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Thread: 3Phase 300KVA Transformer

  1. #1
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    3Phase 300KVA Transformer

    I have six POTs (transformers) that are aerial distribution mounted on a ground pad in a small enclosed fence. This trasformer yard has two sets of 208/120volt, 3 phase, wye-wye connected transformers servicing two separate buildings with a 12,460volt primary incomming from the substation. This is a NEC code violation since these live parts should be mounted on a pole high above the head. I am therefore going underground with the service drops from the two 3 phase transformers and installing a new pad mounted transformer to service both buildings (size to be determined later). Three of the POTS service one building and 3 of the POTS service another building. Each POT is 100KVA, so 6 of them add up to two sets of 300KVA, three phase, 208/120Volt wye-wye connected transformers per building.

    A load study was performed by a consulting firm. This yielded 33amps on the secondary feed for the service drop on one building and 20 amps for the other building. This is demand load amps I am assuming.

    I calculate 278 amps capacity for the secondary for both transformers: 100,000VA/(1.73*208Volts)=278 amps.

    One of these buildings is a large manufacturing facility. I certainly can not image only 33amps of demand load from this type of a building after all single phase and 3 phase loads are added together. The other building is a fairly large office with much equipment. I can not imagine only 20 amps of demand load for that building.

    I know all the loads emanate from the 3 phase main distribuition panel (typically sized for a 400 amp demand load) in parallel paths so that the demand current draws for each loaded circuit add in parallel. It is common to feed 3 phase loads directly from the 3 phase main distribution panel and feed 125amp single phase panelboards from the 3 phase main distribution panel.

    Is there something I am missing here? A 33 amp load draw from a large manufacturing building as determined by the consulting firm in their load study??

  2. #2
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    Re: 3Phase 300KVA Transformer

    I have a hard time understanding your descriptions and your questions, but I'll give it a try.
    Originally posted by eeee: Is there something I am missing here? A 33 amp load draw from a large manufacturing building as determined by the consulting firm in their load study??
    I'll start there. It sounds like they are talking about load on the primary side of the transformer. 33 amps times 12,460 volts times 1.732 equals 712 KVA. 20 amps times 12,460 volts times 1.732 equals 431 KVA.
    Originally posted by eeee: 208/120Volt wye-wye connected transformers per building.
    Why a wye-wye? It is far more common to use Delta-Wye.
    Originally posted by eeee: I calculate 278 amps capacity for the secondary for both transformers: 100,000VA/(1.73*208Volts)=278 amps.
    The transformer is connected as a bank of three single phase units. You must calculate the secondary current in the three-phase configuration. That means you use three times the 100 KVA of a single unit. 300,000 VA divided by (1.732 * 208) equals 832 amps.
    Originally posted by eeee:. . . the demand current draws for each loaded circuit add in parallel.
    I have no idea what you meant by that.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  3. #3
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    Re: 3Phase 300KVA Transformer

    This is a NEC code violation since these live parts should be mounted on a pole high above the head.
    This installation is probably covered by the National Electric Safety Code and not by the NEC. This may be ok if there is a "barrier around the installation (fence)". It needs to be a specific height and width to isolate the energized parts. Is the 12kv overhead or underground. If overhead there is a minimum clearance required.

  4. #4
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    Re: 3Phase 300KVA Transformer

    Question 1: The 12KV primary is overhead. NEC 110-34(e) of the NEC lists clearance for live parts. For a 12.47KV service, unguarded live parts must be 9 feet above areas that could be occupied by personnel given the exception we are operating in at this facility.

    Question 2: This is definately the secondary load since the load study shows the 33Amps and 20 Amps right above the buildings far below the location of the secondary output of the transformer.

    Question 3, Answering the question that amps add in parallel: An example best answers this. If I have a 10volt AC generator connected to one 60 watt light bulb, 60w/120v=1/2 amps existing on this series circuit. If I add another 60watt light bulb, 120w/120v=1 amp exists on this series circuit assuming a purely resistive load with unity power factor. Now if I add another circuit just like it in parallel, 1 amp + 1 amp=2amps since we know amps add in parallel.

    In most of our applications, many parallel circuits will run to a main distribution panel from typically 225amp rated single phase panel boards as well as the 3 phase circuits that are in parallel also. The amps of each of these branche circuits will add together just as they will with my simple light bulb example.

    I have many lights in this manufacturing facility with much equipment in it, some of which are 3 phase. I can not imagine only a 33 amps load draw from this facility to use it as the example. A 1400 square foot house would draw 33amps. A large 25,000 square foot manufacturing facility must surely draw more than 33 amps by comparison.

    I saw in a previous link a gentleman indicating he engineered a 4,000 amp hotel once.

    Anwer to the final Question: The ampacity on the primary is listed as 99 amps. I realize this is just ampacity and not load and is used just for cable sizing purposes.

  5. #5
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    Re: 3Phase 300KVA Transformer

    I am sorry. I knew better. The 300KVA is divided by the 1.73*208volt, not the single phase 100KVA portion of the transformer setup. I was operating too fast on my back check.

  6. #6
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    Re: 3Phase 300KVA Transformer

    Charlie is correct in his ampacity calculation. Since you have 120 volts across the 100 KVA transformer, 100,000va/120v is 833 amps. The 208 and 1.732 only come in when you have the entire 3 phase bank.

    Secondly, your load information seems way off. I would guess an actual demand load of 3va/sq ft to 6 or more depending on what is going on in the building. You have a fraction of that in the measurements. Somehow, you need to have the load survey run again.

    Jim T

  7. #7
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    Re: 3Phase 300KVA Transformer

    Originally posted by eeee: Question 2: This is definately the secondary load since the load study shows the 33Amps and 20 Amps right above the buildings far below the location of the secondary output of the transformer.
    You are definitely misinterpreting the load study. You need to read it again, or perhaps contact the person who performed it. The report does not say what you think it says. As I said earlier, those numbers (20 and 33) make sense only in the context of primary currents at 12,460 volts.
    Originally posted by eeee:The amps of each of these branche circuits will add together just as they will with my simple light bulb example.
    This is going to sound harsh, but I really don't need you to explain to me how to add up the currents drawn by resistors in parallel. I have a masters degree and got as far as passing the PhD qualifying exam. I also have PE licenses in five states. I think I've got the basics down well enough.

    This is going to sound even more harsh, but what you need to do is to learn how to write. Your statement, as written, is wrong. Someone needs to explain to you how to add up the currents drawn by three phase loads. I'll just give it one short attempt.

    What I was asking you is what you meant by asserting that three phase loads add in parallel. They do not. Three phase currents are not added up as though the loads were in parallel. Loads on a three phase panelboard, in general, are not in parallel. Two loads that both are served by Phase A and the Neutral are in parallel with each other. But neither of these would be in parallel with a load connected between Phase B and the Neutral, and neither would be in parallel with a load connected between Phase A and Phase C. The most common approach is to make an effort to balance the loads, then assume that you have succeeded. You convert all loads to units of VA, and find the load on each leg (the three of which you are assuming to be equal) by dividing by the voltage and by the square root of three. If the loads are not balanced, the math gets much harder.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  8. #8
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    Re: 3Phase 300KVA Transformer

    I am sorry Charlie. I was trying to explain what my thought process was which describes how I developed my previous statement, not to try to educate anyone there. I definately needed to think my thought process through better during my writing attempt.

    I am a little over 1 year in to distribution engineering. Although I have a MSEE, my background is in power generation and before that much background in telecom.

    I am working on my PE.

    I'll check in to the load survey result and do appreciate it.

  9. #9

    Re: 3Phase 300KVA Transformer

    A load study was performed by a consulting firm. This yielded 33amps on the secondary feed for the service drop on one building and 20 amps for the other building. This is demand load amps I am assuming.

    EEEE: I believe part of the problem is in the lack of understanding of the Code terminology.
    The 33 amps and 20 amps provided by the consulting firm is the value of load current at the "service drop" See the definition in the NEC Article 100. This is the point where the public utility supplies the energy at 12,460 volts.

    The load study is for the high voltage side of the electrical system (primary side of transformers). The calculations have been properly explained in the preceding messages.

  10. #10
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    Re: 3Phase 300KVA Transformer

    John,


    I have a set of 3 single phase pots rated at 100KVA each for each building. That is a 300KVA three phase transformer set; one 300KVA three phase transformer set of ground mounted pots for each building run aerially. Charlie calculated 712KVA using his calculations for the 33 amps, assuming that is what the load study means for the service entrance on the primary side as you say John and you agree with this.

    The transformer is only 300KVA for the building with the 33 amps as shown on the load study performed by the consultant. (whether this is the secondary load drawn from the building or primary amps delivered from the substation as prescribed by the load study).

    Another EE I know has no faith in this load study I have been told.

    I don't know, it could be the load drawn from the large building is 33amps (although a 1,400 square foot house may draw more than that I expect). There are some computers in the building, four 120volt machines to bead and clean tire parts, a 480 volt machine used for nitro gas filling of large tires, 25,000 square feet of flourescent ceiling lights, possibly some other machines that are used for other purposes servicing some other mechanical operations for other functions in the building I am not yet familiar with. The building has no air conditioning, but does have heat, although that is a natural gas process, not electrical.

    So maybe the 33 amps could be for the load of the building. I have not gotten to the point yet in distsribution engineering that I can look at a facility and ballpark amp load draw for the building to know whether an amp load number I am looking at makes sense.

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