Washington State Admin Exam V.A. load on a group of moto

johnweaver · Feb 5, 2005

The main service is 480/277 volt three phase panel, with a 240/120 volt step down transformer from the main service.
What is the va load on the service conductors with the following 6 motors?

115 volt 1 phase 5 hp cont duty with a name plate of 46.82 amps

230 volt 1 phase 7 1/2 hp 5 minute intermittent duty with a name plate of 33.2 amps

230 volt 1 phase 10 hp cont duty with a name plate of 41.5 amps

460 volt 3 phase 7 1/2 hp cont duty with a name plate of 9.13 amps

460 volt 3 phase 20 hp 5 minuite intermittent duty with a name plate of 22.41 amps

460 volt 3 phase 40 hp cont duty with a name plate of43.16 amps

Notice the question stated 480/277 volt main with a step down transformer of 240/120 volts.
The motor voltages are 230/115 volt and 460 volt. Is that part of the trick?

Also - Do I use the name plate amps or go to the tables in the code book?

Does intermittent duty cycles of the motors have to be taken into consideration?

Has anyone had this question and if so how did they work it?
Thanks John

charlie b · Feb 7, 2005

Re: Washington State Admin Exam V.A. load on a group of moto

First, ignore the nameplate amps. Use the tables to get the amps. Then multiply by the voltage in the table. For the 3-phase motors, you also multiply by 1.732. This gives you the load in VA. I am not at my office, and don?t have my code book. So I can?t give you an answer on the ?intermittent motor? part of your question.

dlock11692 · Feb 12, 2005

Re: Washington State Admin Exam V.A. load on a group of moto

This one gets me as well.MAYBE they tell you about the transformer at the end of the question so you can "guess"what size is needed to run these 3 motors.once you've got that then you can "guess" if that transformer is delta-delta or MAYBE delta-wye and then MAYBE the correct answer is a b c d .if I could get any help on this one that would great.

vsullivan · Feb 12, 2005

Re: Washington State Admin Exam V.A. load on a group of moto

I think the va load on the service conductors is . Here's how I calculated my answer: For all continuous duty motors, I used the table amps and for the intermittent duty motors, I used their nameplate values (see 430.22 (e)). I also used the applied voltage, not the table voltages.
I also included the largest motor 25% factor (430.24) for the motor feeder. Therefore my numbers are: (1) 120 X 56 = 6720 va; (2)240 X 33.2 X .85 = 6773; (3) 240 X 50 = 12,000; (4) 480 x square root of 3 x 11 = 9145; (5) 480 x square root of 3 x 22.41 x .85 = 15,837; (6) 480 x square root of 3 x 52 x 125% = 54,040. This totals to 104,515 va load on service conductors.

vsullivan · Feb 12, 2005

Re: Washington State Admin Exam V.A. load on a group of moto

I think the va load on the service conductors is 104,515. Here's how I calculated my answer: For all continuous duty motors, I used the table amps and for the intermittent duty motors, I used their nameplate values (see 430.22 (e)). I also used the applied voltage, not the table voltages.
I also included the largest motor 25% factor (430.24) for the motor feeder. Therefore my numbers are: (1) 120 X 56 = 6720 va; (2)240 X 33.2 X .85 = 6773; (3) 240 X 50 = 12,000; (4) 480 x square root of 3 x 11 = 9145; (5) 480 x square root of 3 x 22.41 x .85 = 15,837; (6) 480 x square root of 3 x 52 x 125% = 54,040. This totals to 104,515 va load on service conductors.

snapco · Mar 17, 2005

Re: Washington State Admin Exam V.A. load on a group of moto

I would be interested also if anyone could post how this is actually worked out as these are the answer choices-

a) 104,478 b) 113,913 c) 96,384 d) 93,675

charlie b · Mar 17, 2005

Re: Washington State Admin Exam V.A. load on a group of moto

Continuous Duty Motors:

115V, 5 hp, 1 ph = 56 amps = 6,440 VA

230V, 10 hp, 1 ph = 50 amps = 11,500 VA

460V, 7.5 hp, 3 ph = 11 amps = 8,760 VA

460V, 40 hp, 3 ph = 52 amps = 41,430 VA

Intermittent Duty Motors:

230V, 7.5 hp, 1 ph = 40 amps, times 85% = 34 amps = 7,820 VA

460V, 20 hp, 3 ph = 27 amps, times 85% = 23 amps = 18,325 VA

Add 25% of largest load (41,430 VA) = 10,358 VA

Total VA = 104,633 VA

I would choose answer ?A.?

snapco · Mar 17, 2005

Re: Washington State Admin Exam V.A. load on a group of moto

I agree with how you determined total VA and I agree to 104,633 va- but does the State of Washington agree? Why would they give answers to a question like this and expect you to just go with the answer close to thiers. I have worked this question out in every situation I can think of and still have not managed to come up with a answer that is an exact match-

nicknorth · Apr 30, 2005

Re: Washington State Admin Exam V.A. load on a group of moto

Charlie B,

Wouldn't you mulitple by the apllied voltage instead of the rated voltage? The voltage is 480/277 and you are using 460??

dillon3c · Apr 30, 2005

Re: Washington State Admin Exam V.A. load on a group of moto

110.4 Quote:Thoughout this Code,the voltages considered shall be 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.

*220.5 Quote: Unless other voltages are specified,for the purposes of calculating branch circuits and feeders loads,nominal system voltages of 120, 120/240, 208Y/120, 240, 347, 480Y/277, 480, 600Y/347,and 600 volt shall be used..

-Nick,hope this helps you in your question..

nicknorth · May 2, 2005

Re: Washington State Admin Exam V.A. load on a group of moto

So are you saying that Charlie B should of used 120/240 and 277/480 because those are the applied voltages in this question, and not use 115/230??

charlie b · May 2, 2005

Re: Washington State Admin Exam V.A. load on a group of moto

The NEC table gives us the amps that apply at the rated voltage for each motor. It allows us to use the same amps for calculation purposes, even if the actual applied voltage is slightly different. For a motor rated at 115 volts, we can use the tabulated amps for an applied voltage range of 110 to 120 volts. But the rated voltage (e.g., 115) is the value that corresponds to the amps in the table. That is why I multiplied the amps by 115, and not by 120.

BruceH · May 2, 2005

Re: Washington State Admin Exam V.A. load on a group of moto

I don't get it, I posted an answer to a question exactly like this one and the poster was offering $$$ for the right answer. What happened to this post, did it get deleted????

charlie b · May 2, 2005

Re: Washington State Admin Exam V.A. load on a group of moto

It was deleted, by me. I took exception to the notion of one member offering money to any other member for assistance given via this Forum. It was contrary to the intended purpose of this Forum.

BruceH · May 2, 2005

Re: Washington State Admin Exam V.A. load on a group of moto

Oh, I understand. I wasn't concerned with the money. I had just wondered if my answer was correct, it is worded differently now as well. Just curious.

makin_waves1 · May 15, 2005

Re: Washington State Admin Exam V.A. load on a group of moto

I have worked this problem a different way and came up with an exact answer. answer (b) according to article 430.24 which says to use 125% of the full load current of the highest rated motor plus the full load rating of all the other motors in the group. this is what i came up with. i used the 40 hp with 52 Amps x 125%x 831 = 54015va. 20 hp at 27 amps x 831 = 22437va, 7.5 hp at 11 amps x 831 = 9141va, 10 hp at 240 = 12000va, 7.5 at 240 = 9600va, 5 hp at 120 = 6720. these total to 113913. is this not right , am i totally off base. thanks

Washington State Admin Exam V.A. load on a group of moto

johnweaver

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dlock11692

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vsullivan

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vsullivan

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snapco

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charlie b

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snapco

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nicknorth

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dillon3c

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nicknorth

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charlie b

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BruceH

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charlie b

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