

A primary concern
when installing lengths of wire is voltage drop. The amount of voltage lost
between the originating power supply and the device being powered can be
significant. Improper selection of wire gauge can lead to an unacceptable
voltage drop at load end. The following chart is designed to help calculate
voltage drop per 100 feet of paired wire as a function of wire gauge and load
current.
By matching load current (in AMPs) across the top of the chart
with wire gauge (AWG) down the left side of the chart, one can determine voltage
drop per 100 feet of paired wire run.
NOTE: A paired wire run represents
the feed and return line to the load. Therefore, a 500 foot wire pair is
equivalent to 1000 feet of total wire.
EXAMPLE
ONE:
Given a load
current of 1 AMP, and using 18 AWG wire, how much voltage drop can we expect at
the load end for a 350 foot run of paired wire?
Using the chart, we
match the row for 18 AWG and the column for 1 AMP and determine that voltage
drop per 100 feet is 1.27 Volts. By dividing the paired wire length by 100, we
get the factor by which we need to multiply voltage drop per 100 feet to
determine total voltage drop. Therefore, 350 feet divided by 100 equals 3.5.
Multiply 3.5 by 1.27 volts drop per 100 feet to get your total voltage drop.
Thus the total voltage drop is 3.5 times 1.27, or 4.445 voltage drop for 350
feet.
EXAMPLE
TWO:
Given a
camera load of 2 AMPs, that is 400 feet from the power source, which wire gauge
should be selected to keep voltage drop at the camera to less than 3 volts?
To use the chart, we need to determine what the maximum voltage drop per
100 feet is. We calculate that 100 feet is 1/4 of 400 feet, thus the voltage
drop allowed for 100 feet is 1/4 times 3 volts (which is the equivalent of 0.75
volts per 100 feet):
voltage drop per 100 feet = 3/4 = .75 volts per 100
feet.
So, knowing that we can not allow anything greater than a voltage
drop of .75 volts per 100 feet, we can now look at the chart and select the wire
gauges that will give us lower voltage drops per 100 feet at a 2 AMP load
current. In this case, wire gauges of 10 (.40 V), 11 (.50 V), and 12 AWG (.64)
will all suffice, with 13 AWG (.80) being a possibility.
Thus, in order
to keep voltage drop at the camera to less than 3 volts given a camera load of 2
AMPs and a 400 foot paired wire run, we need to use a wire gauge in the range of
1013 AWG.
Voltage
Drop Per 100 FT Run of Paired Wire
FORMULA
METHODS:
These handy equations can be used to determine voltage drop per 100 feet
or wire gauge as an alternative to the chart, even for values that are not on
the chart. To arrive at total voltage drop, always divide paired wire run length
by 100, and then multiply that number by voltage drop per 100 Feet:
1.
To determine voltage
drop per 100 feet given load current and wire gauge:
VD = Voltage drop per 100 feet (Volts) IL = Current load
(AMPs) AWG = Wire gauge
2. To determine wire gauge necessary
given paired wire length, load current, and desired voltage drop per 100 feet:
With these useful tools, voltage drop problems can be
avoided before installation, saving time, money and ensuring a correctly working
system.
