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.
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.
Voltage Drop Per 100 FT Run of Paired Wire
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
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 10-13 AWG.
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:
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
To determine wire gauge necessary given paired wire
length, load current, and desired voltage drop per 100
With these useful tools, voltage drop problems can be
avoided before installation, saving time, money and
ensuring a correctly working system.