Proper feed of 120/240 to transformer

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spectrum24

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I have had some confliciting advice regarding the proper feeding of a transformer from 120/240 service. One camp says to ground the neutral from the service, and just use the 2 hot legs to feed the transformer, with the windings connected in series. The other camp says ground the neutral, connect the windings in series, and then connect it to the center tap of the transformer. I would appreciate input on this question. So, the question is, is a 2 wire or 3 wire connection to the transformer required?
 
Unless you are a utility, you should never, ever, connect to the 'neutral' point on the primary of a transformer.
 
Often times the OP leaves so much to be guessed at.
It was stated what I believe to be a 120/240 1ph3w service entrance and then it was left open ended regarding a transformer of some type that is being feed. Is so we are being asked to take a wild guess at what you are trying to do.
If I were being asked the questions I would want more information regarding this transformer being fed, what is the KVA, the primary and secondary voltages? With this information it is more likely that a specific answer can be provided.
Also, are we to assume that you a versed in safe electrical practices and procedures as well as being familiar with the NEC.
 
I am familiar with NEC but am still learning all the intricacies. Some questions are being raised now as to how things have been done one way once, and other ways some other time.

The feed could be 120/240 single phase, or 240/480. The transformer primary is typically a few feet from the meter in the 120/240 case (step up) but could be a few hundred feet away if the service is 240/480 (step down)

Transformers are typically 5-15 KVA

I have typically not used the neutral to feed the transformer as it seemed pointless... but an EE in the company had it drawn that way. In the case of the 240/480 feeding a step down several hundred feet away, it was drawn as the 2 hots and (grounded) neutral being run the whole way, with no separate ECG. I think that is incorrect.
 
spectrum24 said:
I am familiar with NEC but am still learning all the intricacies. Some questions are being raised now as to how things have been done one way once, and other ways some other time.

The feed could be 120/240 single phase, or 240/480. The transformer primary is typically a few feet from the meter in the 120/240 case (step up) but could be a few hundred feet away if the service is 240/480 (step down)

Transformers are typically 5-15 KVA

I have typically not used the neutral to feed the transformer as it seemed pointless... but an EE in the company had it drawn that way. In the case of the 240/480 feeding a step down several hundred feet away, it was drawn as the 2 hots and (grounded) neutral being run the whole way, with no separate ECG. I think that is incorrect.
Click to expand...

This is commonly done all of the time.

If you have a 1ph transformer that is 240x480-120/240v and would like to use it as a step up transformer by feeding it from a 120/240v 1ph3w source simply bring ount only the 240v L-L and the EGC conductors. No need for the neutral. Normally the secondary of the 240x480-120/240v transformer will have the secondary connections marked x1, X2, X3, and X4. Connect the X2 and X3 together and connect your 240v supply to X1 and X4 connecting the EGC to the frame of the transformer.
I am not sure if your term "if the service is 240/480 (step down)" is correct or not but I may assume that your application is that you intend to end up with 120/240v 1ph3w at the remote location. If so configure the HV side of your 240x480 transformer to be 480v so that you have 480v 2w out.
Then, the remote transformer should have its HV side configured in the same way so that the HV side is also 480v. Then connect the secondary X2 and X3 together. X1 and X4 will then be your lines and X2X3 will be your neutral which also will become your grounded conductor.
Now, transformers of that size should have HV taps which can the used to correct for any voltage drop that occurs and as such you can get a pretty good 120/240 at the remote location.

I have attached a couple of typical transformer wiring diagrams, one that has taps (which is common) and one without taps.
 

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Thanks. I do commonly do what you say, it was the connecting of the neutral on the primary side that concerned me. As someone pointed out to me, When you ground the neutral of the secondary winding (as per NEC), and also ground the neutral of the primary winding, you may establish an unintended circuit from the hot tap of the primary winding, to the neutral of the primary winding, to the neutral of the secondary winding, then to the rest of the secondary winding. That would look like an auto-transformer, or a voltage divider, no more galvanic separation between the primary and secondary windings. I should have thought of that.

Thanks again to all.
 
spectrum24 said:
As someone pointed out to me,When you ground the neutral of the secondary winding (as per NEC), and also ground the neutral of the primary winding, you may establish an unintended circuit from the hot tap of the primary winding, to the neutral of the primary winding, to the neutral of the secondary winding, then to the rest of the secondary winding. That would look like an auto-transformer, or a voltage divider, no more galvanic separation between the primary and secondary windings. I should have thought of that.
Click to expand...

It has nothing to do with creating an auto transformer, utilities do this primary and secondary neutral connections in probably >50% of their installations.

For us, the primary issue has to do with unbalanced loading causing circulating (balancing) currents to flow resulting in overheating and eventual failure.
 
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