1P vs 3P Transformer Amperage Calculation

[OHHV]

I'm looking to calculate the maximum amperage that I can ever read on the following two transformer scenarios.

Scenario 1:

Single phase 25kVA transformer 13.2kV x 120/240V. I calculated 25000VA/240V = 104A as the maximum current reading that I'll measure across the secondary bushings hot leg to hot leg. Is that correct?

Scenario 2:

(3) single phase 25kVA transformers banked together (or one 3 phase 75kVA transformer). 13.2kVA x 120/208V. Here is where I'm confusing myself. When trying to calculate the max. amperage that I can get out of this bank, what values are used? Do I use 75000VA/208V = 360A total or 120A per transformer? In this scenario the secondary windings are paralleled, so I'm getting 120V L-G out of each transformer.

Thanks

 

[OHHV]

Rethinking scenario 2, should it be 75000VA/(208V*1.73) = 208A per phase?

 

[Iron_Ben]

Rethinking scenario 2, should it be 75000VA/(208V*1.73) = 208A per phase?

Yes. You had that wrong in your original post but it's correct now. Several different ways to do the math but they all lead to the one right answer. Not to be too picky, but I wouldn't say "maximum amperage" that you can get out of the transformer. I know what you mean, but "full load current" or some such would be more accurate. Overloading pole top transformers to 125% or 150% of nameplate rating for a few hours here and there is no big deal to a power company. It wasn't for us anyway.

 

[OHHV]

Yes. You had that wrong in your original post but it's correct now. Several different ways to do the math but they all lead to the one right answer. Not to be too picky, but I wouldn't say "maximum amperage" that you can get out of the transformer. I know what you mean, but "full load current" or some such would be more accurate. Overloading pole top transformers to 125% or 150% of nameplate rating for a few hours here and there is no big deal to a power company. It wasn't for us anyway.

Thanks Ben for the reply. So that I understand it correctly, for the 3P transformer bank, where am I measuring 208A? Looking at the attachment, is that measured secondary phase to phase (hot leg to hot leg a,b,c) or line to neutral (a,b,c to n) on the transformer? What I'm trying to get at is if the total full load current for the bank 208A or is that per transformer? I'm not taking 208A * 3 transformers to get 625A right?

 

[gar]

160505-0010 EDT

OHHV:

Your original post was confusing.

Based on your drawing you have three single phase transformers connected in a 3-phase wye to wye configuration. The secondaries are in parallel. Not stated or implied in your original post. In fact the implication of the OP was that the 120 V secondaries were in series.

Each transformer on its own is good for 25 kVA, thus with the secondaries wired in parallel at 120 V the full rated secondary current calculates to 25,000/120 = 208 A for the paralleled secondary.

Using the configuration shown in your diagram does not change this secondary current, nor the wye line current. Whether there is one or three transformers that line current limitation remains the same.

For three phase, if you wired each secondary in series producing 240 V across a secondary and connected those secondaries in delta, then delta line current would not be rated at 208 A, but it would be 208*208/240 = 180 A.

You need to logically think thru a circuit and see how it operates. Don't just plug numbers into an equation.

.

 

[topgone]

Thanks Ben for the reply. So that I understand it correctly, for the 3P transformer bank, where am I measuring 208A? Looking at the attachment, is that measured secondary phase to phase (hot leg to hot leg a,b,c) or line to neutral (a,b,c to n) on the transformer? What I'm trying to get at is if the total full load current for the bank 208A or is that per transformer? I'm not taking 208A * 3 transformers to get 625A right?

Look at it this way:
You have a 75 kVA transformer bank (3 x 25 kVA), 13.2 kV line to neutral (22.86 kV line-to-line) with a rated secondary amps of 75,000/[1.732 x 208] = 208 amperes at 208 volts (line-to-line).

 

[Sahib]

OHHV:
You may utilise the 25 KVA capacity of each transformer in the Y-Y bank configuration either by a set of three phase loads, a set of phase to neutral loads for all the three phases, a set of phase to phase loads or by a mixed set of phase to neutral loads and phase to phase loads, the last being hard to do.

 

[OHHV]

Thank you for the replies. So that I'm clear, in my first scenario i'll be able to feed a Single phase customer who has a diversified actual load draw of 104 A max (theoretically) and for the second scenario i'll be able to supply power to a three-phase customer that would draw 208A total or is it 625A total (208Ax3)?

 

[kwired]

Thank you for the replies. So that I'm clear, in my first scenario i'll be able to feed a Single phase customer who has a diversified actual load draw of 104 A max (theoretically) and for the second scenario i'll be able to supply power to a three-phase customer that would draw 208A total or is it 625A total (208Ax3)?

The single phase customer is limited to 25 kVA.

The three phase customer is limited to 75 kVA, but that is for balanced loading, they still can not put 50 kVA of single phase load on just one of the phases.

Load calculations should be done in VA not amps as a general rule.

 

[Iron_Ben]

Thank you for the replies. So that I'm clear, in my first scenario i'll be able to feed a Single phase customer who has a diversified actual load draw of 104 A max (theoretically) and for the second scenario i'll be able to supply power to a three-phase customer that would draw 208A total or is it 625A total (208Ax3)?

It's 208 amps, not 625 amps. I think it would help you visualize things if you incorporate the phrase "per phase" in your thinking. With this three phase 75 kva bank serving a balanced load at 100% of nameplate, you could put your clamp on ammeter around each of the three phases one at a time and read 208 amps on each of them. So, we say, "208 amps per phase".

 

[charlie b]

It's 208 amps, not 625 amps.

That is correct.

I think it would help you visualize things if you incorporate the phrase "per phase" in your thinking.

And I think our industry would be well served if we all, now and forever, drop the "per phase" from our vocabulary. It is misleading, and indeed tends to make one think of adding the three to get a "total amps." A better way to think of it is that the 208 amps you see on Phase A is the same amps as the 208 amps you see on either Phase B or Phase C. The three just don't reach their peak values at the same time. So when Phase A is at its maximum current, the amps flowing away from the source at that moment are flowing back to the source on Phases B and C. It's the same amps! That is why you don't add the three to get a total.

 

[GoldDigger]

When we talk about a 10A 240V load on 120/240 single phase three wire system, we do not call it 10A per phase either.

 

[Sahib]

OHLV: In your case it does not matter whether you take 208 or 625A for energy consumption calculation provided you take correct voltage in calculations.

 

[kwired]

you can supply up to 625 amps of 120 volt loads, but needs to be balanced across all three phases.

you can supply 208 amps of 208 volt three phase load

you can supply 208 amps of 208 volt single phase load between two lines or between three lines

combination loads (single and three phase) can not exceed 208 amps per line which is 25 kVA @ 120 volts

common practice is to calculate load in VA and to balance load as much as possible.

 

[Tony S]

After looking at the drawing a though occurred to me, is it physically possible to reconfigure the transformers from series to parallel secondary?

Every centre tapped transformer I’ve worked on only one tap was bought out for the mid point.

 

[kwired]

After looking at the drawing a though occurred to me, is it physically possible to reconfigure the transformers from series to parallel secondary?

Every centre tapped transformer I’ve worked on only one tap was bought out for the mid point.

I'm not a utility guy but I believe it is somewhat common for pole mount type transformers to be able to reconfigure them that way. Same unit can be uses as 120/240 single phase or part of a three phase bank or as a 120 unit in a 208/120 wye bank, and the need for stocking more units in the supply yard is reduced because of it. I think you need to open the unit up to reconfigure it though, but is designed to do so.

 

[Tony S]

It was just the way the drawing showed three secondary bushings that made me think of a possible problem.

 

[Smart $]

After looking at the drawing a though occurred to me, is it physically possible to reconfigure the transformers from series to parallel secondary?

Every centre tapped transformer I’ve worked on only one tap was bought out for the mid point.

It was just the way the drawing showed three secondary bushings that made me think of a possible problem.

From the drawing it appears to me that the transformers are configured internally for parallel secondary, and though each transformer has three secondary bushings, only two each are connected.

 

[kwired]

From the drawing it appears to me that the transformers are configured internally for parallel secondary, and though each transformer has three secondary bushings, only two each are connected.

If you forget to reconfigure it internally to parallel coils and connect same way as shown in the drawing things still work, but you only have half the kVA capacity before overloading begins.

 

[Smart $]

If you forget to reconfigure it internally to parallel coils and connect same way as shown in the drawing things still work, but you only have half the kVA capacity before overloading begins.

That is correct. Seems to me that some signage and assured-configuration testing is in order.