Transformer installation

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brian john

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Kilmarnock, Va
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Retired after 52 years in the trade.
Am I being hard headed about my dislike/aversion to "back fed" transformers and what I perceive as inherent issues with this type of installation.

Now for temporary installations I have no problem but with a permanent installation I would prefer to see 208 delta to 480/277 wye.

1. Possible listing issues.
2. Higher inrush currents.
3. 95% plus of all electricians have problems understanding the proper installation of this configuration and when called to a job upon seeing the results of their multimeter FREAK OUT.
4. 208 delta to 480/277 wye are available.
 
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IMHO if the transformer is actually listed for reverse feed, none of your other points are deal killers, although they are significant design issues that should be considered in the installation. Often these design issues are ignored, it seems that we have a question about reverse fed transformers several times a year, and there are always some core issues that are misunderstood.

-Jon
 
brian john said:
the point is they often ground the XO and have no clue about what to do with grounding or not grounding the 480 delta.
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I think you would be the first to admit that is a shortcoming on the installers part not the transformer itself.

Many electricians can not correctly wire the same transformer in the 'normal' direction either. :roll:
 
Bob: I agree, my main point is if I am selling a transformer to a customer, why not sell the one that can make the installation simple for future maintenance personnel and electricians, allow for a grounded system which typically makes for a safer installation and make a few extra dollars in the mix, as a 208 delta - 480/277 Wye is a bit more expensive. Win win situation.
 
brian john said:
... allow for a grounded system which typically makes for a safer installation ...
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Do you have proof that a grounded -Y system is safer than a corner-grounded delta, or an ungrounded with a monitoring system?

If the electricain doesnot understand the system they are working on, then they do not meet the definition of "qualified" and shouldn't be there.
 
brian john said:
Bob: I agree, my main point is if I am selling....
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Brian IMO most of the time this question of back feeding a transformer comes up when the customer already has one available for use.

I will be surprised if many people actually order 'the wrong' (so to speak) transformer.

But it could happen that I ask the supply house for a certain transformer and the lead time is to long and the supply house offers the choice to use one off the shelf if I simply back feed it.

I don't see the issue, yes it may have high inrush, yes the tap changers will be worthless.....I have only twice needed to use them anyway.



It comes down to money and time as it always does.
 
brian john said:
... no clue about what to do with grounding or not grounding the 480 delta.
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I was trying to joke about that exact issue.

To be clear: I far more agree with Brian John than disagree.

I did not mean to devalue the importance of the 'design issues' of such an installation; ground up or down is not a significant design issue, but proper grounding is a _life safety_ design issue, and can make the difference between a compliant installation and an unsafe installation.

In my opinion, all other things being equal, a wye secondary is the best choice for most installations, even when the neutral is not required, and even for an ungrounded system.

In my opinion, a 'reverse connected' transformer should raise a flag that the entire system should get extra attention to locate installation errors. As you note, these systems are often installed incorrectly.

But a 480 delta to 208/120 wye transformer is much more common than the reverse, and when you are talking the used market this makes a very big difference in availability and price. As iwire notes, these last two factors totally swamp the 'all other things being equal' in many situations.

-Jon
 
Several years ago, I installed a back-fed xfmr for some decorative street lights. This was at one of the local casinos, and like everything else at a casino, they expected a crew to arrive within 5 minutes of the phone call.

These were replacements of existing lights, which were 208V. The feeder was 3PH 3W, about 600' form the source. The new lights were 277 only, no taps. Ordered by the casino. They had a 45KVA 480 delta to 120/208 wye xfmr on hand. The actual load was about 20 KVA.

I fed the X side with 208, didn't connect XO at all. I then installed 3 small single phase xfmrs and connected them zig-zag, to generate a neutral. This synthesized neutral was grounded, so the H side of the 45KVA was not grounded. This setup worked as planned, even with all loads on 2 phases turned off, I still had very close to 277 from any phase to neutral.

A few years later, I got a frantic call (don't you just hate those!!) from the casino, saying most of the lights weren't working. When I arrived, I found the 3 zig-zag xfmrs disconnected, and the lights fed directly from the 480 delta side of the 45KVA xfmr. 2/3's of the ballasts were cooked. Even though I had written on one of the small xfmrs (in plain view, still readable) "zig-zag connection to derive neutral from 480 delta", apparently some 'electrician' thought all those xfmrs were unnecessary.

It seems that no matter how carefully something is designed and installed there's always a know-it-all who'll mess it up.
 
Regarding grounding a 480Y/277 or corner grounding a 480v delta. The hint is the rating of the MCCB. Common breaker ratings are 120/240, 240, 277/480, 480, and 600. It's abased upon the L-G voltage. With the 480Y/277 a 277/480v-rated breaker can be used. With 480v delta a 480 would be used. Of course one could use a breaker with a higher rating. Of course the higher the higher the rating the greater the price.
But more importantly what is better having 277v L-G with a 480Y/277 or 480v L=G with a corner grounded delta?
Also, personally, I don't like to use the term "reverse feed" as transformers don't care on which side they are energized from. The HV windings simply have a greater amount of turns and use smaller conductor than the LV windings. Simply speaking a 1ph 480-120 transformers would have a primary with 4x the turns as the secondary. Apply a voltage on the primary and you will get 1/4 the voltage on the secondary. Likewise when you apply a voltage on the secondary you get 4 time that voltage on the primary. The transformer simply doesn't care on which side it is energized.

The common transformer is would such that the HV windings are on the outside of the coil and that is where that HV taps are located. When you use a common 480v D- 208Y/120 transformer as a step up those taps are on the HV side. The below normal taps are now above normal and the above are below. So (2)2-1/2FCAN end up to be (2)FCBN and if you had (4)FCBN taps they would end up being FCAN.

Another issue is magnetizing current inrush when a transformer is first energized. When used as a step down transformer a transformer may have an inrush of 10x the FLC where it commonly is greater that 10x when used as a step up. This is because the LV winding is next to the core on the inside of the HV winding. With a higher proportion of inrush special caution must be observer when choosing a primary OCPD, that is a breaker with a high enough magnetic trip calibration as to not trip when the transformer is energized.
If I were to engineer a transformer as a step up the HV winding would be on the inside of the coil next to the cre with the LV windings on the outside which results in a lower inrush. And, those taps? I can now lut them where they are supposed to be, on the LV windings where we would expect them to be.

And a word of caution. If a transformer's secondary windings are wound to compensate for a voltage drop often referred to as regulation such a with machine tool of control power transformers, that ratio idea doesn't work.
If you apply 120v to a 480-120v cpt you will very likely get a voltage of much less than 480v on the 480v side and wonder what is going on.
 
The only particular problems I've had regarding wiring a transformer "the wrong direction" is with the inrush. It's really high, for some reason, when wired the other way around. I'd rather use the "right" transformer, but when you are a factory electrician in a plant that rearranges and retools often, you use what you got (within code).
 
Do you have proof that a grounded -Y system is safer than a corner-grounded delta, or an ungrounded with a monitoring system?
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No I don't but the NEC leans in that direction, myself for large distribution 480 VAC systems I'd lean towards an impedance grounded safer, at least in relation to ground faults.



If the electrician does not understand the system they are working on, then they do not meet the definition of "qualified" and shouldn't be there.
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Jim: PLEASE, most electricians have NOT A CLUE. I get calls all the time regarding this issue, how to, why. And I answer their questions, but always add if I were in their shoes I would buy 208 delta-480/277 wye, most contractors do not realize this transformer is available.

I am not taking anything away from electricians, BUT most electricians seldom if ever run into this type of installation and just assume installation is simple, it takes extra consideration IMO that is not given.
 
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And a word of caution. If a transformer's secondary windings are wound to compensate for a voltage drop often referred to as regulation such a with machine tool of control power transformers, that ratio idea doesn't work.
If you apply 120v to a 480-120v cpt you will very likely get a voltage of much less than 480v on the 480v side and wonder what is going on.
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Yeah I learned that one the hard way.
 
templdl said:
And a word of caution. If a transformer's secondary windings are wound to compensate for a voltage drop often referred to as regulation such a with machine tool of control power transformers, that ratio idea doesn't work.
If you apply 120v to a 480-120v cpt you will very likely get a voltage of much less than 480v on the 480v side and wonder what is going on.
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That phenomenon seems to be limited to the small control transformers. The 'panelboard sized' transformers don't seem to so suffer.
 
brian john said:
...PLEASE, most electricians have NOT A CLUE. I get calls all the time regarding this issue, how to, why. And I answer their questions, but always add if I were in their shoes I would buy 208 delta-480/277 wye, most contractors do not realize this transformer is available.

I am not taking anything away from electricians, BUT most electricians seldom if ever run into this type of installation and just assume installation is simple, it takes extra consideration IMO that is not given.
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My point is that most electricians need to remember/acknowledge there are voltage systems used in the US that they may not be familiar with and therefore, without getting additional training, they are not qualified to work on.

But, if an electrician wants to install a valid system (i.e. a reverse connected transformer) then I will provide the correct application information. The fact that a different method is available (i.e. a step-up transformer) may be mentioned but I usually do not try to persuade them either way. And, IMO an ungrounded system should never be installed in a non-industrial environment nor in an industrial facility without a "regular" maintenance electrician.
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From what is on the nameplate does tit state form which side the transformer is to be energized?

It is to my understanding that to ?reverse feed? and ?wrong direction? is as one wants to think of it as. The transformer doesn't care. The transformer is marked with 'H' terminations to HV and 'X' for low. There is no primary and secondary. The nameplate should have the wining configuration and phaser diagram with 3ph transformers. It also shows taps, impedance, weight, etc.

If you had an application and had an opportunity to specify the transformer then you would probably spec a 208 delta-480Y/277 to step up from 208-480v. If you have a 480-208Y/120 lying around or you need one that is readily available then the 480 delta must be carefully considered and to alos remember the inrush issue.
As I stated before be HV windings are on the outside and that is where the taps are.
When you feed the transformer as a step up you are energizing the windings on the inside closest to the core. One would think that the inrush of the HV and LV would be simply proportionate with one another but the LV winding is greater.
When we build a step up transformer the HV windings will be inside closest to the core and the LV windings on the outside, which reduces inrush and will also have the taps where you would expect them to be.
 
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