Transformers..

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jpujols01

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Location
Nyc
Hello.....
I just checked a 480v delta/240V Y transformer and found that they fed the transformer through the secondary side in order to get high voltage, but now I don't see the way to protect the conductors from the transformer to the disconnect sw in the line size.
Is it a code violation or a safety issue Reversing the transformer to make work in the opposite way?
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petersonra

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Northern illinois
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engineer
a pretty common practice. I think one of the later versions of the code says the manufacturer has to approve of it, but as best I can tell, they generally do.

the transformer and its conductors would be protected exactly the same way as any other transformer.
 

iwire

Moderator
Staff member
Location
Massachusetts
Notice that the transformer is labeled high voltage and low voltage not primary and secondary.

That being the case there is no 'correct direction' to use it.

You would feed it 208 delta and only get 480 delta out of it. You would have to choose to run it as ungrounded 480 delta or corner grounded 480 delta.
 

jpujols01

Member
Location
Nyc
This transformer is a primary high voltage and the way I know is because in transformer always the taps come in the primary side but please correct me if I'm wrong.


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iwire

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Location
Massachusetts
This transformer is a primary high voltage and the way I know is because in transformer always the taps come in the primary side but please correct me if I'm wrong.

The label does not specify primary or secondary and therefore the NEC allows its use in either direction.

What do you see as a safety issue?
 

big john

Senior Member
Location
Portland, ME
This transformer is a primary high voltage and the way I know is because in transformer always the taps come in the primary side but please correct me if I'm wrong....
Unless a transformer is specifically labeled by the manufacturer with "primary" and "secondary" then that is a designation determined entirely in the field by the end user. Any side that you apply power to becomes the primary by default. And following that, any side that you draw a load from becomes the secondary. For example, in a facility with a grid-tied generation system, the transformer power flow may switch direction through the transformer on a moments notice, so that the "primary" and "secondary" may be swapping sides a dozen times a day.
 

GoldDigger

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Retired PV System Designer
The label does not specify primary or secondary and therefore the NEC allows its use in either direction.

What do you see as a safety issue?
Notice that the voltage selection taps are on the H side. If the transformer is very tightly designed, then it is important to have the taps on the input side to avoid potential core saturation.
In some transformer construction the difference in input surge compared to running current depending on whether the input is on the inner or the outer winding is significant (a factor of two or more).
 

jpujols01

Member
Location
Nyc
What happen if for any reason there is a phase to phase 480v short between the wires that leave the transformer and end into the line side of the disconnect sw where there isn't fuses at all, I think the entire transformer will blow out before the fuses that protects the primary side open and clear any short.



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GoldDigger

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What happen if for any reason there is a phase to phase 480v short between the wires that leave the transformer and end into the line side of the disconnect sw where there isn't fuses at all, I think the entire transformer will blow out before the fuses that protects the primary side open and clear any short.

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The NEC provides requirements for protection of the primary side, whether high or low voltage. If these are met, then the transformer will be protected.
 

don_resqcapt19

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Illinois
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retired electrician
What happen if for any reason there is a phase to phase 480v short between the wires that leave the transformer and end into the line side of the disconnect sw where there isn't fuses at all, I think the entire transformer will blow out before the fuses that protects the primary side open and clear any short.



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How is that any different from when the transformer is fed at 480 and there is a short on the 208 volt side?
 

jpujols01

Member
Location
Nyc
I guess you right don_resq but anyway the arc in short from 480v comparing with 208v is a big proportion, and just prefer to use the secondary side of the transformer in Y system so I can bond the neutral point to the ground and make it work more safety.


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iwire

Moderator
Staff member
Location
Massachusetts
I guess you right don_resq but anyway the arc in short from 480v comparing with 208v is a big proportion, and just prefer to use the secondary side of the transformer in Y system so I can bond the neutral point to the ground and make it work more safety.

And we are trying to explain to you that the transformer in the picture has no set primary or secondary side. It has a high voltage and a low voltage side, how those are used is a design choice not a code one.

It is fine you like to do things a certain way but there is nothing wrong with using it the other way around.

You can run it either direction safely.
 

jpujols01

Member
Location
Nyc
I appreciate the way you guys explain everything here and make me feel satisfied with your answers....thanks all of you very much for your help.


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meternerd

Senior Member
Location
Athol, ID
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retired water & electric utility electrician, meter/relay tech
Notice that the voltage selection taps are on the H side. If the transformer is very tightly designed, then it is important to have the taps on the input side to avoid potential core saturation.
In some transformer construction the difference in input surge compared to running current depending on whether the input is on the inner or the outer winding is significant (a factor of two or more).

I always thought the taps were on the high voltage (H) side because the associated current/wiring would be smaller. Not so?
 

GoldDigger

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Location
Placerville, CA, USA
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Retired PV System Designer
I always thought the taps were on the high voltage (H) side because the associated current/wiring would be smaller. Not so?
Not so. The taps are generally on the source side of any transformer that is designed/optimized for one direction use.
That way the idling flux in the core will always be at the design level in spite of nominal versus actual voltage variations.
 

Jraef

Moderator, OTD
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Location
San Francisco Bay Area, CA, USA
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Electrical Engineer
By the way in case you didn't know (since you appear to not have come across this before), make sure they did NOT connect the X0 terminal to ground when using it as a step-up transformer and that there is no bonding strap or conductor. The frame must be grounded, but there must be no connection of anything to the X0 terminal.

As to the disconnect and OCPD for transformers, there must always be protection for the transformer, but there are numerous code rules that must be followed to the letter regarding protection of primaries and secondaries, based on the specifics of the installation and application. In general though, situations like this, where a machine is being fed via a step-up transformer because the machine needs a higher voltage than the site has, are covered under 240.4 (F). But again, let go of your concept of the primary always being high and the secondary low, it's just relative to what you want to accomplish. The rules I mentioned make no distinction between high and low, only primary and secondary. Primary is the power you bring TO the transformer, secondary is the power you get OUT of the transformer.
 
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