Single Phase Transformer ??

[badgerfan08]

I have a question regarding a transformer installation. This particular building has a 480V 3 phase system, ungrounded delta, and they are putting an addition on. The jobsite trailer needs 240 volts single phase, and I need to install a transformer to accomplish this. It just so happens I had a 25 KVA single phase transformer on the shelf so I decided to use it. The trailer only needs 30 A or so, so it should be more than adequate. I know x-formers operate better when they are closer to 100% loaded but it will have to do. My question is regarding the secondary protection. If I were to use a 3 pole fused disconnect, and only use two poles, one for each hot leg would this be legal? I would tie the neutral straight through, and fuse the two legs to the proper size. The disconnect is rated for 600 Vac and is located directly to the side of the transformer. Just want to check and see if thats OK, or if both legs must open if a problem occurs.

 

[broadgage]

If I have understood the O/P correctly, the intention is to use a single phase transformer with a 480 volt primary and a center tapped secondary to give 120/240 volt service from the secondary.

It is then proposed to use a 3 pole fused disconnect on the secondary of the transformer, with only 2 poles being used in order to protect both hot legs of the 120/240 supply.
Seems OK to me, provided that the transformer frame and the neutral are correctly grounded.
The actual routing of the secondary conductors would require care, in order to avoid eddy current heating.
It would appear that if the dissconect is made of ferous material, then the neutral would have to be routed via the dissconect (though NOT fused !)

 

[jim dungar]

Your idea is fine. Fuses are an acceptable overcurrent protection

The requirement is that both legs be opened/disconnected "manually" at the same time. There is no requirement that they both open during a "fault" condition.

 

[hurk27]

Just to note, if you bond the X/0 at the transformer then make sure you run an EGC with your secondarys to the first panel and treat the panel as a sub panel keeping the neutrals and EGC's seperated.