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

[kwired]

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)

If energy loss is the reason it doesn't belong in NEC it belongs in some energy code. Nothing wrong with NEC putting in an informational note about it though if that were the case. Energy efficiency requirements in the NEC would go against the purpose mentioned in 90.1.

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:

What I posted was additional information provided in my 2014 E book version of NEC. They have it is not part of NFPA 70 itself, may be content printed in the handbook though. They have some of this kind of information after most changes in the E book, but not all the commentary you find in the handbook. What is written there does correspond to the ROP you mentioned.

 

[ggunn]

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

Actually it was a 208D/480Y and a 480Y/208D, i.e., the connections were the same for both transformers but the primary/secondary designations were different. Maybe they were different only in what was stamped on the nameplates, I dunno...

 

[texie]

While you can still use a 480 delta/208 Y in reverse as long as you comply with the new 450.11(B), I just don't get why one would want to do this given all the down sides of this. Whether you operate the now 480 secondary side ungrounded or corner grounded, each triggers other requirements that often don't seem to be considered by the installer and as such many are not safe or compliant.

 

[sandsnow]

Federal Pacific has a nice write up on reverse feeding. Page 16 of the catalog. Go to the link and download 600V Class Transformers.
http://federalpacific.com/literature/dry-type-transformers.htm

I remember something about that reverse feeding is not recommended over 75 kVA, but I can't trace that to a source.

I have seen an old Wye-delta transformer where the HO was bonded to the case at the factory.

 

[iceworm]

...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.

Actually it was a 208D/480Y and a 480Y/208D, i.e., the connections were the same for both transformers but the primary/secondary designations were different. Maybe they were different only in what was stamped on the nameplates, I dunno...

Now that is really weird. I did not expect that.

ice

 

[kwired]

Federal Pacific has a nice write up on reverse feeding. Page 16 of the catalog. Go to the link and download 600V Class Transformers.
http://federalpacific.com/literature/dry-type-transformers.htm

I remember something about that reverse feeding is not recommended over 75 kVA, but I can't trace that to a source.

I have seen an old Wye-delta transformer where the HO was bonded to the case at the factory.

4160/480 volts @ 50Hz? Where did you find this one at?

 

[sandsnow]

4160/480 volts @ 50Hz? Where did you find this one at?

At the now closed El Toro Marine Corp Air Station. In one of the relatively modern hangars, maybe 1950's vintage

 

[sandsnow]

Actually it was a 208D/480Y and a 480Y/208D, i.e., the connections were the same for both transformers but the primary/secondary designations were different. Maybe they were different only in what was stamped on the nameplates, I dunno...

Just a guess, but probably because of this point that templdl made: 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.

 

[GoldDigger]

Just a guess, but probably because of this point that templdl made: 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.

The paper linked earlier shows that instead of the inrush current (early cycles) being roughly 10 times the normal full load current, it can be 55 times the normal full load current.
The magnetizing current (steady state) will be more or less proportional (inversely) to the input side voltage.

 

[Besoeker]

One other point that I add, and forgive if it has already been covered.
If the transformer is designed for 480V primary and 208V secondary on load (and most are in my experience) the secondary turns will be a little greater than 208/480 of the primary the primary turns.
This means that, if you reverse power it you will get a bit less than 480V out unless you can adjust taps to compensate.

 

[templdl]

One other point that I add, and forgive if it has already been covered.
If the transformer is designed for 480V primary and 208V secondary on load (and most are in my experience) the secondary turns will be a little greater than 208/480 of the primary the primary turns.
This means that, if you reverse power it you will get a bit less than 480V out unless you can adjust taps to compensate.[/QUOTE

Yes if it is a Control power or machine tool transformer when the tens ratio helps with voltage drops due to inrush loads such as coils that result in a voltage dips due to energizing coils. If used as a step up transformer (As I prefer not to use the term reverse fed) the ooutput voltage will be surprisingly lower than anticipated.

No if it is a power transformer where a 4:1 ratio is a true 4:1 ratio.
It is also important to note that the LV windings are commonly wound first next to the core with the HV windings on the outside making it convenient to place taps. When used as a step up transformer the taps end up on the output side which can mess with your brain when you try to figure out out to adjust the gasps t compensate for low or high supply voltages.
Also, one would expect that inrush current would be proportional also but because the LV windings are next to the core you can anticipate them to be greater when using the transformer to step up.

 

[ggunn]

I don't remember if I mentioned it earlier, but my application was the interconnection of a bank of 480V native PV inverters with a 208V service.

 

[kwired]

I don't remember if I mentioned it earlier, but my application was the interconnection of a bank of 480V native PV inverters with a 208V service.

If there is increased inrush current as suggested there may be - you may want to find out if the inverters can handle that, though they are more likely to be able to limit current better then other sources may be.

 

[Besoeker]

No if it is a power transformer where a 4:1 ratio is a true 4:1 ratio.
It is also important to note that the LV windings are commonly wound first next to the core with the HV windings on the outside making it convenient to place taps. When used as a step up transformer the taps end up on the output side which can mess with your brain when you try to figure out out to adjust the gasps t compensate for low or high supply voltages.
Also, one would expect that inrush current would be proportional also but because the LV windings are next to the core you can anticipate them to be greater when using the transformer to step up.

It isn't about inrush.
Check out the Steinmetz equivalent circuit. It may help explain.

 

[GoldDigger]

Just an observation about a way in which PV backfeed differs from normal use of a stepdown transformer as a stepup instead:
In the case of a PV interface transformer, regardless of the direction of power flow the inrush current will always be supplied from the POCO side. But problems related to compensated windings will depend on the power flow direction.

 

[tom baker]

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.[/QUOTE

I am glad to see this change - here is why.
Delta 480x 120/208 Wye if reversed is a delta on the secondary and would have to be corner grounded, I suspect there are many that are not.
Sq D and probably others make a step up transformer, delta by wye, which I have used.

 

[ggunn]

If there is increased inrush current as suggested there may be - you may want to find out if the inverters can handle that, though they are more likely to be able to limit current better then other sources may be.

Grid tied PV inverters are by their nature current limited to their published maximum output. As for inrush current to the transformer, that can only come from the service side when the disconnect is closed because the inverters will not fire up until they have seen stable and correct voltage on their output terminals for 5 minutes or so.

 

[kwired]

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.[/QUOTE

I am glad to see this change - here is why.
Delta 480x 120/208 Wye if reversed is a delta on the secondary and would have to be corner grounded, I suspect there are many that are not.
Sq D and probably others make a step up transformer, delta by wye, which I have used.

I agree that there are likely many systems out there that are not grounded. I also believe there are many incorrectly installed SDS that do have a secondary neutral, or are basic two wire or three phase three wire that also are not installed correctly. Installers need to know what they are doing is the bottom line IMO.

Grid tied PV inverters are by their nature current limited to their published maximum output. As for inrush current to the transformer, that can only come from the service side when the disconnect is closed because the inverters will not fire up until they have seen stable and correct voltage on their output terminals for 5 minutes or so.

I'm not really all that familiar with PV equipment, but thanks for the info.

 

[templdl]

It isn't about inrush.
Check out the Steinmetz equivalent circuit. It may help explain.

It will be about inrush when the OCPD is tripped as the transformer is energized. If so how is it addressed?

 

[sandsnow]

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.[/QUOTE

I am glad to see this change - here is why.
Delta 480x 120/208 Wye if reversed is a delta on the secondary and would have to be corner grounded, I suspect there are many that are not.
Sq D and probably others make a step up transformer, delta by wye, which I have used.

I don't have a 2014 as we are on the 2011. Isn't the option still there to either bond and ground a corner of the delta OR provide Ground Detectors?