Transformer Secondary Currents

[dasarmin]

Below is my understanding of a Transformer?s Secondary Currents, I?d like to confirm these assumptions are correct. Any comments?

I have a 480V : 240V/120V Single Phase transformer. The Primary is fed with 2 legs of a 480V supply, the Secondary (X1, X2, X3, X4) has X4 GND/NEU, X2-X3= 120V, X1= 240V.
The available secondary currents:
? 240V @ 41.7Amps
? 120V @ 41.7Amps (this is 41.7 not 83.3 since the 120V is a Center Tap effectively at 5KVA)
The available current to either output 120V or 240V is affected by the other?s current. For example; the 120V output is drawing 15Amps, therefore the current available at the 240V output will be 41.7A ? 15A = 26.7A. The same would be true for the 120V output; 240V output is at 20Amps, therefore the available current at the 120V output would be 41.7A - 20A = 21.7A.

 

[kwired]

Below is my understanding of a Transformer?s Secondary Currents, I?d like to confirm these assumptions are correct. Any comments?

I have a 480V : 240V/120V Single Phase transformer. The Primary is fed with 2 legs of a 480V supply, the Secondary (X1, X2, X3, X4) has X4 GND/NEU, X2-X3= 120V, X1= 240V.
The available secondary currents:
? 240V @ 41.7Amps
? 120V @ 41.7Amps (this is 41.7 not 83.3 since the 120V is a Center Tap effectively at 5KVA)
The available current to either output 120V or 240V is affected by the other?s current. For example; the 120V output is drawing 15Amps, therefore the current available at the 240V output will be 41.7A ? 15A = 26.7A. The same would be true for the 120V output; 240V output is at 20Amps, therefore the available current at the 120V output would be 41.7A - 20A = 21.7A.

I'm going to repeat some of what you said, some with more proper terminology, some maybe just in a little different way that may or may not be easier to understand.

Typical 240/120 secondary has two output coils with leads X1 and X3 for one coil and X2 and X4 for the other coil. These coils will have a nominal 120 volts across them when the primary is supplied with correct nominal voltage (regardless of what that voltage may be). By connecting the X2 and X3 together we make this a multiwire source with 120 volts nominal between X1 and X2/X3, as well as 120 volts between X4 and X2/X3. We have both 120 volt coils in a series so they are additive and we have 120+120=240 volts from X1 to X4. To go off subject just a little you could parallel connect the coils instead of series connecting them and have X1/X2 together and X3/X4 together with only a single 120 volt output but in this configuration you could supply a single 120 volt load that is more then 5kVA where if connected in the 240/120 series connection you can still have 10kVA of 120 volt load but only 5 kVA max can be connected to each "half".

Your 41.7 amp rating is the full current rating of each coil and is 41.7 x 120 = 5000 VA per coil. Each coil doesn't care if it is supplying a 120 volt load or a portion of a 240 volt load it just doesn't want to have more then it's 5kVA rating applied or it will overheat. Loads that can be connected are fairly unimited you just can not have more then 5kVA connected to each half of the transformer. You can have 6 kVA of 240 volt load, and 2 kVA on each "half" of 120 volt load. You can not have 6 kVA of 240 volt load and 4 kVA on just one "half" of the secondary or that one half will be overloaded while the other half has some capacity left - though the primary still only sees a net of 10 kVA - this is part of why secondary overcurrent protection is almost always necessary on a multiwire secondary.

 

[dasarmin]

Transformer Secondary Currents

I'm going to repeat some of what you said, some with more proper terminology, some maybe just in a little different way that may or may not be easier to understand.

Typical 240/120 secondary has two output coils with leads X1 and X3 for one coil and X2 and X4 for the other coil. These coils will have a nominal 120 volts across them when the primary is supplied with correct nominal voltage (regardless of what that voltage may be). By connecting the X2 and X3 together we make this a multiwire source with 120 volts nominal between X1 and X2/X3, as well as 120 volts between X4 and X2/X3. We have both 120 volt coils in a series so they are additive and we have 120+120=240 volts from X1 to X4. To go off subject just a little you could parallel connect the coils instead of series connecting them and have X1/X2 together and X3/X4 together with only a single 120 volt output but in this configuration you could supply a single 120 volt load that is more then 5kVA where if connected in the 240/120 series connection you can still have 10kVA of 120 volt load but only 5 kVA max can be connected to each "half".

Your 41.7 amp rating is the full current rating of each coil and is 41.7 x 120 = 5000 VA per coil. Each coil doesn't care if it is supplying a 120 volt load or a portion of a 240 volt load it just doesn't want to have more then it's 5kVA rating applied or it will overheat. Loads that can be connected are fairly unimited you just can not have more then 5kVA connected to each half of the transformer. You can have 6 kVA of 240 volt load, and 2 kVA on each "half" of 120 volt load. You can not have 6 kVA of 240 volt load and 4 kVA on just one "half" of the secondary or that one half will be overloaded while the other half has some capacity left - though the primary still only sees a net of 10 kVA - this is part of why secondary overcurrent protection is almost always necessary on a multiwire secondary.

Your explanation makes sense, thanks for the response.