WHERE DOES THE NEC ADDRESS STEP UP TRANSFORMERS

[mta145]

I ran into a job where a 150kw transformer is being used to create 480v from 208v the nameplate on the transformer says PRI: 480v SEC: 208v so obviously they are feeding it backwards. I have read article 450 completely and cannot find anything about whether this is legal or not?? My common sense tells me that the nameplate should reflect what the transformer is rated to do. I would think that it should say PRI: 208v SEC: 480v. Hoping that someone can lead me to where the code addresses this??

Thank you

 

[iwire]

Some, including some inspectors might fail it based on 110.3(B) which basiclly requires we use listed equipment as intended.

However a call to the transformer maker would probaly result in them saying it is OK but not the best option.

If wired correctly (if it was a 480 Delta - 208Y/120 XO must be left unconnected) it is not unsafe so I see little reason to fail it.

 

[jim dungar]

In the recent past (maybe 10 years or so) some manufacturers have changed their nameplates to simply say high voltage and low voltage, they no longer identify the primary and secondary.

 

[tom baker]

Here is what you may run into
480 x 120/240 is delta wye
Reverse it and you get wye delta.
Now on the seconday you will to ground and bond and will end up with a corner grounded delta. Your distribution panels will have to use straight rated breakers, very expensive over slash rated.
And I say its a violation unless the transformer is labled for this use.
Go to Sq D and purchase a step up transformer.

 

[templdl]

Here is what you may run into
480 x 120/240 is delta wye
Reverse it and you get wye delta.
Now on the seconday you will to ground and bond and will end up with a corner grounded delta. Your distribution panels will have to use straight rated breakers, very expensive over slash rated.
And I say its a violation unless the transformer is labled for this use.
Go to Sq D and purchase a step up transformer.

This is the way that I look at it:
I believe that should the transformer be labeled as primary and secondary with the primary being the HV side then I would think it would be difficult to explain that the secondary is actually the input side. I would conclude that it would be similar to "reverse feeding" a breaker that is labeled with line and load. You could do it, the breaker doesn?t care, but it would be misleading to those who may work on the breaker assuming that the end marked line is the end that is being feed. If the breaker is not marker with line and load I would like to think that a qualified person who if knowledgeable in safe electrical practices and procedure would never is assume that the "on" end of the breaker is the end that is being feed.

Should the transformer be labeled simply as HV and LV which way you feed the transformer should not make a difference should one pay attention to how a 'Y' is to be feed correctly. As such a qualified person should never assume the HV side is the input side but should understand that the transformer doesn't care which way that it is used unless the NP specifically states other such as step up use or pri and sec. I believe that 'primary' is commonly understood to be the HV side, the side that is feed if I'm not mistaken and the secondary, the LV load side.

The one thing that using a common D-Y transformer as a step up transformer caused is inrush currents that a proportionately higher on the LV side. This is because the LV windings are wound in the inside of the HV windings and, as such, are closer to the core. If one was to expect a 12x the FLC for the HV windings when used s a step down then anticipated that inrush for the LV windings will be greater than 12x the LV FLA.

 

[iwire]

And I say its a violation unless the transformer is labled for this use.
Go to Sq D and purchase a step up transformer.

Square Ds own web site FAQs says it is OK to use their transformers in either direction.

If you are suggesting a safe installation should fail strictly because of how the transformer is labeled I find that somewhat troubling. In my opinion that is putting the cart before the horse.

It would also mean that all my past buck boost installations should fail as the transformers are labeled differently then they are being used for.

 

[hardworkingstiff]

This is because the LV windings are wound in the inside of the HV windings and, as such, are closer to the core. If one was to expect a 12x the FLC for the HV windings when used s a step down then anticipated that inrush for the LV windings will be greater than 12x the LV FLA.

How do they construct a step up transformer to avoid this issue?

 

[ceb58]

The last one I did was a 480 to 208 but the mfgs. tag plainly stated "suitable for reverse connection" which is what I needed to do. Made it a corner grounded delta and was good to go.

 

[templdl]

How do they construct a step up transformer to avoid this issue?

Wind the HV windings first and the LV over them which is how the manufacturer that I represented built them.
This may make many look stupid as I had thought myself as but on a common step-up transformer where are the HV taps? Obviously they are on the outside HV winding where they are accessible. And, there are commonly (2) 2-1/2% FCAN taps and with (2) or (4)2-1/1/2 FCBN taps. When uses as a step up transformer those taps have to be looks at in the opposite direction, + (4) and - (2). And is the supply voltage normally higher than the nominal voltage of lower? Most commonly lower and you only have (2) BN taps that you can use. The (4) AN are most likely going to be useless as it?s not often the AN taps are used. Trying to make sense of using the taps plays with one?s mind as it?s somewhat confusing.
So, when building a step-up transformer with the LV windings on the outside the taps are actually in the conventional location on the input side of the transformer, +(2), and -(4) and you have access to (4) BN taps as well as the added benefit of having a reduced inrush on this design.

 

[iwire]

I would conclude that it would be similar to "reverse feeding" a breaker that is labeled with line and load. You could do it, the breaker doesn’t care,

It was my understanding that some breakers do 'care' which direction they are supplied and that reverse feeding them would actually be a dangerous practice. If that is correct I see a big difference between the breaker and a transformer.

On the other hand I could be misinformed.

 

[hardworkingstiff]

It was my understanding that some breakers do 'care' which direction they are supplied and that reverse feeding them would actually be a dangerous practice. If that is correct I see a big difference between the breaker and a transformer.

On the other hand I could be misinformed.

I assume you are talking about large (as in ampacity) breakers and not the QO or I-Line (yea, I like SQ D) type breakers.

 

[templdl]

It was my understanding that some breakers do 'care' which direction they are supplied and that reverse feeding them would actually be a dangerous practice. If that is correct I see a big difference between the breaker and a transformer.

On the other hand I could be misinformed.

That could be an old wive's tail. Please provide an example of this as you have alluded to the fact that there are breakers that are sensitive to being reverse feed. There isn't a big difference between a breaker and a transformer when you have to go by the way they are marked and which end they are feed from. How is the breaker marker and how is the transformer marked. If the transformer is marker with the HV and LV unless stated specificaly that the transfomer can not be energized fron the LV end there is not reason which prevents you from doing so.
I personally was a breaker application engineer for one of the largest breaker manufactures and feeding a breaker from the load end was never an issue when breakers had their own protection that was integral to that breaker. The only exception that there may be today is if there was a feature available that would respond to reverse power flow. Yes, some LV and HV (especially HV breakers) have external relays that with a reverse power feature which has the capability of controling the breaker.
What is dangerous about feeding a breaker from the "load end" is when the breaker is MARKED with load end. This you never do. Otherwise breakers that are not marker can be feed from either end. As an example these breakers are those that are sealed with no non interchangeable or electronic trip units. The only danger there is when you have an incompetent person working on the breaker who is no familiar with safe electrical practices and procedures.
Even when a person is qualified it is a dangerous thing when you turn your brain off putting into the automatic mode not being aware of things other than the status quo.

 

[hurk27]

That could be an old wive's tail. Please provide an example of this.

Thats easy

GFCI
AFCI:happyyes:


Kidden
Most breakers I have seen that don't have some form of electronic trip can be fed in reverse as far as I know?

 

[templdl]

Thats easy

GFCI
AFCI:happyyes:


Kidden
Most breakers I have seen that don't have some form of electronic trip can be fed in reverse as far as I know?

Never crossed my mind about AFCI and GFCIs but I must admit one would have to be pretty creative to make that happen. But I'm sure there are those that would try to find a reason to do so.
When you say "most breakers" that don't have some sort of electronic trip, if your area of expertise/specialty is residential breakers I can see your point with electronic trips being GFCI and AFCIs you are correct.
But in the industrial/commercial breakers there are more options. Those breakers are commonly available with frames from 100-2500a. Frames 225-250a, 400a, 600a, 800a, 1200, and 2500a most are available with 3 basic trip unit option.
1) The interchangeable (IT) old school thermal magnetic (TM) trip where you could buy a 600a frame and install a 400AT in it in the field and then change it to a 500at later with the options of adding aux switches, shut trips, etc in the field marked with line and load ends.
2) Non interchangeable trip unit breaker (NIT) where you musto order the breaker complete with TM and options which would be sealed and have no line/load markings.
3) The electronic trip unit breaker which most if not all that I'm aware are sealed with no line/load markings where accessories can be added in the field and trip unit ratings can be changes by exchanging a simple rating plug.

 

[iceworm]

It was my understanding that some breakers do 'care' which direction they are supplied and that reverse feeding them would actually be a dangerous practice. ...

That could be an old wive's tail. Please provide an example of this ....

Disclaimer: The inner guts of CBs are out of my area of expertise. CBs are "black box" to me. They are a part with specs and instructions.

How about a CB fitted with a UV trip. All but one I have seen take the UV input from the line side. They would not be able to be closed if the line side were not powered. Not dangerous - just wouldn't work.

ice

 

[iwire]

It was my understanding that some breakers do 'care' which direction they are supplied and that reverse feeding them would actually be a dangerous practice. If that is correct I see a big difference between the breaker and a transformer.

On the other hand I could be misinformed.

That could be an old wive's tail. Please provide an example of this as you have alluded to the fact that there are breakers that are sensitive to being reverse feed. There isn't a big difference between a breaker and a transformer when you have to go by the way they are marked and which end they are feed from. How is the breaker marker and how is the transformer marked. If the transformer is marker with the HV and LV unless stated specificaly that the transfomer can not be energized fron the LV end there is not reason which prevents you from doing so.
I personally was a breaker application engineer for one of the largest breaker manufactures and feeding a breaker from the load end was never an issue when breakers had their own protection that was integral to that breaker. The only exception that there may be today is if there was a feature available that would respond to reverse power flow. Yes, some LV and HV (especially HV breakers) have external relays that with a reverse power feature which has the capability of controling the breaker.
What is dangerous about feeding a breaker from the "load end" is when the breaker is MARKED with load end. This you never do. Otherwise breakers that are not marker can be feed from either end. As an example these breakers are those that are sealed with no non interchangeable or electronic trip units. The only danger there is when you have an incompetent person working on the breaker who is no familiar with safe electrical practices and procedures.
Even when a person is qualified it is a dangerous thing when you turn your brain off putting into the automatic mode not being aware of things other than the status quo.

Can you tell me why the maker would chose to mark a breaker line and load if it did not matter?


My feeling is that a modern breaker with electronic controls must get its operating power from the supply.

If you reverse feed it would the controls still be supplied with control power with the breaker open?

For me at an installer I do not turn my brain off, I know that the transformer manufactures have been stating that either way is fine, they have also switched from labeling them primary / secondary to High / Low.

I have never seen any statement from anyone (other than yourself above) that a breaker marked line and load can be used in the opposite fashion. As a matter of fact UL white book specifically tells us we can't do that with breakers but has no such restriction on transformers.

 

[jim dungar]

How about a CB fitted with a UV trip. All but one I have seen take the UV input from the line side. They would not be able to be closed if the line side were not powered. Not dangerous - just wouldn't work.

This is a design issue. The vast majority of UV trip breakers I have specified involved 'separate' external wiring of the UV trip. The trip could just as easily be wired to the bottom of the breaker as to the top.

UL and the NEC are very clear about it, all breakers can be fed from either top or bottom, unless they have specific line and load markings. Probably more than 98% of breakers have no restrictions.

 

[templdl]

"Can you tell me why the maker would chose to mark a breaker line and load if it did not matter?"
*Did I imply that they did?

"My feeling is that a modern breaker with electronic controls must get its operating power from the supply.
If you reverse feed it would the controls still be supplied with control power with the breaker open?"
*Electronic breakers that I have worked with do not use the supply and control power to operate. It is strictly by Cts unless technology has changed as of late. Have you ever taken any apart? Maybe ones that must have a voltage reference that measure/monitor power would. But, think about it, trip units are on the lower end of the breaker below the toggle. With a breaker feed from the opposite end turning off the breaker and no power to the trip unit. When feed from the load end power will remain present. So, does it really matter which end a breaker is fed? (This could end up to be a good philosophical discussion.)
Anytime that you have the chance to get your hands of a breaker that is going to be scrapped tear the thing apart and disect it.

"For me at an installer I do not turn my brain off, I know that the transformer manufactures have been stating that either way is fine, they have also switched from labeling them primary / secondary to High / Low."
*I am pleased that you don't turn your brain off but as an application engineer I have learned not to assume that those who I converse with haven't done as much. I only wish that all others were like you but I have learned not to take anything for granted as safety is my priority. I have to make sure that those that I talk to have knowledge of safe electrical proactices and procedures and are qualified. All to often people tell me just enough to get the answers that they want to hear and not enough to get the correct answer. If there were more technicions like you it would have made my job much easier to say the least.

"I have never seen any statement from anyone (other than yourself above) that a breaker marked line and load can be used in the opposite fashion. As a matter of fact UL white book specifically tells us we can't do that with breakers but has no such restriction on transformers."
*Did I say that you could feed a breaker from the end marked as "load " and that they could be used in th opposite fashion? Unless I missed something I don't believe that I did. I appologize if what I said was misleading.
And, yes, there are no transformer manufacturers that mark their transformers as primary and secondary that I am aware of anymore. As such it allows the transforemr to be installed as a step up or down provided that the transformer is not marked to the contrary and that the connections are made correctly such as transformer that is supplied from the wye side and the installer knows the implications and disavantages of suppying a load with a delta.