Transformer Hot, 3 phase supplying a 1 phase panel

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091207-0822 EST

augie47:

I agree with you that 10 kVA is the view point to use. In our area whether Y or delta for residential or commercial up to maybe 100,000 sq-ft you see three single phase transformers. However, more usually they are delta secondary with one secondary center tapped for 120/240.

However, for your earlier calculation I would change to 5,000/120 = 41.7 A for maximum line current in contrast to 10,000/208 = 48.1 .

Power dissipation in a true three phase transformer would be different than three single phase transformers, and I have no idea how much this would change the maximum current.

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gar, The last thing I wish to do is enter a war of with with you as I am essentially unarmed against an engineer. That said, this information comes from a transformer mfg. instructions: View attachment 4072

That was why I expressed my concern.
I have no idea what the 3 phase current would be in such an unbalanced situation, but I had concerned that, based on the document, the OP might be able to obtain 15 kva of single phase load from that transformer.
 
Last edited:
091207-1943 EST

091207-1938 EST

augie47:

Your concern was important, because the transformer would be overload if a 15 KW load was put on the transformer such that only two hot legs were used.

In your reference, the loads 10, 8, 5, and 4, produce some of the following combinations of resistive loads, and therefore KW not any KVA:

These are OK.
1. 10, 13, 4; 14, 13
2. 10, 12, 5; 15, 12
3. 15, 8, 4
4. 14, 8, 5

Some invalid combinations are:
5. 18, 5, 4
6. 18, 13
7. 17, 10


For those interested in why the line to line load vs line to neutral loads totaling the same power have different line currents consider the following:

Neither two 7.5 KW loads (120 v * 62.5 A) or one 15 KW load (208 V * 72.11 A ) could be safe loads on the two legs of the Y supply. These would greatly overheat the single phase transformers used to supply each phase of the Y, or the two used phase windings of a single three phase transformer.

The VA rating of a single three phase transformer is the total VA capability of the transformer when supplying a balanced load. For three single phase transformers performing the same function it is still the total VA capability, and therefore each of the single phase transformers would be rated at VAtotal / 3.

For a resistive load connected line to neutral the load current. line current and line to neutral phase voltage are all in phase with each other. Thus, a 5 KW or 5 KVA resistive load connected line to neutral has a load and line current of 41.7 A. When a 10 KW resistive load is placed line to line one has a 208 V source, not 2*120 = 240, and therefore the line current has to be larger, 48.1 A, than the line current for the two 5 KW loads placed line to neutral.

When the resistive load is line to line the line current is in phase with the line to line voltage, but not in phase with the line to neutral voltage.

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Sorry but I'm think headed. So in this situation we should limit to load to no more than 5 kw on each 120 volt leg or 10 kw on a 208 load is that correct ?
 
091208-2138 EST

augie47:

Between the two hot lines with 208 between them you are limited to 41.7 A on each hot line and since the voltage is 208 this means the maximum load is 208*41.7 = 8.674 KVA and not 10 KVA.

The reason is the internal temperature rise of the transformer wire is determined by the current in the wire.

Since we had no other information I viewed the transformer of one phase as a single phase transformer with a resistive load whose secondary was between neutral and a hot leg. By choosing this location for the transformer secondary and load resistance it made the voltage and current in phase and thus produced the maximum power transfer for the transformer rating. From this circuit configuration the maximum secondary load current could be calculated.

This calculated value of current becomes the maximum secondary current no matter what is the phase angle of the current relative to the voltage.

Next we put a load between the hot lines. We are still limited to the current calculated above. But now our voltage across the load is not 2*120 = 240 but just 208. The VA capability in this case has to be limited to (208/240) of the sum of the two loads between hot and neutral.

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Since the unit is a 15kVA the first question that needs to be answer is the unit ventilated or encapsulated.

If encapsulated - the unit is most likely a SCOTT-T design, which will be shown on the wiring diagram or phase diagram. Then this is a miss-application, and could be why you are seeing extra heat.

If ventilated - the unit would be a Delta to Wye, and have three separate 5-kVA single phase devices, 480 to 120. In that case you can do the unbalance loading as long as you are energizing the unit with 3-phase.
 
fordaputz said:
If encapsulated - the unit is most likely a SCOTT-T design,
Click to expand...

No. A Scott is a special Tee connection that is used to generate 2-Phase power from 3-Phase systems.

Other than that I agree, the connection is most likely a T (up to maybe 30kVA) for most dry-type designs, in particular, encapsulated ones.

The difference in a Scott connection and a general T can be noticed at the outer ends of the coils.
 
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