Transformer Polarity
Transformer polarity is fundamental to grasping how transformers
function and how they’re utilized. Understanding polarity is essential
to properly paralleling single-phase transformers and connecting
instrument transformers (current and potential) to metering devices
and protective relays. It has always been a challenge to explain
transformer polarity in a manner easily understood by students.
Occasionally, trainees will ask why there are two transformer
polarities rather than one. This is a logical question for which the
answer has been shrouded in mystery. This paper is intended to
clarify various technical elements of transformer polarity in a
manner useful for training specialists teaching transformer classes.
Let’s start with the definition of polarity: The electrical property of a
body that either develops magnetic poles or has terminal points
between which exists a difference in potential. The word “polarity”
itself refers to these poles, meaning positive and negative (or north
and south, as with Earth’s magnetic poles). Poles are given terminals
that designate direction of current flow. Poles of electrical polarity
(positive and negative) are present in every electrical circuit. In
theory, electrons flow from the negative pole to the positive pole. In
a direct current (DC) circuit, one pole is always negative, the other
pole is always positive, and electrons flow in one direction only. In
an alternating current (AC) circuit, the two poles alternate between
negative and positive, and electron flow reverses back and forth. In
every situation, there are two poles: positive and negative—or, in
transformers, additive and subtractive.
Transformer terminal markings are another key to understanding
polarity. They have been standardized for many years (IEEE Std
C57.12.70-2000) in the following manner: Terminals shall be
distinguished from one another by marking each terminal with a
capital letter (H for the primary winding and X for the secondary)
followed by a subscript. Terminal designation for primary terminals
is easy to remember, since it never changes. When facing the
transformer from the front side, the H1 terminal is always on the left
and the H2 terminal is always on the right. Secondary terminal
markings are different depending on the polarity of the transformer,
as confirmed on the nameplate. If the polarity of the transformer is
subtractive, the X1 terminal is on the left and the X2 terminal is on
the right. Conversely, if the transformer polarity is additive, the X2
terminal is on the left and the X1 terminal is on the right.
When only one lead of the high-voltage winding is brought out (the
other being connected to the tank internally), it shall be designated
as H1. For polarity marking and testing, the H1 terminal shall always
be located on the left when facing the front side of the transformer.
The terminals of any winding whose leads are brought out of the case
shall be numbered 1,2,3,4, etc., the lowest and highest numbers
marking the full winding and the intermediate numbers marking
portions or taps. Thus, If the transformer has a center tap used as a
neutral, it shall be designated as X2. Internally, secondary winding
leads are marked A,B,C,D from left to right.
Transformer polarity depends on which direction the coils are wound
around the core (clockwise or counter clockwise) and how the leads
are brought out from the winding ends to the terminals. The two coil
windings have distinct orientation with respect to one another—each
coil can be wound around the core clockwise or counter clockwise. If
the primary and secondary coils are wound in opposite directions,
the polarity is additive; if wound in the same direction, it’s
Another element in determining transformer polarity is noting how
power flows through the windings. To understand this, voltage and
current that flows across the transformer windings must be observed
when peak positive voltage is being produced. In a 60 Hz AC circuit,
voltage changes polarity 120 times per second. Any time the current
is flowing into one of the primary terminals, it will be flowing out of
one of the secondary terminals.
When the potential of primary terminal H1 “goes positive” and the
secondary terminal on the right also goes positive, current flow
is in on the H1 terminal and out on the secondary terminal on the
When the potential of primary terminal H1 “goes positive” (i.e.
during the first half-cycle of AC) and the secondary terminal on the
left goes positive at the same time, the current flow is in on
the H1 terminal, and out on the secondary terminal to the left.
Note that the secondary terminal designation depends on polarity;
with subtractive polarity X1 is on the left, and with additive polarity
it is on the right.
It is unusual for a line crew to need to conduct a polarity test, since
polarity is confirmed on nameplates; however, there could be a
situation where a nameplate is missing and polarity needs to be
verified. Verifying the polarity of a transformer can be accomplished
with a simple voltage test using the following steps:
1. Make a temporary connection between the primary and
secondary terminals of the left side of the transformer
(when facing the front of the transformer).
2. Connect a portable voltmeter across the primary and
secondary terminals on the right side of the transformer.
3. Apply a low voltage (around 120 volts) to the primary
terminals; this will result in a voltage of about 12 volts
across the secondary winding (provided the turns ratio is
4. If the voltmeter indicates the sum of the voltages (120 + 12
=132) the polarity is additive.
5. If the voltmeter indicates the difference in the voltages
(120-12 = 108) the polarity is subtractive.
Voltage readings may vary somewhat depending upon the turns ratio
of the transformer. If the indicated voltage is more than the applied
voltage, the polarity is additive; if it’s less than the applied voltage,
the polarity is subtractive. Care should be taken to avoid
connecting the 120 volt source across the secondary
terminals, as high voltage will be present at the primary
Another way of understanding polarity is to view the windings on the
same horizontal plane (similar to a single-winding autotransformer)
along with the direction of current flow in each winding.
Transformer polarity became an inherent electrical consideration
when the first AC transformers were developed back in the late
1800s. At that time, pioneers learned what polarity meant when they
attempted to parallel transformers for more capacity. They quickly
discovered that the transformers had to be the same voltage, and
would only operate properly in parallel when the terminals were
connected a certain way. There were no standard markings for the
transformer terminals, and nameplates did not include any
indication of polarity. Frequently, connecting of these early
transformers was done by trial-and-error, and electrical workers
were exposed to hazards created by short-circuits and damaged
transformers. Eventually the industry recognized the need for
clarification and standardization of various aspects of transformer
manufacturing, including polarity.
In 1918, the American Institute of Electrical Engineers and other
organizations established standards for external transformer lead
markings. These markings served as the basis for establishing
polarity as we know it today. The basic standard was as follows: The
leads of any winding (high voltage or low voltage) brought out of
the case shall be numbered 1,2,3,4 etc. The lowest and highest
numbers represent the full winding and the intermediate numbers
represent fractions of the winding or taps.
The first transformers were simply wound with no consideration of
polarity. The origin of the polarity concept is obscure, but
apparently, early transformers having lower primary voltages and
smaller kVA sizes were first built with additive polarity. In the early
1900s, almost all transformers were manufactured with additive
polarity. When the kVA and voltage values were extended, a decision
was made to switch to subtractive polarity.
As the industry became more familiar with transformers, it was
determined that with a two-winding transformer, voltage stress
occurs between the two windings as a result of the difference in
potential (voltage) of the two windings. The magnitude of the voltage
stress is affected by the transformer polarity or direction of current
flow in the two windings. Engineers discovered that as voltage stress
increases, the life of the transformer is shortened. Winding
insulation failures were the main result of the increased voltage
stress. It was discovered that subtractive transformers produce less
voltage stress than additive transformers.
As an example, assume we have a two-winding transformer with a
primary voltage of 25,000 volts and a secondary voltage of 7,200
volts. A comparison of the voltage stress between the windings for
additive and subtractive polarities can be determined as follows:
As you can see by the voltage, stress between the windings is
considerably higher with an additive transformer. This was a factor
in establishing a standard that transformers over 8,660 volts would
have subtractive polarity. The reduction in voltage stress would
result in longer life for transformers with higher voltages. The
obvious question that comes to mind is, “Why not make all
transformers subtractive polarity?”
Since there were large numbers of additive transformers in service,
it was decided to continue manufacturing additive polarity
transformers for voltages below 8,660 volts. Manufacturing
transformers with different polarities is strictly a U.S. standard.
Canadian standards are additive and Mexican standards are
subtractive (not the clearest answer to the lineworker’s question, but
hopefully somewhat understandable). Lineworkers should be
informed that polarity differences are not a significant problem in
the field, as transformer nameplates confirm polarity, and it is rare
lineworkers are confronted with paralleling or banking transformers
of different polarities.
The voltage stress between windings is considerably higher with an
additive transformer.
Today’s standard has evolved over time from ANSI (American
National Standards Institution) to the IEEE (Institute of Electrical
and Electronic Engineers).
By Alan Drew, Vice President of Research & Development