Autotransformer Selection Guide

Autotransformer Selection Guide

An autotransformer is a transformer which has only one winding per phase, part of which is common to both primary and secondary circuits.

Using an autotransformer is an economical and compact means of connecting electrical equipment to a power supply or a different voltage. Part of the winding is common to both primary and secondary circuits so there is no isolation between the two. This may be acceptable on some power systems that do not have a grounded neutral on the secondary side of the main power transformer. Typical applications include motor loads of industrial machinery, electric heating, air conditioners, etc.


• Performs the same function as an isolation transformer of same KVA and voltage rating without the isolation feature
• Ventilated openings in the enclosure are coordinated with the cooling ducts in the coils to ensure proper natural circulation of air for long life and minimum losses.
• NEMA 3R ventilated steel enclosure suitable for dry locations. After fabrication, the enclosure is finished in ASA61 grey powder paint suitable for most industrial and commercial installations.
• Most units have lifting lugs, conduit knockouts and removable front and back covers for easy access to terminals.
• Class 220 insulation
• CSA certified File No. LR34493, UL Listed File No. E108255, ISO9001 Quality Certification


Glossary of Technical Terms

How to Select a Transformer
Product Selection
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How to Select a Transformer

Selecting the right voltage transformer is simple. Review the following considerations to determine the best fit for your application. Then, select a transformer from one of the tables below.

Input Voltage

Select a transformer that will operate on the supply voltage available at your facility (Example: 120V, 240V, or 480V). To ensure compatibility, check the wiring diagram by clicking a part number and viewing its product page.


All the transformers in this section are rated for both 50 and 60 Hz, for use worldwide.


Copper wound models cost more than the equivalent aluminum wound versions. The primary advantage of copper is its superior corrosion resistance. Copper wound transformers are commonly used for corrosive or high humidity environments like marine applications, and when cost is not a factor. Our single phase models are available only with copper windings.

kVA or Amp Output

Select a transformer with kVA based on your load requirements. It is fine to oversize significantly, but never to undersize for your load. For motor loads, do not exceed 60% of the maximum capacity of the transformer, because electric motors have start up requirements significantly higher than their running requirements.

Autotransformer Product Selection

Below is a list of all the TEMCo autotransformer configurations we offer. First, find the primary voltage you need. Then, click on the primary voltage you need to view the full product selection of secondary voltage configurations.

Single Phase Primary Voltages
A. 600 » L. 240 »
B. 575 » M. 230 »
C. 550 » N. 220 »
D. 480 » O. 208 »
E. 460 » P. 200 »
F. 440 » Q. 190 »
G. 416 » R. 120 »
H. 400 » S. 115 »
I. 380 » T. 110 »
J. 347 » U. 100 »
K. 277 »
Three Phase Primary Voltages
Aluminum & Copper
A. 600 » K. 230 »
B. 575 » L. 220 »
C. 550 » M. 216 »
D. 480 » N. 208 »
E. 460 » O. 200 »
F. 440 » P. 190 »
G. 416 » Q. 120 »
H. 400 » R. 115 »
I. 380 » S. 110 »
J. 240 » T. 100 »

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