3 phase Transformer Calculations
- Thread starter laketime
- Start date
- Status
- Location
- Illinois
- Occupation
- retired electrician
208/480 x 100
victor.cherkashi
Senior Member
- Location
- NYC, NY
Use 100% rated cb in order not to deal with 1.25 factor
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Sent from my Nexus 5 using Tapatalk
- Occupation
- Licensed Electrician
Just pick one of those up at........where? You tell me and I will get a sackful of them.
kwired
Electron manager
- Location
- NE Nebraska
First, one normally figures out what capacity one needs to supply the intended load instead of matching a load to an available output.
Second thing is this a transformer designed for step down and you wish to "back feed" it or is it a step up transformer? 2014 added wording that effects use of a "back feed" transformer, does not prohibit it but says something to the effect it must be designed to do that. Back feeding does present some challenges as typically the 480 volt side is delta connected and only has three terminals - no neutral so you will likely need to corner ground it. You also do not want to connect the 120 volt neutral to the X0 terminal - it can/likely will cause high currents to circulate in the 208/120 side.
Tony S
Senior Member
- Location
- Resting under the Major Oak UK
Without more details this isn’t an easy one.
I’ve had this before, a DYn transformer used in reverse. It’s OK if the connected 480V system can absorb the back feed, the normal supply transformer provides the earth reference.
If the main 480V is from a MV/480V DY the corner grounding of the 2nd back feed transformer is out of the question
I’ve had this before, a DYn transformer used in reverse. It’s OK if the connected 480V system can absorb the back feed, the normal supply transformer provides the earth reference.
If the main 480V is from a MV/480V DY the corner grounding of the 2nd back feed transformer is out of the question
- Location
- Massachusetts
Not following you here but typically we would corner ground the secondary of a backfed transformer.
- Location
- Massachusetts
I think you will find that the dry type transformers being installed today are no longer labeled primary and secondary but high voltage and low voltage. Feed them either direction with out a code issue.
kwired
Electron manager
- Location
- NE Nebraska
You probably still find the high voltage side is delta connected and has no neutral unless it was intended to be a step up transformer, then the low voltage side may be delta with no neutral.
Still is more installations involving step down from 480 to 208/120 AFAIK then the other way around.
- Location
- Massachusetts
And the enclosures may be painted gray but I don't see how either of those issues has to do with the code section you mentioned.
kwired
Electron manager
- Location
- NE Nebraska
The way OP is stated this may be an older transformer customer just has laying around and they want to know what they can run with it. If buying a new transformer you usually figure out the load needs then select a suitable transformer. Also trying to reinforce the idea that there is a good chance there is no neutral to the high voltage side of the transformer.And the enclosures may be painted gray but I don't see how either of those issues has to do with the code section you mentioned.
laketime
Senior Member
The only power requirement I have is that the transformer needs to be supplied with 100 amps at 208v. We will not be pulling a neutral just 3 phase. Transformer is a "custom built drive isolation transformer". Spec says "75kva 120/240/480 to 480Y/277 volt. Does this information help?
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
I don't see 208V as option. This looks like a multi-tap delta input, stepping up to a 480Y/277 output, which is not an uncommon arrangement for drive isolation.
100A @208V will not provide 75kVA of power.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
instead of 480 you will get about 416 volts if you use the 240V taps.
- Status