3 phase Delta High Leg Single Phase Load

[mivey]

Good point on the multi-pole breakers.

A lot who have posed this question in the past are trying to make use of those unused spaces in a full panel and are looking for the 1p breaker to do it with.

I wonder if there is any advatage to being the only load on the high bus (say a UPS or sensitive equipment or something)?

 

[jumper]

Mivey and Smart$,

Are you saying that it is okay to hook up a 208v l-n load? Say I have a multi- voltage flourescent ballast, can I hook it up if I use a full 240v rated breaker? One pole of a 2 or 3 pole breaker.

 

[mivey]

I'm saying that it will not necessarily "burn stuff up". Too much of this high leg-N unbalanced load might not be good at any 3-phase delta configuration because other windings share the load unevenly.

 

[nakulak]

the attached pic is from the Lineman and Cableman's Handbook 11th edition figure 5.11. It shows a delta 240v 3p coming off the primaries to a 240/120 3p service. note the configuration of the secondary windings.

If the pic is an accurate representation of a utility derived 3p service like this, then could someone please explain to me how the secondary windings are overloaded ?

I understand that the breaker would have to be non-slash rated, but I cannot understand how the use of the 208 N-C phase overloads any winding more than any other winding.

Mr Shumaker, please forgive my unauthorized use of your diagram, if you want it removed please post and I will do so.

 

[mivey]

I have not run the numbers but just think about the pic. The L-N load would "travel" from "c" to "n" by splitting between "c-b" and "c-a", then from "b-n" and "a-n". There are different amounts of load running through different amount of windings.

Simply put, the "b-n" and "a-n" windings would take more "abuse" than the "c-b" and "c-a" windings.

 

[jumper]

I'm saying that it will not necessarily "burn stuff up". Too much of this high leg-N unbalanced load might not be good at any 3-phase delta configuration because other windings share the load unevenly.

Understood. I am talking about 1 20amp circuit in a building with a 800amp 240/120 service. This is just an experiment I want to try.

 

[mivey]

Understood. I am talking about 1 20amp circuit in a building with a 800amp 240/120 service. This is just an experiment I want to try.

How much is the service loaded now?

add: I would not do it if you were close to fully loaded or you are already having unbalance issues.

 

[jumper]

How much is the service loaded now?

add: I would not do it if you were close to fully loaded or you are already having unbalance issues.

I do not have exact numbers, but I know the service is way under loaded compared to what it was designed for. The building was originally a small college dining hall, now it is a Fine Arts building. Most the panels in the building are completely unused now.

 

[Smart $]

...could someone please explain to me how the secondary windings are overloaded ?

...

Because the windings see the load as having a power factor. Refer to the following vector diagram. Two windings see the load as having a power factor of .87, leading on one and lagging on the other. Because of this, all three windings have to carry a current equal to that of the load divided by the √3. In essence, the system sees it as a full-load-amps, 3? load. The diagram below depicts a 208V 20A 1? load. The system sees it as a 20A 240V 3? load, but only 20A@208V is real power. The rest is all reactive power, with the exception of the part that is dissipated as heat, but it all counts towards loading of the system. The green dim's represent the current of real power while the purple dim's represent the current of apparent power.

 

[Smart $]

Mivey and Smart$,

Are you saying that it is okay to hook up a 208v l-n load? Say I have a multi- voltage flourescent ballast, can I hook it up if I use a full 240v rated breaker? One pole of a 2 or 3 pole breaker.

AFAICT, yes... but I'm open to someone presenting an authoritative refutal.

That being said, doesn't one winding a Scott-T transformer connect across the hi-leg to neutral???

 

[ohmhead]

Well looking at the hi leg B phasor at 90 deg away from the 120 deg Phasors A-C how does this effect the correct wattage on a wattmeter .


We are at 208 volts on b-n but 240 volts from a-n c-n would this be incorrect in saying it could effect wattage of kw to kvar meaning not measure correctly the total wattage used in circuit unbalance as a wattmeter uses voltage and current to calculate each phase of a three phase system ?


Just wondering how i can save my energy bill with that scott t connection .

 

[jim dungar]

That being said, doesn't one winding a Scott-T transformer connect across the hi-leg to neutral???

A Scott-T transformer is used to convert between 3-phase and 2-phase systems. I believe you want to discuss a standard T connection.

But are you asking about connecting the primary of a T transformer to a 240/120V high leg system or, about the 240/120V system being created by a T-connected transformer or, something else.?

 

[nakulak]

Ok, so I understand that connecting the 208 will produce more load across the split phase windings. So

1) As long as the loading is somewhat balanced, and an allowance is made for the heavier loading on the center-tapped winding,
2) the correct breaker is used for the 208 loads


why is there any problem using some 208 legs from the service ? (Or alternatively, IS there some other valid reason(s) why 208 should not be used for loads off this service ?)

 

[Smart $]

A Scott-T transformer is used to convert between 3-phase and 2-phase systems. I believe you want to discuss a standard T connection.

But are you asking about connecting the primary of a T transformer to a 240/120V high leg system or, about the 240/120V system being created by a T-connected transformer or, something else.?

Just thinking out loud, so to speak.

Anyway, not the same thing. Refer to upper diagram here.

Similar connection for one winding, but on the primary side, not the secondary.

 

[ohmhead]

Ok, so I understand that connecting the 208 will produce more load across the split phase windings. So

1) As long as the loading is somewhat balanced, and an allowance is made for the heavier loading on the center-tapped winding,
2) the correct breaker is used for the 208 loads


why is there any problem using some 208 legs from the service ? (Or alternatively, IS there some other valid reason(s) why 208 should not be used for loads off this service ?)

Well first again iam a electrician not a expert or engineer just what i have been shown or heard so thats not what you need .

But this is what i understand the connection to the b phase to ground uses part of c winding at any point in the rotation if its a balance transformer your fine .

But if the transformer is loaded with different loads single phase 240 volts between phases and or 120 volts to center tap and these are rotating off in cycles of different times the load resistance on each winding changes this can effect the voltage on that high leg to a degree of higher or lower voltage its not just 208 volts but can exceed that amount up or down .

Thats how i was schooled i maybe wrong ?

 

[jim dungar]

...
and these are rotating off in cycles of different times the load resistance on each winding changes...

I am not sure what you are describing here. I think you are trying to say if the loads on the phases are out of balance.

...this can effect the voltage on that high leg to a degree of higher or lower voltage its not just 208 volts but can exceed that amount up or down .

Yes, the loading (balance and amount) of a transformer can affect the actual voltage at any point in time, however nominal voltage does not change.The voltage of B-N/G will always be a nominal 208V, just like the L-L will always be nominal 240V and the other L-N = 120V.

 

[Smart $]

Well looking at the hi leg B phasor at 90 deg away from the 120 deg Phasors A-C how does this effect the correct wattage on a wattmeter.

We are at 208 volts on b-n but 240 volts from a-n c-n would this be incorrect in saying it could effect wattage of kw to kvar meaning not measure correctly the total wattage used in circuit unbalance as a wattmeter uses voltage and current to calculate each phase of a three phase system ?

It shouldn't affect it at all. You have to remember only 20A 1? flows outside the transformer windings on the conductors. The extra current just circulates in the transformer windings. The meter measures uses the voltage across conductors and the current passing through them to determine power consumed.

But to say so with certainty I cannot, for I do not know enough about the internal workings of the meter.

Just wondering how i can save my energy bill with that scott t connection .

The Scott-T don't work that way, as noted in my reply to Jim.

 

[Smart $]

I am not sure what you are describing here. I think you are trying to say if the loads on the phases are out of balance.

Yes, the loading (balance and amount) of a transformer can affect the actual voltage at any point in time, however nominal voltage does not change.The voltage of B-N/G will always be a nominal 208V, just like the L-L will always be nominal 240V and the other L-N = 120V.

I think ohmhead is thinking of an open delta service, though what you say still holds true for that, too.

 

[ohmhead]

Well iam thinking delta this is just for B phase to common center tap winding on c winding if a motor load single phase is installed on a high leg it is a winding itself meaning its inductive or coil its a motor this is a auto transformer in theory it will backfeed into system and aid voltage to system in polarity and give a increase in voltage to the 1/2 part c and full b coils which at times can effectively add to that voltage of the high leg .

Now in the field we see 190 volts or 208 volts on a high leg its different then calculations in a book depends on your power company or location .

Iam thinking motor generator like backfeeding into c to b but changes in resistance of other loads on other phases of that transformer to me can effectively drop load current and increase voltage of other loads by resistance to me a delta transformer is a bridge its all connected together like a bigg voltage divider ?

Has anyone ever monitored voltage for months on a delta transformer ?

But its just a electrician thinking so please be nice !

 

[jim dungar]

Well iam thinking delta this is just for B phase to common center tap winding on c winding if a motor load single phase is installed on a high leg it is a winding itself meaning its inductive or coil its a motor this is a auto transformer in theory it will backfeed into system and aid voltage to system in polarity and give a increase in voltage to the 1/2 part c and full b coils which at times can effectively add to that voltage of the high leg .

No it is not an auto transformer, there is no addition. There is no backfeed into the system, that does not exist in any other transformer.

Now in the field we see 190 volts or 208 volts on a high leg its different then calculations in a book depends on your power company or location .

This is no different than the voltage difference you see on a 120/240V or a 208Y/120 system. This is why the NEC says to use nominal voltages. the laws of physics dicates that the high leg will be a nominal 208V if the L-L is nominal 240V.

to me a delta transformer is a bridge its all connected together like a bigg voltage divider

It may look like that, but it definitely does not behave like one. Again, it has to do with the laws of physics.

Has anyone ever monitored voltage for months on a delta transformer ?

There is no need to do this for most installations. Delta connections, either closed, open, or high-leg, have been used for more years than have wye connected systems. The phase voltages will typically be affected by the overall loading and balancing.