Motor Voltage Rating +/- 10% ?.....

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ibew441dc

ibew441dc

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Can someone provide me with a valid reference that states that motors can be installed at voltages other than stated on the nameplate?

I.E.- 230 volt (nameplate) on a 240 volt nominal system
460 volt (nameplate) on a 480 volt nominal system

(note**I fully understand the intent of 110.4 and 110.3(B), as well as the theory behind why it can be done.)

I was teaching a NEC CEU course and the question came up. Rather than provide an opinion, I told him somebody off this forum has to have the answer. I have checked the UL White Book....but came up short.

I really don't want to bye a bunch of expensive UL books if I dont have to......

Thanks,
 
OOps! I forgot.....

The person asking me the question was very committed to the fact that it is a code violation of 110.4.

I told him, no it isn't ...that motors are generally rated at +/- 10%.

He of course said, show me!

I said ...I'll get back to you on that...
 
Like this? -

Variation from Rated Voltage:

In accordance with NEMA MG 1, 12.44, motors shall operate successfully under running conditions at rated load with variation in the voltage up to the following percentages of rated voltage:

Universal motors except fan motors - plus or minus 6 percent (with rated frequency).
Induction motors - plus or minus 10 percent (with rated frequency).

For more of the story- Go to this link :)

Say thanks to a bro in the frozen north....forgot, snowbirdin in Vegas :)!
 
Motors are not rated the same as nominal voltage, IE a 208 V system has a motor rated 200 V. This allows for voltage drop to the motor. Using a 200V motor on a 240 volt system will result in an increase in current at the magnetic fields become saturated.

And this NEC does not allow this:
NEC 110.4 Voltages.
Throughout this Code, the voltage considered shall be that at which the circuit operates. The voltage rating of electrical equipment shall not be less than the nominal voltage of a circuit to which it is connected.

Using a 230 volt motor on a 208 volt system would be a violation of 110.3(B), and probably be below the 10% tolerance of the motor.
The NEC does not allow motors to be used at other than nameplate voltage, and its hard on the motor. If the current increases, the heat increases and motor life decreases.
 
Thanks DC. Better pay attention to that ol Tom Baker too.

Hopefully this will will overwhelm your student enough that he knows you walk on water (AK winter joke...)

footnote - In real life, I've clled the manufacturer before to find out if what I have in mind, is real world, or not. They are always willing to help, they know that you may want to use there product line more than once.
 
tom baker said:
Motors are not rated the same as nominal voltage, IE a 208 V system has a motor rated 200 V. This allows for voltage drop to the motor. Using a 200V motor on a 240 volt system will result in an increase in current at the magnetic fields become saturated.

And this NEC does not allow this:
NEC 110.4 Voltages.
Throughout this Code, the voltage considered shall be that at which the circuit operates. The voltage rating of electrical equipment shall not be less than the nominal voltage of a circuit to which it is connected.

Using a 230 volt motor on a 208 volt system would be a violation of 110.3(B), and probably be below the 10% tolerance of the motor.
The NEC does not allow motors to be used at other than nameplate voltage, and its hard on the motor. If the current increases, the heat increases and motor life decreases.
Click to expand...

The student of mine that asked the original question to me was questioning the fact that it is perfectly legal, and intended, that a 230 volt motor be used on a 240volt nominal system. He was basing his argument on the language in 110.4.

I agree with the technical points you have made.....Do you have a good reference/standard that is in addition to the one Rockyd provided?
 
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When asked tis question I take students to the Tables in 430, IE...

Table 430.250 Full-Load Current, Three-Phase Alternating-Current Motors

The voltages listed are rated motor voltages. The currents listed shall be permitted for system voltage ranges of 110 to 120, 220 to 240, 440 to 480, and 550 to 600 volts.

and note for them the the seperate Voltage tables in...

Table 430.248 Full-Load Currents in Amperes, Single-Phase Alternating-Current Motors
115 200 208 230
Volts Volts Volts Volts

then to

220.5 Calculations.
(A) Voltages. Unless other voltages are specified, for purposes of calculating branch-circuit and feeder loads, nominal system voltages of 120, 120/240, 208Y/120, 240, 347, 480Y/277, 480, 600Y/347, and 600 volts shall be used.
 
Must be I just don't understand something. Rated namplate= 230v . Nominal voltage of the transformer in question unloaded = 260 volts. 230 volts plus 10%= 253 volts, 7 volts less than 260v. It is close, but is it a violation?
 
macmikeman said:
Must be I just don't understand something. Rated namplate= 230v . Nominal voltage of the transformer in question unloaded = 260 volts. 230 volts plus 10%= 253 volts, 7 volts less than 260v. It is close, but is it a violation?
Click to expand...


It was not a transformer voltage rating that was in question.

Transformer ratings, in my experience have been pretty cut and dry, as far as matching them with the proper nominal voltage.
 
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