reverse use of standard delta Y three phase transformer. 208 in for 480 out

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infinity

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In general it's permitted, what will you use on the Delta secondary, corner grounded?
 

templdl

Senior Member
Location
Wisconsin
I've been in the industry since 1982 and worked for a DTDT manufacturer for 3 years. There is no derasting required in any of my application notes.
However you will note that the OP does not include enough information in order to provide a difinitive response without assuming what the actual application is such as what the supply voltage is. One could assume that it is as common 208y/120. If so a neutral is not required to be run, only the lines and EGC only.
The eye of the xfmr should be configured in this way: Remove the connection from the X0 to the enclosure if it is connected. There should be no connection to the X0. The connect the lines to the X1, X2, and X3 connection the EGC to the xfmr encl.
As one of thepreviouse posters pointed out it is important that you recognize that your secondary is a delta ad now you must decide on f you want to corner ground it.
In addition you must recognize that the taps are now on the secondary and as such you have to think backwards when connecting then which can play with your brain a bit.
The last thing that is overlooks is that HV windings as wound on the outside which is where the taps are connected to the windings. Since the LV windings are wound next to the core you will find the the magnetizing current (inrush) is not proportional but will be greater. As such be aware of undersdizing the PRI. of the OCPD and as you may trip it instantaneously when the xfmr is energized.
 
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kwired

Electron manager
Location
NE Nebraska
Note there was a change in 2014.

450.11(B) says : "A transformer shall be permitted to be supplied at the marked secondary voltage, provided that the installation is in accordance with the manufacturer?s instructions."

Basically means you can only "back feed" a transformer if the transformer is specifically intended for reverse wiring.
 

infinity

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Note there was a change in 2014.

450.11(B) says : "A transformer shall be permitted to be supplied at the marked secondary voltage, provided that the installation is in accordance with the manufacturer?s instructions."

Basically means you can only "back feed" a transformer if the transformer is specifically intended for reverse wiring.

The way I read that the instructions would have to say that it's only permitted to wire it using the marked primary voltage for that to prohibit a "back fed" arrangement.
 

kwired

Electron manager
Location
NE Nebraska
The way I read that the instructions would have to say that it's only permitted to wire it using the marked primary voltage for that to prohibit a "back fed" arrangement.

A comment after the end of the section in my E-book version says "Change from 2011 NEC:Revised into subsections with a new list format in subsection (A), and new subsection (B) now requires marking for transformers that can be reverse wired."


Either way one has to give some second thoughts before "backfeeding" any ol transformer you have laying around.
 

iceworm

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The way I read that the instructions would have to say that it's only permitted to wire it using the marked primary voltage for that to prohibit a "back fed" arrangement.

...Either way one has to give some second thoughts before "backfeeding" any ol transformer you have laying around.

The same transformer that was safe and legal yesterday, is now unsafe and illegal - unless the mfg writes "Okay to backfeed" on it. Kind of makes one hope the mfgs had sufficient heads up to purge their system of all the old xfm on the shelf and all the old data plates.

Makes me curious what prompted the change. Mfgs to sell more xfm? Doesn't seem likely - they are selling the same amount of Al and steel. Bodycount/fires? Haven't heard of any. Lack of following code on ground detectors? No way to fix stupid that I know of. Finally getting around to fixing a section that has slways been wrong - like the dryer/stove issue? Maybe.

It's a puzzle.

ice
 

infinity

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Is what kwired posted actually part of the actual NEC wording? If not then it's anyone's guess as to whether or not an off the shelf transformer can still be reverse wired. If I had to take a guess I would say that nothing has changed but my guesses usually are in the 50-50 range. :roll:
 

augie47

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The same transformer that was safe and legal yesterday, is now unsafe and illegal - unless the mfg writes "Okay to backfeed" on it. Kind of makes one hope the mfgs had sufficient heads up to purge their system of all the old xfm on the shelf and all the old data plates.

Makes me curious what prompted the change. Mfgs to sell more xfm? Doesn't seem likely - they are selling the same amount of Al and steel. Bodycount/fires? Haven't heard of any. Lack of following code on ground detectors? No way to fix stupid that I know of. Finally getting around to fixing a section that has slways been wrong - like the dryer/stove issue? Maybe.

It's a puzzle.

ice

Is there a greater energy loss wheh "reverse wiring" as opposed to a "designed" transformer. (Just thing the change might be based on energy loss)
 

Nom Deplume

Senior Member
Location
USA
The same transformer that was safe and legal yesterday, is now unsafe and illegal - unless the mfg writes "Okay to backfeed" on it. Kind of makes one hope the mfgs had sufficient heads up to purge their system of all the old xfm on the shelf and all the old data plates.

Makes me curious what prompted the change. Mfgs to sell more xfm? Doesn't seem likely - they are selling the same amount of Al and steel. Bodycount/fires? Haven't heard of any. Lack of following code on ground detectors? No way to fix stupid that I know of. Finally getting around to fixing a section that has slways been wrong - like the dryer/stove issue? Maybe.

It's a puzzle.

ice

All of the transformers that I have seen only mark the HV and LV and not Line or Load. So if marked HV and LV they can be backfed.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
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All of the transformers that I have seen only mark the HV and LV and not Line or Load. So if marked HV and LV they can be backfed.

450.11.B is new in the 2014. Looks like they were trying to make a point about something. Have you read it?

ice
 

Nom Deplume

Senior Member
Location
USA
So now with 450.11(B) you cant backfeed a transformer unless the manufacture says it's OK?
I guess transformers have been bursting into flames all around us and we haven't noticed.

(B) Source Marking.
A transformer shall be permitted to
be supplied at the marked secondary voltage, provided that
the instal lation is in accordance with the manufacturer?s
instructions.
 

infinity

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Location
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Journeyman Electrician
Here's the 2014 new section. I don't see where marking of the transformer is required to permit reverse wiring.

(B) Source Marking. A transformer shall be permitted to
be supplied at the marked secondary voltage, provided that
the installation is in accordance with the manufacturer?s
instructions.
 

infinity

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Here's the ROP:

9-145 Log #1138 NEC-P09 Final Action: Accept in Principle
(450.11)
________________________________________________________________
Submitter: Gaylord Poe, Inspection Bureau, Inc.
Recommendation: Revise text to read as follows:
2014 NEC Proposed - 450.11 - Marking. Each transformer shall be
provided with a nameplate giving with the following information:
1. Tthe name of the manufacturer.,
2. Rrated kilovolt-amperes.,
3. Ffrequency.,
4. Pprimary and secondary voltage.,
5. For dry type transformers, if the transformer is permitted to be connected
in either direction, it shall be marked as ?Bi-Directional?. Installation
instructions shall be provided by the manufacturer detailing how the
transformer should be connected when the primary and secondary are reversed.
6. The impedance of transformers 25kVA and larger.,
7. Rrequired clearances for transformers with ventilating openings., and
8. Tthe amount and kind of insulating liquid where used.
In addition, the nameplate of each dry-type transformer shall include the
temperature class for the insulation system.
Substantiation: Although it is common industry practice to reverse wire dry-
type transformers, the marking requirements in the current NEC do not make
the installer aware of the fact that UL 1561 does not support this practice. UL
1561 provides that the testing of these transformers includes ?...step up, step
down, and autotransformer type...? with the supply being connected to the
primary and the load being connected to the secondary, thus, making
connections in any other manner a violation of 110.3(B). Changing the marking
requirements of 450.11 will provide clarification and enhance electrical safety.
Panel Meeting Action: Accept in Principle
Revise the text to read as follows:
450.11 - Marking.
(A) General. Each transformer shall be provided with a nameplate giving the
following information:
1. The name of the manufacturer
2. Rated kilovolt-amperes
3. Frequency
4. Primary and secondary voltage
5. The impedance of transformers 25kVA and larger
6. Required clearances for transformers with ventilating openings
7. The amount and kind of insulating liquid where used
8. For dry-type transformers, the temperature class for the insulation system
(B) Source Marking. A transformer shall be permitted to be supplied at the
marked secondary voltage provided the installation is in accordance with the
manufacturers? instructions.
Panel Statement: The Panel action corrects a violation of the NEC Style
Manual relative to the formatting of lists and meets the submitter?s intent. CMP
9 concludes that a specific marking as opposed to general instructions from the
manufacturer is excessive.
Submitter?s proposal was limited only to dry-type
transformers without adequate substantiation.
Number Eligible to Vote: 12
Ballot Results: Affirmative: 12
 

iceworm

Curmudgeon still using printed IEEE Color Books
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Occupation
EE (Field - as little design as possible)
Here's the 2014 new section. I don't see where marking of the transformer is required to permit reverse wiring.

yep. However, following Nom's reasoning (which is good): If it isn't marked on the nameplate the instructions would have to be taped to the side of the xfm. If they were to fall off, the xfm could well burst into flames.

ice
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
yep. However, following Nom's reasoning (which is good): If it isn't marked on the nameplate the instructions would have to be taped to the side of the xfm. If they were to fall off, the xfm could well burst into flames.

ice

FWIW, when I was designing a PV system with 480V inverters connected to a 208V service, the transformer vendor I was dealing with told me that their 480V to 208V step down transformer was not interchangeable with their 208V to 480V step up transformer.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
... the transformer vendor I was dealing with told me that their 480V to 208V step down transformer was not interchangeable with their 208V to 480V step up transformer.

That's a good vendor. I would sincerely hope a vendor would spec a 480D/208Y and a 208D/480Y depending on which direction one wished to go. And those two are certainly not interchangable.

ice
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
...a 480D/208Y and a 208D/480Y ...are certainly not interchangable.

Just an opinion. My preference would be to standardize:
transformer is marked "Primary" and "Secondary" - It is a one-way xfm
transformer is marked "Line" and "Load" - It is a one-way xfm
transformer is marked "HV" and "LV" - It is okay either direction.

I don't see it happening. What do they do for:
Taps (saturation when reverse fed)
Inrush 3(+) X normal when reversed
Compensated windings (low voltage)

The only time I would ever recommend using a 208Y/408D as a step-up is if that was all that was available in the alloted time.

ice
 
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