There was a commercial client who converted their building’s electrical services from 240 volts 3-wire to 208Y/120 volts 4-wire. They checked the nameplates on their HVAC system’s motors and saw that they were rated for “208- 230” or “208/230” volts. So they assumed that they would have nothing to worry about.

Soon after their power conversion their motors failed.  The reason, supplied by the repair shop, was that the voltage to the motor was too low. If the motor nameplate showed 208/230 volts what was the problem? Why did the motor fail?

Danger Will Robinson!

Suppliers sell motors and guarantee them for voltages no lower than 10% under the value on the motor nameplate. This is required by NEMA. Manufacturers make standard motors rated for “230” (or 230/460) volts. What this means is that the lowest permissible terminal voltage (not back at the controller, or service entrance) is 90% of 230, or 207 volts.

Also an American National Standard (ANSI C84.1) prescribes the lowest allowable power system or supply voltage. For a “240” volt system, the service entrance voltage can be as low as 220; at motor terminals, the minimum is 212. Thus, the 207 volt motor minimum will always be delivered.

But suppose the same motor is connected to a “208 volt” system. The ANSI standard then allows minimum service entrance voltage of 191; 184 is permitted at the motor.

Those values are well below the minimum 207 volts that the motor requires. Low voltage causes the motor’s current to rise.  The higher amps increase heat in the motor which will cause a failure over time.  This will also void the motor’s warranty and degrade the motor’s performance reducing its startup torque. Without full torque the motor may no longer be able to start its load.

So what’s the meaning of equipment nameplates reading “208-230” or “208/230” volts? This is confusing. What this means is that the basic motor design (most often made for 230 volts) is satisfactory at a terminal voltage of 208 – no less. That’s not the same as saying “this is good for use on a 208 volt system,” where supply voltage can drop below what the motor will handle.

What’s the solution?

Because of the confusion this has caused over the years, NEMA motor standards are now in revision to make this distinction clearer. But the changes may not appear for some time. Remember this; a motor intended for use on any 208 volt system is one designed and nameplated for “200” volts. This is an existing NEMA standard motor nameplate voltage intended for this purpose.

If your motor was oversized for its application and it operates in a cool location, it may operate at the reduced voltage. However, it is far safer to just use a motor properly rated for your building’s voltage.

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