WHITE PAPER:
MEASURING NEUTRAL VOLTAGE DROP WITH THE EAGLE 120
Contributed by Cowles Andrus March 2013
i
ABSTRACT
This paper shows why the Eagle 120 measures a different
voltage potential between the neutral and ground and
why the neutral’s voltage is elevated above ground under
a load condition. It goes on to explain why neutral-line
voltage drop is larger than the neutral-ground voltage.
VH/N
Neutral
Hot
Ground
RESISTANCE AND VOLTAGE DROPS
In a properly wired system, the neutral is only connected
to ground at the power transformer and the transformer’s
case and at the service entrance. In many homes and
businesses, there is considerable wire length between
where the neutral and ground are tied together at the
service entrance and the wall receptacle. Where there is
wire, there is resistance, and, the more wire length, the
more resistance. As loads are applied, current flows, and
this line resistance always causes voltage drops. As the
diameter of wire decreases, the resistance increases,
and subsequently an increase in voltage drops is seen.
The Eagle 120 plugs into a standard wall receptacle and
makes it very easy to evaluate and resolve neutral to
ground voltage elevation issues.
On a standard 120 volt receptacle, by convention and
electrical code, the left terminal, where the white wire is
connected, is the neutral, and the right terminal, where
the black wire is connected, is the hot. The ground is
of course the connection in the middle below the two
blade connections, and is connected to the green wire as
shown in Figure 1. As the load draws current, a voltage
drop develops across the supply and return conductors.
neutral to ground voltage measurements at the load
will reflect the voltage drop across the return (neutral)
conductor.
In the diagram shown in Figure 1, if the load is 10 ohms
and the wire resistance is 1 ohm in the hot and neutral
line, and the source voltage is 120, then the voltage
across the load would be 100 volts with 10 volts dropped
in the hot and another 10 volts dropped in the neutral
wire. The voltage measured between the neutral and
ground should also be 10 volts. This is because unless
there is a ground fault, there is no current flowing in the
ground line, therefore no voltage to drop, so ground is
by definition at 0 volts and neutral would be elevated
to 10 volts due to the voltage drop that occurs across
the 1 ohm wire resistance with 10 amps flowing through
it. Another interesting fact is that if the resistance is
indeed 1 ohm in the hot line, with the 10 amp load, 10
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VH/G
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VN/G
volts is also the drop on this leg before getting to the Figure 1. Basic singleload. This example uses line resistance that is much phase circuit and load
greater than what would normally be found to simplify
the math and demonstrate the concept of load voltage
drop due to resistance in the hot and neutral wire and the
relationship when the measurement is being made if the
ground is used as the reference. At first glance, a lot of
technicians pick up on the voltage drop in the hot without
realizing that there is also a drop in the neutral under
load conditions. Basic knowledge of Ohm’s law is all
that is required to understand why the neutral potential
changes in respect to the ground potential under a
loading condition. To get 1 ohm worth of resistance in
#12 gauge copper wire, it would have to be 630 feet long.
To get a 1 volt drop, however, it would only require 63
feet of #12 gauge copper wire with a 10 amp load. As a
reference, measuring directly at the source and directly
across the load, this would be a 2 volt drop.
The Eagle 120 receptacle recorder is designed to plug
into a normal 120 wall outlet. A load can then be plugged
into the Eagle and its current can be measured (see
Figure 2). Factors including distance from the source to
the wall plug, the wire material, the wire diameter, and
the load applied will determine the amount of voltage
drop the Eagle 120 will measure from ground to neutral
and ground to hot.
When is the voltage difference between the neutral and
ground an issue? That depends on how sensitive the
equipment that is plugged in is to changes in voltage
between the two lines. For example, when two devices Figure 2. The Eagle 120
are trying to communicate via a USB circuit and the is designed to plug into a
power supply and circuit boards are not tied to the standard 120V wall outlet.
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WHITE PAPER:
MEASURING NEUTRAL VOLTAGE DROP WITH THE EAGLE 120
464uH
1
have been more common than in today’s world. With
strictly 60 Hz, the inductive reactance of the low value
i
of the wiring inductance was not much. However, with
the advent of semiconductors creating nonlinear loads,
this low inductance needs to be considered. Nonlinear
loads create harmonics of many multiples of the base
60 Hz frequency. As the frequency increases due to
VH/G=100V
120V
VH/N=100V
the harmonic content, so does power line inductive
reactance. XL = 2πFL, Inductive Reactance is equal
Neutral
to ~6.28 times frequency in Hz times the inductance in
Hot
henries. As shown in Figure 3, the inductance of a 630
Ground
464 uH
1
foot piece of #12 gauge copper wire is around 464 x 10 -6 H
or 464 uH. At 60 Hz there would be very little impedance
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added, in the order of 0.175 ohms. As the harmonic
VN/G=10V
content becomes greater, at some point, the line’s
inductive reactance could increase to add significantly to
the overall impedance and thus the overall voltage drop
between the neutral and ground. This could only happen
Figure 3. Inductance on chassis ground and the USB circuitry does have ground in a situation where the nonlinear load generates a lot
a 630 foot peice of #12 connection, if the difference between the ground and of high frequency harmonics. Typically this still is not
gauge copper neutral becomes too great, communication can be much of an issue but could explain some of the neutral
affected and data errors introduced.
to ground voltage.
Up until this point, neutral to ground voltage difference
has been discussed as if it were only caused by wire
resistance. This is a good assumption if the load is
purely resistive. A few decades ago this scenario would
The graph in Figure 4 shows how the Eagle 120 displays
what happens when a heavy ~5.5 amp and 11 amp
load are placed on a long run to a power receptacle.
Channel 1 shows what happened between the hot or line
Figure 4. Graph resulting from ~5.5 amp and 11 amp loads placed on a long run to a 120V receptacle
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WHITE PAPER:
MEASURING NEUTRAL VOLTAGE DROP WITH THE EAGLE 120
voltage in reference to the neutral. Channel 2 displays
the voltage difference between the ground and the
neutral. Notice that the total voltage drop across the line
is approximately 2 times the ground to neutral voltage
as shown in Figure 5. If the hot and neutral wires are
the same size, thus the same resistance, then half of the
supply voltage is dropped across each wire, and since
the ground wire is not loaded, the potential on it stays at
zero. With no load applied, the neutral and ground are at
the same potential. When the current flow in channel A is
compared with the voltage, as shown in Figure 6, there is
a clear relationship between the load and the change in
potential between the ground and the neutral.
RI=0.8
i =10.74A
VS = 121.9V
VH/N=104.5V
Neutral
Hot
Ground
RI=0.8
i =10.74A
CONCLUSION
VH/G =113.1V
VN/G=8.6V
It is normal for the voltages measured on the neutral line
to be slightly elevated above the system’s ground due to
current flow in the neutral to hot circuit. The more load Figure 5. 60 Hz is the resistive load. The inductive reactance is negligible with a pure
that is applied, the more current flow in the neutral wire resistive load.
Figure 6. Current flow compared with voltage
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WHITE PAPER:
MEASURING NEUTRAL VOLTAGE DROP WITH THE EAGLE 120
and the more this voltage is elevated. The difference is
voltage drop due to the current flowing in the neutral line,
due to its impedance, compared to no voltage drop in the
ground line. No current should be flowing in the ground
line unless there is a ground fault condition, so some
difference is to be expected under normal conditions.
It is usually only when equipment is very sensitive to
these variations that this slight elevation would present
a real issue. With the Eagle 120, it becomes very easy
to evaluate neutral to ground voltage elevation issues
and correlate this phenomena to problems such as USB
communication issues. Under normal conditions, expect
the ground to neutral potential to be approximately one
half the total voltage drop from the source where the
ground and neutral are tied together, to the load.
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One way to reduce the neutral to ground differential is to
reduce the resistance in the neutral wire. First make sure
that all electrical connections are solid on the breaker
panel and on the receptacle. Then increase the wire
gauge and if possible shorten the wire length between
the breaker box and the wall receptacle. Make sure
that the ground and neutral are bonded at the service
entrance and arev in compliance with electrical code.
Cowles Andrus, III
Communications Specialist
candrus@powermonitors.com
www.powermonitors.com
1.800.296.4120
© Power Monitors, Inc. 2013 • 800.296.4120 • www.powermonitors.com