3 phase transformer

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shaw0486

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baltimore
If i am using a 3 phase delta wye transformer to feed a 120/208v single phase panel, is there any code that states i still have to take all 3 phases to the transformer or can i just feed it with a 2 pole breaker?
 
You talking Primary or Secondary?

Primary yes. Need it there to work.

Secondary I would take it to the first OCP. But not sure if it is mandatory.
 
shaw0486 said:
If i am using a 3 phase delta wye transformer to feed a 120/208v single phase panel, is there any code that states i still have to take all 3 phases to the transformer or can i just feed it with a 2 pole breaker?
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I am curious why you would want to feed it with only 2 of the phase conductors?
As Brian has mentioned, you will need 3-phase to produce 3-phase.


You mention a 3-phase transformer...does your building not have 3-phase available (for the primary side)?
 
That would really bother me to place just a single phase panel on the load side of a three phase transformer. It makes no sense whatsoever.

That said, I do not know of any requirement to do anything with the unused phase, just leave it unconnected at the transformer.
 
brian john said:
Primary yes. Need it there to work.
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If he only wants single phase on the secondary then he will only need single phase on the primary.

LarryFine said:
All of which would lead to "Why have a 3-phase transformer?"
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I'm thinking the same thing. OP, are you just trying to use what you have available or is there some other reason you are asking this question?


A three-phase transformer is just three single-phase transformers inside the same box. If you don't want to use all three of them, the others will still work fine. The worst thing I could say about it would be that it is wasteful. If you're OK with that, go for it.
 
drbond24 said:
If he only wants single phase on the secondary then he will only need single phase on the primary.



I'm thinking the same thing. OP, are you just trying to use what you have available or is there some other reason you are asking this question?


A three-phase transformer is just three single-phase transformers inside the same box. If you don't want to use all three of them, the others will still work fine. The worst thing I could say about it would be that it is wasteful. If you're OK with that, go for it.
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The issue is that 120/208 sinle phase is not true single phase. Call it psuedo-single-phase perhaps... since there is still a phase relationship for line1-to-neutral-to-line2. Using a delta-wye configured transformer to derive the 120/208 requires all three phases to be present on the primary. If the design permits, the unused windings should be disconnected and shorted end-to-end.
 
A 3-Phase Y secondary produces 208 volts phase to phase, and 120 volts phase to ground. If you only connect one winding, you may (I don't know) have 208 volts across that winding, but there would be no way to get a neutral. You would have to have two secondary windings, and I don't see how you can connect just two delta primary windings on a 3-phase transformer.
I don't think what you're proposing will work.
I also don't think I've ever seen a 208/120 single phase panelboard.
As far as the Code issue - the fact that it is not a 'standard' voltage (as defined by IEEE) may have some bearing.
I recommend you connect it the way it was designed, and use a 3-Phase panelboard.
good luck
db
 
barclayd said:
...You would have to have two secondary windings, and I don't see how you can connect just two delta primary windings on a 3-phase transformer.
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Many dry-type transformers, especially those that have voltage taps on the primary, would permit one winding to be disconnected and shorted.

I don't think what you're proposing will work.
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It'll work when fed and connected properly.

As far as the Code issue - the fact that it is not a 'standard' voltage (as defined by IEEE) may have some bearing.
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TTBOMK, 120/208V 1? 3W is a standard IEEE voltage.

PS: I forgot to add in my first post to this thread that I would only use this configuration if the equipment was already "on hand" and saved someone the purchase of new, more suitable equipment.
 
I would not drive a 3-phase transformer (at or near full kVA load) as a single phase with out engineering input.

my concern is the unknown of a unbalnced load (using two ratjer than three phases)
Will it creat circulating current in the pri that exceeds the design feature.

Will it impact core saturation or uneven heating??

I do not have the answers, but my guess is a email to a mfg will get a tech support response. As a inspector I would request such information.
 
Smart $ said:
The issue is that 120/208 sinle phase is not true single phase. Call it psuedo-single-phase perhaps... since there is still a phase relationship for line1-to-neutral-to-line2.
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Yes, but that manifests itself by producing only 208v line-to-line, which IS single phase. Granted, that is offest slightly time-wise from the rest of the system, but that doesn't matter; only the voltage does.

As long as you treat it as two 120v supplies, and/or as a single 208v supply, the relative phasing doesn't have any effect that matters to the equipment being supplied.
 
barclayd said:
I also don't think I've ever seen a 208/120 single phase panelboard.
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I have, in an apartment building. Each tenant receives 120/208 1ph. It looks and works just like your home's service, with the exception that line-to-line loads receive only 208v.

Most 240v equipment, such as an AC unit, is rated for 208v to 240v. A clothes dryer has only the heater element connected line-to-line, and the motors wired line-to-neutral.

Heater elements, such as the dryer, or a water heater element, will operate at approximately 3/4 the power on 208v, while a motor will use a bit more current on 208v for the same power.
 
LarryFine said:
... Each tenant receives 120/208 1ph.
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They are all fed from a 3-phase 208Y/120v service, right?
(Does the panelboard namplate say 120/208 1-phase?)
No problem - 2-pole breakers in a 3-phase panelboard.
IJS - you need two phases on the secondary to do that, and I don't think you can energize two phase windings on the secondary by energizing only one primary winding.
SIMUMOA
db
 
brian john said:
You talking Primary or Secondary?

Primary yes. Need it there to work.
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Do you though?
If you apply phase to phase input voltage across just one primary delta connected winding the secondary wye (star) winding in phase with that will produce its rated voltage.
You can load that winding to its capacity.
 
Besoeker said:
the secondary wye (star) winding in phase with that will produce its rated voltage.
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If that one secondary winding has 208 volts measured from one end to the other, where would you get 120v?

I still don't think this will work real good.
Where is the neutral?
db
 
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