Center tap transformer

hhsting

Senior Member
Please see attached three phase transformer its primary is 480/277V and secondary is 240/120V i.e delta with 120V center tap.
Spec sheet says primary connection Delta and secondary connection is delta w/center tap.

In addition to attachment please see below link that has wiring diagram of transformer.


Questions:

1. Would the primary windings be delta or wye? Would secondary windings be delta or wye?

2. Do I need to ground both primary and secondary or just secondary or just primary per NEC 2024 Section 250.30?

3. How does one ground this transformer if windings are delta?

4. Is transformer separately derived or not?

View attachment 2554956

View attachment 2554957
 
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winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
1. Would the primary windings be delta or wye? Would secondary windings be delta or wye?
The datasheet says this. The primary is wye, the secondary is delta with a center tap.

2. Do I need to ground both primary and secondary or just secondary or just primary per NEC 2024 Section 250.30?
For 99% of NEC applications, you would only ground the secondary. The supply system to this transformer would generally already be grounded (eg from a 480/277V grounded source) or be intentionally ungrounded (where you don't want to add any grounding connections).

The only exception is if this transformer were being used to 'derive a neutral' on an existing ungrounded system for purposes of grounding it. But this is a specialized 'grounding transformer' application, not what you would do to supply 240V loads.

3. How does one ground this transformer if windings are delta?
In this case you would ground the center tap terminal on the secondary.

4. Is transformer separately derived or not?
This transformer is separately derived.

-Jon
 

hhsting

Senior Member
The datasheet says this. The primary is wye, the secondary is delta with a center tap.


For 99% of NEC applications, you would only ground the secondary. The supply system to this transformer would generally already be grounded (eg from a 480/277V grounded source) or be intentionally ungrounded (where you don't want to add any grounding connections).

The only exception is if this transformer were being used to 'derive a neutral' on an existing ungrounded system for purposes of grounding it. But this is a specialized 'grounding transformer' application, not what you would do to supply 240V loads.


In this case you would ground the center tap terminal on the secondary.



This transformer is separately derived.

-Jon
250.20(B)(2)(3) wye, delta needs to be grounded. So primary is wye windings then why would it not be grounded? Also spec sheet say primary connection is delta?
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
250.20(B)(2)(3) wye, delta needs to be grounded. So primary is wye windings then why would it not be grounded? Also spec sheet say primary connection is delta?
The system the primary is connected to is already grounded per 250.20(B)(2).

The spec sheet is incorrect. The description of the transformer and the wiring diagram show a Wye connected primary.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
250.20(B)(2)(3) wye, delta needs to be grounded. So primary is wye windings then why would it not be grounded? Also spec sheet say primary connection is delta?
250.20 is talking about systems as a whole.

The transformer primary is connected to a supply system. So the transformer primary 'gets its grounding' from that supply system (with the exception that I mentioned of 'grounding transformers' specifically used to ground the supply system).

Say your service is a 480/277V wye service. The service will be grounded at the service entrance, and you don't want to 'reground' it at this transformer. Say your service is an intentionally ungrounded 480V delta. You don't want to unintentionally create a ground connection at this transformer.

The transformer is an SDS, so the secondary is the origin of a new system. So the secondary gets grounded per 250.20.

I am not sure about the spec sheet saying that the primary connection is delta. I believe that this means that only H1, H2, and H3 get connected and H0 is supposed to 'float'.

-Jon
 

kwired

Electron manager
Location
NE Nebraska
250.20(B)(2)(3) wye, delta needs to be grounded. So primary is wye windings then why would it not be grounded? Also spec sheet say primary connection is delta?
The secondary is a separately derived system. Nothing on secondary is grounded until you ground it. The secondary won't care which primary conductor or if any are grounded either as it is a stand alone system which only association with the primary is via the magnetic coupling of the core/coils.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
This is what I don’t understand then if you have wye to wye transformer you would ground both primary and secondary correct? So why the exception here?
For NEC applications, you don't ground the primary in a wye:wye transformer.
With a wye:wye transformer, the primary _neutral_ needs to be connected to a feeder neutral, or the secondary line to _neutral_ voltage will not be stable. This is not _grounding_ the primary H0, this is connecting the primary H0 to a neutral.

Of course since the supply neutral is grounded, this has the effect of connecting H0 to ground, _via_ the neutral.

If you were to connect the primary H0 to a GEC then the effect would be grounding the neutral multiple times.

When you have a wye:delta transformer, you must not connect the primary neutral H0 to anything. This is because the delta connection will have circulating currents if the wye voltages are not perfectly balanced.

Note: for non-NEC applications (say power company distribution) the above rules often get turned on their head; that is a whole story that I'd like to learn more about.

-Jon
 

SSDriver

Senior Member
Location
California
Occupation
Electrician
Here is the wiring diagram of the transformer you listed above. Just a note but you will only have 120v from x1 to x4(neutral only to X1 and X2) and X2 to X4. X3 to X4 would be a stinger leg at 139 (138.5) volts. X4 is not a neutral and is only a grounded conductor.

1610554279388.png
 

hhsting

Senior Member
Here is the wiring diagram of the transformer you listed above. Just a note but you will only have 120v from x1 to x4(neutral only to X1 and X2) and X2 to X4. X3 to X4 would be a stinger leg at 139 (138.5) volts. X4 is not a neutral and is only a grounded conductor.

View attachment 2554962
Which would be grounded with grounding electrode conductor to grounding electrodes X2 or something else? If i am using 240/120V single phase
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Here is the wiring diagram of the transformer you listed above. Just a note but you will only have 120v from x1 to x4(neutral only to X1 and X2) and X2 to X4. X3 to X4 would be a stinger leg at 139 (138.5) volts. X4 is not a neutral and is only a grounded conductor.
Small correction:

The X3 to X4 is a 208V stinger leg.

While I absolutely agree that X4 is _not_ neutral to the complete set of X1, X2, X3, since it would supply the grounded conductor and is neutral to X1, X2, it is commonly called the 'neutral'. This is analogous to the grounded leg of a stand alone 120V supply being called 'neutral' even though it really is not a neutral point.

To answer hhsting's question: the GEC would be connected to X4. The reason is that while physics permits any transformer terminal to be grounded, code requires that we select the terminal that gives the lowest voltage of the ungrounded terminals. If you ground X4 than the ungrounded terminals are 120, 120, and 208V to ground. If you ground X2 than the ungrounded terminals are 240, 240 and 120V to ground.

-Jon
 

SSDriver

Senior Member
Location
California
Occupation
Electrician
X4 would be your grounded conductor. Heres a picture, I added the x1, x2, x3, x4 (IN RED). Hopefully this helps.

X1 to X4 - 120v
X2 to X4 - 120v

EDIT: Your transformer is labeled X4 and not X0. X0 is generally used for a true nuetral on a secondary. X4 lets you know that is not a neutral point but a tap in the center of one of the coils.


transformer.jpg
 
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hhsting

Senior Member
Small correction:

The X3 to X4 is a 208V stinger leg.

While I absolutely agree that X4 is _not_ neutral to the complete set of X1, X2, X3, since it would supply the grounded conductor and is neutral to X1, X2, it is commonly called the 'neutral'. This is analogous to the grounded leg of a stand alone 120V supply being called 'neutral' even though it really is not a neutral point.

To answer hhsting's question: the GEC would be connected to X4. The reason is that while physics permits any transformer terminal to be grounded, code requires that we select the terminal that gives the lowest voltage of the ungrounded terminals. If you ground X4 than the ungrounded terminals are 120, 120, and 208V to ground. If you ground X2 than the ungrounded terminals are 240, 240 and 120V to ground.

-Jon
Transformer secondary conductors would feed have secondary single phase panelboard 240/120V that takes 3 wires + EGC.

The wiring diagram shows four conductors x1 to x4. How does that work?

X4 you say have to be grounded thats neutral but then would I get 240/120V single
Phase?
 

SSDriver

Senior Member
Location
California
Occupation
Electrician
Transformer secondary conductors would feed have secondary single phase panelboard 240/120V that takes 3 wires + EGC.

The wiring diagram shows four conductors x1 to x4. How does that work?

X4 you say have to be grounded thats neutral but then would I get 240/120V single
Phase?
If you had to use this transformer(already had it) for that panel you would only run wires from X1(L1), X2(L2), and X4(would go to neutral bar). X3 would be left disconnected. The transformer would be unbalanced. You would be better off buying a Single phase 480 to 240/120 transformer. You can run 480v from a 2 pole breaker from your 3 phase panel to power the single phase transformer.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I dont see it in manufacturer data sheet or code thats the problem.

If the designer is providing it and says if he brings in x1,x2,x4 + EGC to single phase 240/120V panelboard and leaves x3 float would that still be wrong?
We discussed this in a previous thread.

IMHO it is code compliant as long as it is understood that the transformer will be derated from its 3 phase capacity and less efficient than the 'correct' transformer.

-Jon
 

SSDriver

Senior Member
Location
California
Occupation
Electrician
I dont see it in manufacturer data sheet or code thats the problem.

If the designer is providing it and says if he brings in x1,x2,x4 + EGC to single phase 240/120V panelboard and leaves x3 float would that still be wrong?
This transformer is a better design/fit for a service that is using mostly 240v single phase loads with some small 120v loads as the transformer would be unbalanced with a lot of 120v loads. A single phase transformer would be cheaper, smaller and accomplish what you are looking for. You would also have one less wire on the primary.
 

hhsting

Senior Member
This transformer is a better design/fit for a service that is using mostly 240v single phase loads with some small 120v loads as the transformer would be unbalanced with a lot of 120v loads. A single phase transformer would be cheaper, smaller and accomplish what you are looking for. You would also have one less wire on the primary.

Its for cell tower site which mainly feeds 2 poles 240V single phase 2 rectifiers, 240V surge, and 120V single phase 2 receptacles, 2 AC power alarms
 
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