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Parallel operation of transformers //

In perfect parallel operation of two or more transformers, current in each transformer would be directly proportional to the transformer capacity, and the arithmetic sum would equal one-half the total current.

5 conditions for perfect parallel operation of single phase transformers
5 conditions for perfect parallel operation of single phase transformers (photo credit:
In practice, this is seldom achieved because of small variations in transformers.

However, there are 5 conditions for operating transformers in parallel and they are:

Condition #1

Any combination of positive and negative polarity transformers can be used. However, in all cases, numerical notations must be followed on both primary and secondary connections. That is H1 connected to H1, H2 connected to H2, and X1 connected to X1, X2 connected to X2, X3 connected to X3.

Note that each subscript number on a transformer must be connected to the same subscript number on the other transformer as shown in figure 1.

Paralleling single phase transformers
Figure 1 – Paralleling single phase transformers

With positive and negative polarity transformers, the location of X1 and X2 connections on the tanks will be reversed. Care must be exercised to ensure that terminals are connected, as stated above. See figure 1 above.

Condition #2

Tap settings must be identical.

Condition #3

Voltage ratings must be identical; this, of course, makes the turns ratios also identical.

Condition #4

The percent impedance of one transformer must be between 92½% and 107½% of the other. Otherwise, circulating currents between the two transformers would be excessive.

Condition #5

Frequencies must be identical. Standard frequency in the United States is 60 hertz and usually will not present a problem.

One will notice, from the above requirements, that paralleled transformers do not have to be the same size.

However, to meet the percent impedance requirement, they must be nearly the same size! Most utilities will not parallel transformers if they are more than one standard kVA size rating different from each other. Otherwise, circulating currents are excessive.

Reference // Transformers: Basics, Maintenance, and Diagnostics – U.S. Department of the Interior Bureau of Reclamation

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About Author


Edvard Csanyi

Edvard - 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 fascilities. Professional in AutoCAD programming. Present on


    Jan 24, 2016

    sir, what about the phase correction? Which groups of 3 phase transformers can be connected in parallel?

  2. Arun Nair
    Jan 23, 2016

    What about the phase shift ??

  3. Mario Maggi
    Jan 23, 2016

    Hallo, there is probably a mistake in the Figure 1, third schematic.

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