Jeez, simple help on KVA calc

[fastline]

Maybe I need sleep but here is the simple deal. 150kva Xfrmr. 208/120 sec. What are the available amps at 120? I have 1250A. basically on single phase, you don't divide by sq3. It just seems really high but it does make sense as the 3P side is 416.37 and that *3 is about the 1250A. It's late and I haven't done this in a few.

 

[topgone]

At 1.5% Z, you'll have 27.8 kA.

 

[petersonra]

150000 / 480 / sqrt(3) = 180 A on primary side

150000 / 208 / sqrt(3) = 416 A on secondary side

Per phase

I suppose you could look at this as 1250 A of total available 120 V power on the secondary, but that is not so because it is three phase. Some of the loads might be 208 V three phase, others might be 208 V (L-L) single phase, and some might be 120 V single phase (L-N).

 

[Jraef]

150000 / 480 / sqrt(3) = 180 A on primary side

150000 / 208 / sqrt(3) = 416 A on secondary side

Per phase

I suppose you could look at this as 1250 A of total available 120 V power on the secondary, but that is not so because it is three phase. Some of the loads might be 208 V three phase, others might be 208 V (L-L) single phase, and some might be 120 V single phase (L-N).

Looked at another way:
You feed a panelboard with that transformer, that panelboard has 3 bus bars, each rated for 400A, not 1200A…. But if you have 1200A of 120V load, this would be the best way to distribute it.

I suppose you COULD use a single phase transformer rated 150kVA, but that would be a severe imbalance in the 3 phase distribution feeding it.

 

[d0nut]

You can also think of the three phase transformer as three, single phase transformers if that makes more sense. The math works out the same.

150000 VA / 3 = 50000 VA
50000 VA / 120 V = 417 A

 

[fastline]

OK, so "technically" my math is correct? I do fully understand it is not very practical for huge 120V loads, I was just trying to get the math worked up. But it does bring me to another point I never have had to deal in much because most of my loads are of course 3P. What is the Neutral current on 120V loads? It's been 20yrs since I had to do this. Assume L1 20A, L2 15A, L3 0A.

 

[wwhitney]

OK, so "technically" my math is correct?

Your math is correct in the sense that if you have only 120V loads, then you can supply up to 1250A worth of those from a 150 kVA transformer, as long as they can be divided into 3 groups of 417A in each group. So the largest single 120V load you can supply is 417A, not 1250A.

What is the Neutral current on 120V loads? It's been 20yrs since I had to do this. Assume L1 20A, L2 15A, L3 0A.

Depends on the relative power factor (current phase angle) of the two different 120V loads. But if they are the same (e.g. both power factor 1), then the neutral current for this case is sqrt(20²+15²-15*20) = 18A. For equal current phase angle, the neutral current will never exceed the maximum of the line currents, but may equal the maximum (e.g. L1=20, L2=20, L3=0 gives N=20).

Cheers, Wayne

 

[tortuga]

I suppose you COULD use a single phase transformer rated 150kVA, but that would be a severe imbalance in the 3 phase distribution feeding it.

A Scott-T configuration is best when you need to balance a massive single phase load on a 3-phase system, such as electric rail.
You of course get no 3-phase out of it, but if you needed the most possible single phase (or split phase 120/240 ) and had limited capacity on the primary side that would be my way to do it.

 

[wwhitney]

A Scott-T configuration is best when you need to balance a massive single phase load on a 3-phase system, such as electric rail.

Say you start with a 208Y/120V source limited to 100A (36 kVA). I'd be interested to see the math on the largest 2-wire 120V load you could serve via a Scott-T transformer that makes 170V/120V 2P5W (not sure if that voltage nomenclature is standard, those are the two different L-L voltages).

For common transformer arrangements, the best case would be to use a 208V : 120V single phase transformer, that would let you supply a 173A 120V load. If I'm not mistaken, that matches using a Scott-T transformer to make 120V/85V 2P4W and supplying the 120V load from the "main" transformer, and it beats using the "teaser" transformer from such an arrangement.

For uncommon transformer arrangements, I believe the best case would be to use (3) 120V:60V transformers, one on each phase, and put the outputs in series to get 120V single phase (60V ∠ 0 + 60V ∠ 60 + 60V ∠ -60 = 120V ∠ 0 ). That would let you supply a 200A 120V load.

Cheers, Wayne

 

[tortuga]

Say you start with a 208Y/120V source limited to 100A (36 kVA). I'd be interested to see the math on the largest 2-wire 120V load you could serve via a Scott-T transformer that makes 170V/120V 2P5W (not sure if that voltage nomenclature is standard, those are the two different L-L voltages).

For common transformer arrangements, the best case would be to use a 208V : 120V single phase transformer, that would let you supply a 173A 120V load. If I'm not mistaken, that matches using a Scott-T transformer to make 120V/85V 2P4W and supplying the 120V load from the "main" transformer, and it beats using the "teaser" transformer from such an arrangement.

For uncommon transformer arrangements, I believe the best case would be to use (3) 120V:60V transformers, one on each phase, and put the outputs in series to get 120V single phase (60V ∠ 0 + 60V ∠ 60 + 60V ∠ -60 = 120V ∠ 0 ). That would let you supply a 200A 120V load.

Cheers, Wayne

would all those balance the on the primary side though?

 

[wwhitney]

would all those balance the on the primary side though?

None of them will, it's impossible to take a single phase load and turn it into a balanced 3 phase load using passive components. The P vs t waveforms are different between the two cases; balanced 3 phase is constant, while single phase is periodic of twice the fundamental frequency.

Cheers, Wayne

 

[tortuga]

None of them will, it's impossible to take a single phase load and turn it into a balanced 3 phase load using passive components. The P vs t waveforms are different between the two cases; balanced 3 phase is constant, while single phase is periodic of twice the fundamental frequency.

Cheers, Wayne

My understanding is the scott-t creates a balanced 3-phase side, provided the two single phase feeders are balanced

Scott-T transformer - Wikipedia

en.wikipedia.org

CASE STUDY: Scott-T Transformer - Carroll & Meynell

This is a Scott-T transformer as it's wound using only 2 interconnected coils, enabling 3-phase input but single phase output. This arrangement results in more balanced loading as seen by the supply compared to a single phase transformer powered by two live lines only. Primary voltage is 440V...

www.carroll-meynell.com

 

[wwhitney]

My understanding is the scott-t creates a balanced 3-phase side, provided the two single phase feeders are balanced

True. But we were discussing the case of a single large single phase load, so the two phase side would not be balanced.

If you have multiple single phase loads and can divide them into balanced sets, the only advantage of the Scott-T is when you can do that into 2 sets but not 3 sets. I guess if you had 5 large single phase loads you absolutely had to balance over a 3 phase supply, you could probably come up with a transformer arrangement to do that, too.

Cheers, Wayne