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240 V Motor Operation on 208 VAC

Thomas Mason, PE

Course Outline

How do you safely reconnect an existing 240 V motor to operate on a new 208 VAC system? This problem frequently arises when replacing power transformers or installing generators. There is a proven method, recognized by the National Electric Code, which is inexpensive and reliable. The application of auto transformers is the topic of this 2-hour PDHonline course. Examples are included.

The course is made up of the following parts:

1. Learning Objective
2. Course Introduction
3. Course Content - summary and detailed narration, graphics and samples.
4. Course Summary
5. Related Links

This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

Learning Objective

At the conclusion of this course, the student will:

Intended Audience

This course is intended for professional engineers, architects and contractors. It will also be of value to persons with technical background who wish to extend their knowledge into new realms. It does not replace a PE license. Some topics are presented only in brief reference. Correct technical terms are used so that an internet search will produces many sources for further information.

Course Introduction

For many, many years, 120/240 V was the only electric system for lights and receptacles. It used three wires - two "hots" and a "neutral". Today, new systems rarely use this form for commercial buildings, mostly because of the availability of high efficiency metal halide and fluorescent lighting at 277V. Another reason that building distribution of 120/240V is becoming rare is that it is inherently inefficient. At low voltage, the wire size must be larger and the losses are larger, nonetheless. One result is that old buildings with 120/240 distribution are being converted to 277/480V. Receptacles are powered by small 120/208V transformers in the electric closets. It is possible, but undesirable, to install small 120/240V transformers.

This leaves a large number of pieces of 240V HVAC equipment incompatible with the new 208V system. A very common decision is to "band-aid" the 208V to make 240V and keep operating the old equipment until it fails. At that time, it will be replaced by 208V equipment and the low-cost, temporary transformers trashed.

Another very common occurrence is the installation of an emergency generator and the transfer of critical loads to an automatic transfer switch which chooses between the utility power and the generator power. (Please see the PDHonline course on generator installation.) Old buildings have multiple services, very often 120/240V, along with newer 120/208V. The generator will be selected for the newer 120/208V, but the existing loads at 120/240 or 240 three-phase must be supported somehow. Again, temporary, low cost auto transformers solve the problem.

At least three serious problems with auto transformers must be addressed. First, auto transformers are confusing. Most technicians who successfully install them consider them magic. For a recent project, 6 PE's at two engineering firms argued for months on sizing the auto transformers for an air compressor. Nonetheless, it takes only a little effort to understand how a transformer works and to understand this specialized application of a transformer. We will succeed is dispelling confusion.

Second, stepping 208V up to 240V is not tidy. It introduces a high-impedance into the circuit for starting a small motor. This means that during the high inrush current of the motor, a high line impedance drops the voltage. This limits starting capability. The solution is to strictly follow the design guide. Give the motor a full 240V before it starts. During inrush, the voltage may drop to 230V, or even 220V, but these are within the specifications of the motor. If you design it to have 230V before it starts, it may drop to 215 or 205V, below the motor rating.

Third, the standard form for three-phase auto transformer application does a very good job for two phases and a poor job for the third phase. Public utilities use this "open delta" connection with the poor third phase, but always warn customers that they should pay $10,000 extra to upgrade the system. The principle applies to small motors, too. You can upgrade a $150 "band-aid" to a $1,000 permanent solution. And, just like the utility choice, the short-term small motor problem may not justify the first-class solution.

Course Content

The course content is in 240 V Motor Operation on 208 VAC (PDF File). You need to open or download any of these documents to study this course.

Course Summary

Auto transformers neatly solve the problem of old motors that match an old electric power distribution system but not the new system. For a small cost, individual motors or groups of motors can keep running well on 240V after the building is converted to 208V.

There are problems and a potential for confusion, but substantial savings are available and the resulting "band-aid" will surely function longer than the load equipment.

Related Links

For additional technical information related to this subject, please visit the following websites or web pages:


Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

Take a Quiz

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.