Center Tapped Delta Transformer 5% Limitation

[mull982]

Can someone please the 5% limitation on the secondary of a 480-240/120V delta center-tapped transformer?

Is this 5% of rated kVA limitation in reference to the neural current on the center tap from unbalanced 120V L-N loads or is this in reference to the total connected kVA of 120V L-N loads connected between adjacent phases and centerpoint?

Transformer manufacturers seem to reference this in relation to total connected load but I’m wondering if this is a conservative rating that accounts for maximum unbalance current flowing on neutral?

 

[electrofelon]

Can someone please the 5% limitation on the secondary of a 480-240/120V delta center-tapped transformer?

Is this 5% of rated kVA limitation in reference to the neural current on the center tap from unbalanced 120V L-N loads or is this in reference to the total connected kVA of 120V L-N loads connected between adjacent phases and centerpoint?

Transformer manufacturers seem to reference this in relation to total connected load but I’m wondering if this is a conservative rating that accounts for maximum unbalance current flowing on neutral?

The former. The transformer cannot differentiate a combination of L-N loads that are balanced from a L-L load of the same value.


There was a recent thread that touched on this:

delta high leg and receptacles

Good afternoon, If you add a Delta Hight leg transformer and a 120/240 delta panel to provide power for some specific Dehumidifiers in a building. Can somebody help me understand, why I'm being told not to add 120v circuits, to this panel? thank you!

forums.mikeholt.com

 

[don_resqcapt19]

If you need a high leg system and you need to supply a lot of line to neutral loads, you can build a transformer bank using two or three single phase transformers. I have never actually seen a 3 phase high leg transformer.

The high leg services around here are typically two transformer open delta systems, with the transformer having the center tap to supply the line to neutral loads having a larger kVA rating than the transformer that supplies the 3rd phase.

 

[kwired]

If you need a high leg system and you need to supply a lot of line to neutral loads, you can build a transformer bank using two or three single phase transformers. I have never actually seen a 3 phase high leg transformer.

The high leg services around here are typically two transformer open delta systems, with the transformer having the center tap to supply the line to neutral loads having a larger kVA rating than the transformer that supplies the 3rd phase.

Where you are more likely to find them is where there was a high leg system to begin with, then came the need for more VA capacity but wanted to stick with the high leg system for various possible reasons.

Had grain storage facilities that were 240 delta high leg, adding enough load POCO needed more transformer plus wanted to get rid of overhead primary that was nearby a facility that often needs to use cranes - somewhat forced to use a padmount transformer. Did not want to rework all the existing equipment to work on 208 volts so POCO gets padmount with 240 high leg delta secondary. Is usually longer lead time from my understanding for them to get those and they typically do cost more from my understanding then a same KVA 208/120 secondary.

 

[Hv&Lv]

I have never actually seen a 3 phase high leg transformer.

Here...
We have several hanging. This one happened to be on the yard.

I don’t know why it’s sideways. It’s correct on my phone and changing it on the phone doesn’t fix orientation in the post.

 

[jim dungar]

If you need a high leg system and you need to supply a lot of line to neutral loads, you can build a transformer bank using two or three single phase transformers. I have never actually seen a 3 phase high leg transformer.

Pole top style transformer banks are typically 2 single phase units in open delta and only rarely 3 units in a closed delta. Dry type and padmount styles are usually closed deltas. Of course there are exceptions.

 

[kwired]

Here...
We have several hanging. This one happened to be on the yard.

You forgot to link or insert image, I think.

Pole top around here is almost always multiple single phase units connected together to make a three phase bank though.

 

[Hv&Lv]

You forgot to link or insert image, I think.

Pole top around here is almost always multiple single phase units connected together to make a three phase bank though.

I was still editing

 

[mull982]

Where you are more likely to find them is where there was a high leg system to begin with, then came the need for more VA capacity but wanted to stick with the high leg system for various possible reasons.

This is exactly the situation we have run into. A customer has a facility which was previously fed by a 500kVA 240/120V center tapped delta transformer. The facility had 120/240V loads which were served directly from transformer as well as 480/277V loads which were fed via step-up transformers.

The customer wanted to increase service to 750kVA for increased capacity but utility would only provide new 480/277V service. The existing 480/277V loads can now be fed directly from service however all the existing 240V 3-Ph/1-Ph loads and 120V 1-Ph loads must now be fed from new 300kVA and 500kVA Center Tapped Delta secondary's (480V primary) connected to existing distribution panels.

The contractor was going to connect these new transformers in the field and noticed the 5% limitation (and full size neutral conductors way to big for neutral lug) and questioned if there would be issues or not. The challenge now is to try to quantify all of the existing 120V L-N loads that will be fed from these transformers and evaluate weather or not the neutral current will exceed the 5% limitation on these. As you can imagine there is no easy way to evaluate these loads so we are trying to put together as best of an analysis as we can based on panel schedules and perhaps even measuring the existing neutral currents on the (2) panels that will be connected on secondary of these new transformers (panels still in service) to get representation of unbalanced current.

Once we arrive at some understanding of these L-N load profile that then leaves us with evaluating if we consider what we think is the "connected kVA" of the 120V L-N loads or if we can prove that there will always be some degree of balancing/canceling that would allow adequate loading on the transformer.

Converting system to 120/208V was considered but would require replacing several 240V loads that are depended on 3-Ph 240V and thus no longer any option.

Open to any ideas/suggestions anyone has on trying to evaluate this situation.

 

[synchro]

Have you considered adding a 480V to 240/120V single phase transformer to handle some or all of the 120V loads? Typically it would support 50% of the kVA rating for 120V loads that are completely unbalanced to one side, because in this case only one of the X1-X2 or X3-X4 secondary windings is being utilized.
You would only need a 50kVA single phase 480 to 240/120V transformer to provide the same neutral current as a 500kVA 3-phase high leg delta unit can support (because 50% of 50kVA = 5% of 500kVA).

 

[mull982]

E

Have you considered adding a 480V to 240/120V single phase transformer to handle some or all of the 120V loads? Typically it would support 50% of the kVA rating for 120V loads that are completely unbalanced to one side, because in this case only one of the X1-X2 or X3-X4 secondary windings is being utilized.
You would only need a 50kVA single phase 480 to 240/120V transformer to provide the same neutral current as a 500kVA 3-phase high leg delta unit can support (because 50% of 50kVA = 5% of 500kVA).

Unfortunately the 120V loads are not centralized but rather are dispersed in the form of branch circuits from multiple 3-Ph distribution panels. There are multiple feeders to branch circuit panels so it would be difficult if not impossible to isolate these and feed them with single Phase transformer

 

[don_resqcapt19]

The two utilities local to me use multiple single phase pad mounts for a high leg service, just like they do for the pole mounted transformers.

 

[electrofelon]

This is exactly the situation we have run into. A customer has a facility which was previously fed by a 500kVA 240/120V center tapped delta transformer. The facility had 120/240V loads which were served directly from transformer as well as 480/277V loads which were fed via step-up transformers.

The customer wanted to increase service to 750kVA for increased capacity but utility would only provide new 480/277V service. The existing 480/277V loads can now be fed directly from service however all the existing 240V 3-Ph/1-Ph loads and 120V 1-Ph loads must now be fed from new 300kVA and 500kVA Center Tapped Delta secondary's (480V primary) connected to existing distribution panels.

The contractor was going to connect these new transformers in the field and noticed the 5% limitation (and full size neutral conductors way to big for neutral lug) and questioned if there would be issues or not. The challenge now is to try to quantify all of the existing 120V L-N loads that will be fed from these transformers and evaluate weather or not the neutral current will exceed the 5% limitation on these. As you can imagine there is no easy way to evaluate these loads so we are trying to put together as best of an analysis as we can based on panel schedules and perhaps even measuring the existing neutral currents on the (2) panels that will be connected on secondary of these new transformers (panels still in service) to get representation of unbalanced current.

Once we arrive at some understanding of these L-N load profile that then leaves us with evaluating if we consider what we think is the "connected kVA" of the 120V L-N loads or if we can prove that there will always be some degree of balancing/canceling that would allow adequate loading on the transformer.

Converting system to 120/208V was considered but would require replacing several 240V loads that are depended on 3-Ph 240V and thus no longer any option.

Open to any ideas/suggestions anyone has on trying to evaluate this situation.

There is the possibility that there wont be an issue. Think of it this way: Without the 5% limitation, you would still be limited to 1/6th of KVA capacity for a delta with a center tap. That is 16.6% which granted is three times higher, but still not "a ton". You are talking a 300KVA and 5% of that is 125 amps of imbalance allowed. That is quite a lot of 120V stuff, although I have no idea what your facility is like. I have a client with a 1000 amp 120/208 service, mixed lab and offices with mostly L-N loads (other than HVAC), peak demand of about 450 amps. I have a lot of time at this site and have taken extensive measurements over the years, and I dont recall ever seen more than 50 amps on the neuter.

If that didnt allay your fears, What about making a three phase bank out of three single phase units? (did anyone mention that yet?)

 

[kwired]

I was still editing

I see it now.

Though not surprised such a thing is available, never seen one like that before. Such a source around here is always built with two single phase units. Even with padmount units. Though a full delta secondary for padmount would typically be a single core type unit, especially if over 112.5 kVA. Most those would be limited to upgrade of a facility already using a high leg 240 delta, new services where all three primary phases are available they would push for using 208/120 or 480/277 on new facility installations over 112.5 kVA.

 

[electrofelon]

I see it now.

Though not surprised such a thing is available, never seen one like that before. Such a source around here is always built with two single phase units. Even with padmount units. Though a full delta secondary for padmount would typically be a single core type unit, especially if over 112.5 kVA. Most those would be limited to upgrade of a facility already using a high leg 240 delta, new services where all three primary phases are available they would push for using 208/120 or 480/277 on new facility installations over 112.5 kVA.

One job I had we were considering going new high leg delta service. POCO would do it if overhead, but would only do 120y/208 for a padmount.

 

[kwired]

One job I had we were considering going new high leg delta service. POCO would do it if overhead, but would only do 120y/208 for a padmount.

Thing here is that there are remote areas that don't have all three primary phases, but as long as load isn't too high they can bring two phases plus neutral and build an open delta bank. But they are doing less of that than they used to, pretty much has to be pretty limited load anymore. But there are still some open delta banks out there with higher loads than they would build new supply for today. One I am pretty familiar with has 100 HP irrgation well on an open delta bank. funny thing is the substation is on opposite side of that section but on this side of the section is a single phase line coming from north and single phase line (a different phase) coming from south to allow for this open delta to exist. If that same location were to be requesting new service for 100 HP motor today they would have built full three phase primary line from whichever direction was most economical and gone with 480/277 wye secondary.

Same POCO and only about a mile or so from where I live has a single phase only supply to a 50 HP irrigation well. That one the POCO has a static phase converter installed and the POCO maintains it. They want to get rid of it, but still waiting for other nearby customers to subscribe to three phase service before they will do anything. They only continue to maintain this phase converter because of original agreements when it was installed, at least 35-40 years ago. There is three phase lines by my place, added about ten years ago, but it ends at an adjacent neighbor's service for an irrigation well. Existing single phase at that other location comes from a different direction so not even partial components present between the two points. They won't build the mile and a half needed on their own dime to do this and will continue to maintain the phase converter when needed.

 

[mull982]

In trying to quantify the existing L-N load profile I've been able to get ahold of the various panel schedules for each of the branch circuit panels (Just load assignments not actual load or breaker rating). With this I've at least been able to segregate all the respective L-N loads to both their respective A and C Phases (B is wild-leg). With this I'm not trying to determine a way to perhaps estimate loads for each of these circuits to determine the maximum kVA on one of these phases to use as a starting point. These loads consist of mostly lighting and receptacle circuits as well as some other auxiliary circuits so hard to estimate what load may be associated with each of these. Any ideas?

I've also gotten ahold of some metering data for each of the (2) main panels that will be re-fed from these transformers. This metering data gives min/max/avg voltage and current readings with individual L1, L2, L3 currents. I'm trying to determine how I may be able to use this data to somehow back calculate the existing unbalance on neutral but cant seem to see a way to do it unless I'm missing something?

My initial thought was to take the difference on the L1 & L3 line currents (A & C Phases) but that would assume balanced 3-Phase loading and that all other 240V L-L loads were balanced so that difference was resulting from L-N loads fed from these phases.

 

[hillbilly1]

Can the poco leave the existing service, and add a seperate bank of transformers for the new 277/480 service? Code does allow (or used to) a second service for different voltages, upon approval by the AHJ. Its been a long time since I had to do that, but may be practical in this instance.

 

[electrofelon]

Can't you just put a logger on the neutral?

In trying to quantify the existing L-N load profile I've been able to get ahold of the various panel schedules for each of the branch circuit panels (Just load assignments not actual load or breaker rating). With this I've at least been able to segregate all the respective L-N loads to both their respective A and C Phases (B is wild-leg). With this I'm not trying to determine a way to perhaps estimate loads for each of these circuits to determine the maximum kVA on one of these phases to use as a starting point. These loads consist of mostly lighting and receptacle circuits as well as some other auxiliary circuits so hard to estimate what load may be associated with each of these. Any ideas?

I've also gotten ahold of some metering data for each of the (2) main panels that will be re-fed from these transformers. This metering data gives min/max/avg voltage and current readings with individual L1, L2, L3 currents. I'm trying to determine how I may be able to use this data to somehow back calculate the existing unbalance on neutral but cant seem to see a way to do it unless I'm missing something?

My initial thought was to take the difference on the L1 & L3 line currents (A & C Phases) but that would assume balanced 3-Phase loading and that all other 240V L-L loads were balanced so that difference was resulting from L-N loads fed from these phases.

 

[kwired]

Can the poco leave the existing service, and add a seperate bank of transformers for the new 277/480 service? Code does allow (or used to) a second service for different voltages, upon approval by the AHJ. Its been a long time since I had to do that, but may be practical in this instance.

Nothing in NEC prohibits that. Reading some member's posts on this site, appears that some POCO's will refuse to supply customers with multiple sources though.

Here there are many places that have multiple sources, and not always the same voltage or number of phases either. Not quite so often is it two multiple supplies to one building, but on farms or larger commercial or industrial or even a campus type application you might see multiple services on the property.