Motor Calcs for service

[wannawaterski]

This is a question that is similar, if not the same, as a previous question, except I have specifics!

The question is stated as such: There are 6 motors on a 480V, 120/240V delta 3 wire system, served by a single service with the 120/240 loads being served through a transform. What is the load on the service in VA?

Motor #1: 115V, single phase, 2 Hp, continuous duty, nameplate 19.92 A

Motor #2: 115V, single phase, 3 Hp, intermittent duty rated at 5 min., nameplate 28.22A

Motor #3: 230V, single phase, 5 Hp, continuous duty, nameplate 23.24A

Motor #4: 460V, 3 phase, 30 Hp, continuous duty, nameplate 33.2A

Motor #5: 460V, 3 phase, 40 Hp, intermittent duty rated at 5 min., nameplate 43.16A

Motor #6: 460V, 3 phase, 50 Hp, continuous duty, nameplate 53.95A


OK, so I know you're supposed to use the tables in Article 430 for the currents. For single phase it's I X E and for 3 phase it's I X E X sq.rt. of 3. Do you use the nameplate voltage (115,230,460) or the input voltage (120,240,480)? Also, I'm not certain if you need to consider the duty rating of the intermittent motors, which I don't think so, and of course, use the largest motor demand at 125%. I assume when you figure the VA of the 115/230V motors, you total them up, regardless of the fact that they are fed through a transformer since there is no mention of a power factor, and then add them to the total of the VA's for the 460V motors.

Believe me, I've tried several different ways to try to get an answer that was even close to what they offer for choices, and I'm completly frustrated. HELP ME PLEASE SOMEONE!!!!!

 

[charlie b]

Re: Motor Calcs for service

For starters, you don?t start with Article 430. It covers ?motors, motor branch-circuit and feeder conductors and their protection, motor overload protection, motor control circuits, motor controllers, and motor control centers.? It does not, on its own, address the loads that you assign to a motor, when performing a service calculation.

You start with Article 220.2, ?Computations,? part (A), ?Voltages.?

Unless other voltages are specified, for purposes of computing 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.

220.3(B)(3) tells you that to find the VA for a motor, to use the same rules that 430 uses to calculate the branch circuit conductor sizes. That is the place where you see the addition of 25% to the largest motor load. But nowhere in 430 (or at least nowhere that I can find) tells you to use a voltage other than the nominal system voltages shown above. Therefore, I conclude that, for the purposes of load calculations, you do not use the nameplate voltages of 115, 230, and 460, but rather the 120, 240, and 480 volt nominal ratings.

By the way, I agree that you ignore the existence of a transformer, when performing a load calculation. Also, since we are talking in terms of VA, and not in terms of watts, there is no need to consider power factor.

 

[loren]

Re: Motor Calcs for service

Try this.
Use nominal voltages as in 220.2 (A)for final va calculations.

What are the total va's of the 115 and 230 motors as stated in the example.

This transformer has a 4:1 ratio. There for divide the secondary va's by 4.

Add the 460 VA's

Add the the?secondary va and the primary va (after dividing) together.

Motors are va related machines. You lower the voltage and it will draw more amps to do the same work. Excluding the losses of the transformer what ever the total va that is calculated above will be present even at higher voltages.

I could be wrong but that is how I would calculate it.

 

[loren]

Re: Motor Calcs for service

sorry forget the first part of that reply.
The question only asked what the load was to be in va.