St of Washingon Test- Motor Rating

[Radial12]

Does anyone know if testing agencies, like LaserGrade for the State of Washington, would use the 115v motor rating as a basis for the calcs instead of the 120V nominal system voltage?

This applies when finding amperage when only given VA or for finding VA when only HorsePower is given.

I am taking the General Administrator test on Thursday and would appreciate some feedback if anyone knows.

Thank you,

 

[iwire]

Have you looked at the tables in the rear of Article 430 and the voltage ratings that are used in those tables?

 

[charlie b]

Bob, those tables presume that you know the motor HP. The question here involves a test question in which a motor is described only in terms of VA. I don?t know if they would actually create a test question like this.

If the test question gave you a motor that is powered from a single phase 120 volt system, and if they gave you the motor HP and asked for the VA load that should be included in the overall calculation, I would use the amps from the table and multiply by 115 (not by 120). If they did not give a value for HP, but only gave you amps, then the issue of whether to multiply by 115 or 120 would rest on the exact wording of the question.

 

[iwire]

Bob, those tables presume that you know the motor HP. .

My point was only that motors are typically rated odd voltages,

115 in place of 120

460 in place of 480

It seems to be some sort of standard for motors.

 

[kingpb]

Standard motors are rated for plus or minus 10% motor nameplte voltage so they can provide adequate performance during system voltage variations.

Now, think of the fact that there is a recommended maximum voltage drop of 5% from nominal, this means that the motor will see approximately 100% of it's rated voltage, thusly operating in the most efficient range.

Also, with a voltage drop on the bus where large motors start, limitingthe drop to -15% of nominal (which is very typical), then the voltage at the motor is approx. -10% and should run adequately. This is why it is very common to limit bus voltage drops to 85% of nominal during large motor starting. At lower voltages you really need to look at other factors and parameters.

To try and answer the OP, If you want MOTOR amperage, you have to use the MOTOR nameplate voltage and horsepower and VA . If you want amp draw on the system due to the motor, you have to first get the motor nameplate amps, then ratio it down. e.g. 115V motor draws 5 amps, the current at 120V is A= FLA *115/120V.

 

[iwire]

Standard motors are rated for plus or minus 10% motor nameplte voltage so they can provide adequate performance during system voltage variations.

Yeah thats true.

Now, think of the fact that there is a recommended maximum voltage drop of 5% from nominal,

I think your forgetting the power company gets a 5% VD gift as well.

5% drop for service voltage and an additional 5% drop to the utilization equipment

So the NECs recommended 5% drop along with the power companies allowable drop can use up the entire 10%

ANSI C84.1?1989 covers all this.

http://www.pacificpower.net/File/File22465.pdf

 

[Radial12]

Thank you all for the lightning fast responses. I must say that I am impressed in the level of knowledge offered here to people like my self trying to make sense of the Code(s).

My apologies for not pinning my question down to related solely to the Major Calcs section of the Administrator exam. I have been working with the exam prepbook and came accross an error i had been using for the feeder/service calcs. Instead of using the customary or common nameplate rated voltage of 115v, I had used the 120v from the specific question for a VA calc for a 1/2 dishwasher. For VA I went to Table 430.248 and found an amperage of 9.8 amps times 120v gave me 1,176VA instead of the text books answer of 115v times 9.8A = 1,127VA. Where at face value it would not seem like much of a problem but when you add the wrong VA to the other VAs and doing likewise with the AC load at 240volts instead of 230volts, I had ended up with a higher circuit breaker then the text book had in the answer key. That is the root of my question, whether to use the customary 115v or to use the nominal system voltage from the question of 120volts.

It seems to me I should use 115v (or 230volts for line-to-line) for two reasons:
1) the table's column heading shows 115volts
2) in my experience anyways, most motor nameplates in real life are in fact rated at 115v

Comments?

Thanks again!

 

[tom baker]

Motors use a lower voltage than the system nominal voltage to account for voltage drop.
120 nominal = 115 motor
480 = 460
208 = 200

This is a NEMA standard.

 

[infinity]

Motors use a lower voltage than the system nominal voltage to account for voltage drop.
120 nominal = 115 motor
480 = 460
208 = 200

This is a NEMA standard.

Are 200 and 208 interchangeable? Table 430.250 lists both.