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Home / Technical Articles / Understanding Transformer Polarity
Dry-type transformer by Trasfor (company acquired by ABB)
Dry-type transformer by Trasfor (company acquired by ABB)

With power or distribution transformers, polarity is important only if  the need arises to parallel transformers to gain additional capacity or to  hook up three single-phase transformers to make a three-phase bank.   The way the connections are made affects angular displacement, phase  rotation, and direction of rotation of connected motors.

Polarity is also  important when hooking up current transformers for relay protection  and metering.  Transformer polarity depends on which direction coils  are wound around the core (clockwise or counterclockwise) and how  the leads are brought out.

Transformers are sometimes marked at their  terminals with polarity marks.  Often, polarity marks are shown as white paint dots (for plus) or plus-minus marks on the transformer and  symbols on the nameplate.  These marks show the connections where  the input and output voltages (and currents) have the same  instantaneous polarity.

Figure 1 - Polarity Illustrated
Figure 1 - Polarity Illustrated | Test connections for determining polarity using alternating currentwith reduced voltage for excitation on the primary side. (Note that the position od X1 and X3 are reversed)

ANSI designations

More often, transformer polarity is shown simply by the American National Standards Institute (ANSI) designations of the winding leads as H1, H2 and X1, X2.  By ANSI standards, if you face the low-voltage side of a single-phase transformer (the side marked X1, X2), the H1 connection will always be on your far left.  See the single-phase diagrams in figure 1.

If the terminal marked X1 is also on your left, it is subtractive polarity.  If the X1 terminal is on your right, it is additive polarity.  Additive polarity is common for small distribution transformers.

Large transformers, such as GSUs at Reclamation powerplants, are generally subtractive polarity. It is also helpful to think of polarity marks in terms of current direction.  At any instant when the current direction is into a polarity marked terminal of the primary winding, the current direction is out of the terminal with the same polarity mark in the secondary winding.  It is the same as if there were a continuous circuit across the two windings.

Polarity is a convenient way of stating how leads are brought out.  If you want to test for polarity, connect the transformer as shown in figure 14.

A transformer is said to have additive polarity if, when adjacent high and low-voltage terminals are connected and a  voltmeter placed across the other high- and low-voltage terminals, the voltmeter reads the sum (additive) of the high- and low-voltage windings.  It is subtractive polarity if the voltmeter reads the difference (subtractive) between the voltages of the two windings.  If this test is conducted, use the lowest AC voltage available to reduce potential hazards.

An adjustable ac voltage source, such as a variac, is recommended to keep the test voltage low.

SOURCE: Transformers: Basics, Maintenance and Diagnostics

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Ameer Hussain
    Nov 23, 2022

    I can’t understand the mitigation steps for transformer noise or humming.

  2. Charles Maresca
    Dec 28, 2018


    Hope all is well?

    I’m looking for a robust training / explanation to interpreting distribution transformer nameplates.

    The nameplate interpretation (pictorially, polarity, and etc.) by many in the field is confusing and I haven’t seen any comprehensive training literature on the internet or trying to solicit from transformer manufacturers has been unsuccessful.

    Can you lead me in a direction where I could find this requested information?

    PS One linemen asked a very simple question: “Where did the number 8660 come from as it relates to transformer polarity? How was it derived?”

    I has to do with less transformer internal stresses by using subtractive polarity but why 8660 was choose and/or how was it derived?

    Thanks in advance.CM

  3. Girish phatak
    Jan 08, 2018

    Articles are interesting and useful.can we download in PDF form?

  4. Michael
    Aug 19, 2014

    Hello, I have a question which has troubled me through my studies to get a diploma in electrical power. the question is: Which polarity test is best suitable and or preferred between additive and subtractive?

    It would make me happy if you can answer the question with simplified answer

    Thank you

  5. Rajendra Kumar gupta
    Jan 02, 2014

    I am an electrical engineer, i like reading technical articles & would like to share my experience with you guys. It’s a good platform to share experiences of our field of engineering


    R K

  6. Rafay khan
    Oct 10, 2011

    Thnx for share. But where is figure14 which is mentioned in the end of third last paragraph??
    I would like to know the details.
    Will wait for your reply

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