 gauge to the pump
enclosure and what looks like 14 or 16 gauge wire on the pump itself.
The wire run from the front pump, through the fuse block and to the battery
bank is about four feet - no big thing.
You can see by this picture the sizes of the wiring.
pump fuse block connections

Pump and connections for the front yard.

12 volt d.c. pump for front yard

On the other hand, the pump for the backyard needs a wire run of about twenty feet.
In this case I dug out some 8 gauge two conductor tray cable.
I had enough to make two runs to feed the back pump to prevent a damaging voltage drop when the pump was started.
Low voltage pumps can draw 100 to 200 percent over their name plate amp rating just to start.
In a low battery voltage condition, the pump can over-amp which will cause over-heating and ruin the pump.

You can see the light gauge wire coming from the pump into the left side of the feed through splicer block.

On the right side are two positive 8 gauge wires coming from the switch and two negative 8 gauge wires coming the battery.

12 volt d.c. pump with bypass     

The black and red wires in the gray jacket are a piece of marine cable from the cut-off wire pile.

splicer blodk

You can see how the cables to and from the switch are doubled up - in my case,
I use what I can find.

switch connections

These are the 8 gauge tray cables, positive from the fuse block and negative from the
battery bank.

input to switch and splicer block

This battery system also powers up a 16" 12 volt d.c. venturi attic fan.
It is a long wire run but the amp draw is very low.
If the attic fan is running and one of the pumps is turned on it shuts down
almost instantly.
Like plumbing, you only have so much pressure to go around.