480 Volt single phase to 120/240 V single phase

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Kenneth Goetz

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Port Lavaca, Texas USA
I work at an industrial facility where we use 480V three phase for most installations. Panels in buildings are 480V to 120/208 3 phase. We have an upcoming installation where the portable building being received has a 120V 240V panel installed so the contractor needs single phase. Our electricians say just use 2 legs of the 480V 3 phase Y system and use a 480V to 120/240 V single phase transformer. I saw where you can get a 37.5 KVA transformer. Is this proper to do?
 
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At first the electricians said to use a 480V to 120/208 transformer and just run 2 legs of the 480 and don't worry about the other one. I told them you could not do that due to the fact a 30KVA transformer would not be 30KVA and the transformer would run hot due to not being balanced also. I asked the question due to the fact the 480V is only 120 deg difference in phases and not 180 deg. Thanks for the welcome and the answer.
 
Kenneth Goetz said:
At first the electricians said to use a 480V to 120/208 transformer and just run 2 legs of the 480 and don't worry about the other one. I told them you could not do that due to the fact a 30KVA transformer would not be 30KVA and the transformer would run hot due to not being balanced also. I asked the question due to the fact the 480V is only 120 deg difference in phases and not 180 deg. Thanks for the welcome and the answer.
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Since 37.5 kVA single phase transformer was mentioned, your load must be more than 25 but less than 37.5. If you used a 30 KVA wye secondary unit and only supplied two phases and neutral - you really only have 20 kVA available and the remaining 10 kVA is not being tapped into.

Even if you went with larger three phase transformer, the source still will see it as an unbalance on two of the three phases. Might as well go with a single phase transformer if you are going to purchase one, if you have something that may work already on hand, that can be something to consider though.
 
infinity said:
Just buy the proper 1Ø, 480-240 volt transformer.

View attachment 23267
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Thank you personally for posting this. I did not have a photo of a similar installation and have used this exact scenario before multiple times for a lot of remote wastewater applications that have 3 phase available at a site, bu have to run a few thousand feet to an OSSF area with control station, but we have had electricians question the wiring setup and asked a similar question to the OP.
 
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