Transformer- Single phase and Three phase kVA calculations

[LincHawk]

I had a question about calculating kVA for a three phase to single phase transformer. I know the formula for calculating the kVA. My question is since I’m stepping down to a lower voltage and single phase, do I just do the single phase calculation and get my kVA? Then based off the kVA I calculate, that will determine my primary three phase amps?

Example

I need a 200A 120/240v single phase panel. So based off the kVA calculation I come up with 240*200 = 48,000/1,000 = 48 kVA

I’m feeding the primary side with 480v 3 phase. So do I take 48,000/830.4 = 57.8A?




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[broadgage]

There is no simple way to obtain single phase from 3 phase whilst spreading the load across all 3 phases of the primary.

Therefore a single phase step down transformer is needed with a 480 volt primary and a 240 volt secondary. The primary current would be 100 amps in theory, or a little more in practice due to losses in the transformer.
Connect the primary between any two lines of the 480 volt system.

This presumes that the 480 volt three phase system is either 480, volt 3 phase, 3 wire, or 277/480 volt, 3 phase, 4wire.

In the unlikely event that you have a 480/830 volt 3 phase, 4 wire system, then you could use EITHER a transformer with a 480 volt primary , connected between any phase and neutral, OR a transformer with an 830 volt primary, connected between any two lines. Such systems are uncommon.

 

[LincHawk]

There is no simple way to obtain single phase from 3 phase whilst spreading the load across all 3 phases of the primary.

Therefore a single phase step down transformer is needed with a 480 volt primary and a 240 volt secondary. The primary current would be 100 amps in theory, or a little more in practice due to losses in the transformer.
Connect the primary between any two lines of the 480 volt system.

So I’m just taking 48,000/480=100A ?

And only using 2 of the 3 phases?


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[oldsparky52]

So I’m just taking 48,000/480=100A ?

And only using 2 of the 3 phases?


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Correct. Then of course the "balance" on the 3-phase service gets a little off.

 

[broadgage]

Yes, only two of the three available phases can be used to produce a 240 volt single phase secondary.

 

[LincHawk]

Yes, only two of the three available phases can be used to produce a 240 volt single phase secondary.

I see. Thanks


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[Tony S]

A Layton 3/2 transformer can balance a single phase supply across three phases. Whether it would be economical is another matter.

 

[broadgage]

A Layton 3/2 transformer can balance a single phase supply across three phases. Whether it would be economical is another matter.

I know, which is why I stated that there is no SIMPLE way to obtain single phase service from a three phase supply whilst spreading the load equally over the three phases.
Special transformers, which may need custom manufacture are only justified in special cases.

 

[kwired]

So I’m just taking 48,000/480=100A ?

And only using 2 of the 3 phases?


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Power is power - your 48 kVA does the same work whether it is three phase, single phase or the output of a steam boiler, though boilers are not typically rated in kVA or kW units.

Without some more complex method to convert all the power the difference is the three phase is carried by more conductors than the single phase.

If your source is wye connected you are utilizing two of the three source coils to supply typical setup like described, you only have 2/3 of source available to you for what you are utilizing.

If your source is delta connected you are connecting across only one of the three source coils and only have one third of source available, presuming all three source coils are of same capacity sometimes they are not same size in delta systems and single phase loads were intended to be connected to the larger coil by design of the system.

 

[synchro]

If your source is delta connected you are connecting across only one of the three source coils and only have one third of source available, presuming all three source coils are of same capacity sometimes they are not same size in delta systems and single phase loads were intended to be connected to the larger coil by design of the system.

Consider a delta transformer with equally sized secondary legs and a single phase load across one of the three windings. Half of the load current will be supplied by that winding, and the other half of the load current will be supplied through the other two windings that are connected together but without a load at that connection point. However, even when the power factor of the single phase load is unity the other two windings will see a 50% PF due to the 60 degree phase shift. So the KVA on each of the three windings will be the same but two of them will only contribute 1/2 the KW as their rated KVA. So the total available KW delivered to the single phase load is 2/3 of the total KVA in the windings.

I think the OP was interesting in sizing a single phase 480 to 240/120 transformer. So my comments might not be relevant to his situation unless there is a concern about the loading on the 480V delta supply.

 

[kwired]

Consider a delta transformer with equally sized secondary legs and a single phase load across one of the three windings. Half of the load current will be supplied by that winding, and the other half of the load current will be supplied through the other two windings that are connected together but without a load at that connection point. However, even when the power factor of the single phase load is unity the other two windings will see a 50% PF due to the 60 degree phase shift. So the KVA on each of the three windings will be the same but two of them will only contribute 1/2 the KW as their rated KVA. So the total available KW delivered to the single phase load is 2/3 of the total KVA in the windings.

I think the OP was interesting in sizing a single phase 480 to 240/120 transformer. So my comments might not be relevant to his situation unless there is a concern about the loading on the 480V delta supply.

Single vs multi- core source makes a difference.

Typical open delta with a smaller high leg pot - it is smaller because it only carries load of the high leg. If you have no high leg load at any given time you can disconnect supply to that pot and never know anything is wrong.