Single phase transformer question.

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rrc14

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What is the max secondary current of a 7.5kVA, 240/120V, transformer, if I'm only using a line, neutral, and ground conductor? I'm thinking the capacity of the transformer will be reduced by 1/2 since I'm only using one line instead of two. Am I visualizing this correctly?

Thanks,

RC--
 
Yes

Yes

RC,
Yes, you are visualizing correctly.
The center-tapped transformer is only using half of the windings, or 3,750VA, so the current will be 3,750VA/120V=31.25A.
JM
 
transformer

transformer

JM,
Would that not depend on what he is actually looking for on the secondary and how it is wired?.
rrc states he's only using "line, neutral and grounded"...if he wants to wire the secondary for 120 volt output (vs 240/120) would he not have the full capacity of the transformer ?
 
.... agreed

.... agreed

augie47,
Good point!
If he were to parallel the windings such that there were now (2) 3750 VA transformers in parallel, that would make for 7500/120=62.5 amps.
I hadn't considered that!
JM
 
The portable building I am working on to provide electricity to has been
prewired for 120V, 3-wire supply. The load of the building is 55A at 120V. I am stepping down 480V to 120V. A standard size 7.5kVA single phase can provide 31.25A at 240V & 62A at 120V using both hot wires.

Since the building has been prewired for a 120V, 3-wire supply I was originally thinking about using a 15kVA transformer to supply my 55A at 120V, 3-wire, using only half the windings. I did not even consider paralleling the two half-windings to allow the full capacity of the 7.5kVA transfomer at 120V, 3-wire. I'll contact our transformer supplier tomorrow and verify if this is possible. Anyways, thanks for your replies and I'll let you know what the outcome is.

RC-
 
As has been described a single winding secondary (240/120) with a center tap with a L-N-L you are able to get 1/2 of the rating of the transformer by using only one L-N connection.
Transformers also have dual winding secondaries (120/240) have secondary connections that are marked X1, X2, X3, and X4. X1-X2 in one 120v winding and X3-X4 is the other. If X2 and X3 are tied together that connection becomes essentially a center point, as the neutral would be for a 240/120 secondary. Again you have only 1/2 of the KVA of the transformer available from X1- X2X3 and X4-X2X3.
Although not often used it is possible to get 100% of the capacity of the transformer with a 120/240 secondary on a single 120v circuit by connecting the X1 and X3 together and the X2 and X4 together which parallels the windings. The 120v load would be taken from the X1X3 and the X2X4.
 
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