Transformer Secondary Wiring

[KBElectric]

My situation is this. I have 480Y/277 3-ph primary. I need 240 Delta w/ lighting tap for secondary(3-ph). I have the correct transformer rated at 75 kva. I am using a 200 amp, 3-ph MB panel. I have the 480 on H1,H2,H3 and ground to case. X1,X2,X3 go to the breaker lugs in the panel with X3 being the "wild" leg. My question is, where does X4 go in the panel? This is my largest transformer I've installed. I usually deal with 3kva-6kva. I understand the principles, just not the application of them. Thanks for the help.

 

[GoldDigger]

My situation is this. I have 480Y/277 3-ph primary. I need 240 Delta w/ lighting tap for secondary(3-ph). I have the correct transformer rated at 75 kva. I am using a 200 amp, 3-ph MB panel. I have the 480 on H1,H2,H3 and ground to case. X1,X2,X3 go to the breaker lugs in the panel with X3 being the "wild" leg. My question is, where does X4 go in the panel? This is my largest transformer I've installed. I usually deal with 3kva-6kva. I understand the principles, just not the application of them. Thanks for the help.

The X4 terminal of the secondary is the center tap of the 240V winding that connects between X1 and X2 (given that X3 is the high leg).
So in your 240/120 single phase 3 wire panel, X4 will provide the neutral.
At the transformer or the panel you will also have to bond X4 to the ground/GES/EGC. The details of this in terms of what each bonding jumper is called and where it can be located are all in the code, but the underlying principle is that X4 needs to be grounded as well as run to the neutral bar of the lighting panel.

 

[Strathead]

X4 is your neutral for 120 volt loads. Don't forget to phase the high leg orange.

 

[pjholguin]

That orange(wild leg) conductor will need to be landed on the "B" phase of the breaker.

X4 is your neutral for 120 volt loads. Don't forget to phase the high leg orange.

 

[augie47]

Curious. How much of your load is 120 volt ?

 

[Cow]

Be careful with those 240v high leg transformers, some of them have a reduced neutral and will only take a MAXIMUM of 5% neutral load. We just ran into this issue recently on a 75kva unit. We had to yank a brand new one out, just to replace it with a Square D transformer that had no restrictions on the maximum neutral load.

 

[augie47]

Hummm wonder why I asked that question

 

[GoldDigger]

Hummm wonder why I asked that question

But you just asked about 120V loads and not whether they were balanced.
Only the unbalanced current flows in the neutral, but you may have to assume a worst case of only one line having loads turned on.
Also, not that I would recommend it, any high leg to neutral load will also cause current in the neutral and in both halves of the secondary.

 

[kwired]

Be careful with those 240v high leg transformers, some of them have a reduced neutral and will only take a MAXIMUM of 5% neutral load. We just ran into this issue recently on a 75kva unit. We had to yank a brand new one out, just to replace it with a Square D transformer that had no restrictions on the maximum neutral load.

Not trying to disagree with you, but just exactly what is reduced that can only handle 5%? Other question is 5% of what? 5% of the 75kVA? That would mean (I think) you could have 3750 VA on the neutral.

Now assuming three 25 kVA windings making up a full delta and a 120 volt load across half of the center tapped winding, that half winding should be able to handle 12.5 kVA before it is exceeding it's rating. I would have to assume if anything the difference for this 5% neutral transformer is maybe the size of conductor used to extend the center tap. I just don't see why the winding wouldn't be able to carry its full rating. This 12.5 kVA load is about 16.6% of the 75 kVA of the whole secondary system.

That is for a full delta, if you have open delta - IDK, but I think it would depend if the two sides of the delta are equal capacity to begin with. Is it common to even find open delta in a dry type self contained unit, not that I have seen a lot of delta secondary dry type transformers but all of them I ever recall seeing were full delta.

 

[KBElectric]

Be careful with those 240v high leg transformers, some of them have a reduced neutral and will only take a MAXIMUM of 5% neutral load. We just ran into this issue recently on a 75kva unit. We had to yank a brand new one out, just to replace it with a Square D transformer that had no restrictions on the maximum neutral load.

That is exactly what I need. My current 120v load is <20amps, but would be convenient to have more available. The xfmr they sent does have the 5% lighting tap, which to my calc is roughly 10amps. That's where my problem is. I've talked to eaton directly and they are telling me I can only get one with the tap. I will give Sq D a look. Thanks.

 

[jim dungar]

Not trying to disagree with you, but just exactly what is reduced that can only handle 5%? Other question is 5% of what? 5% of the 75kVA? That would mean (I think) you could have 3750 VA on the neutral.

Now assuming three 25 kVA windings making up a full delta and a 120 volt load across half of the center tapped winding, that half winding should be able to handle 12.5 kVA before it is exceeding it's rating. I would have to assume if anything the difference for this 5% neutral transformer is maybe the size of conductor used to extend the center tap. I just don't see why the winding wouldn't be able to carry its full rating. This 12.5 kVA load is about 16.6% of the 75 kVA of the whole secondary system.

That is for a full delta, if you have open delta - IDK, but I think it would depend if the two sides of the delta are equal capacity to begin with. Is it common to even find open delta in a dry type self contained unit, not that I have seen a lot of delta secondary dry type transformers but all of them I ever recall seeing were full delta.

The loading problem has to do with three sets of windings on a single core, it does not exist with (3) individual transformers.

For simplicity we usually apply the 5% limit on the full load current of the transformer.
For example: 75kVA 3Phase 240V is roughly 180A per leg, 5% of 180A is roughly 9A,
So, we would say, this transformer would be limited to a neutral current of about 9A.

 

[Strathead]

That orange(wild leg) conductor will need to be landed on the "B" phase of the breaker.

What is even more confusing, most utilities require the wild leg to be connected to the C phase on the meter, but the panelboards are listed for the wild leg to be connected to the B phase.

 

[kwired]

The loading problem has to do with three sets of windings on a single core, it does not exist with (3) individual transformers.

For simplicity we usually apply the 5% limit on the full load current of the transformer.
For example: 75kVA 3Phase 240V is roughly 180A per leg, 5% of 180A is roughly 9A,
So, we would say, this transformer would be limited to a neutral current of about 9A.

The common core reason was something I didn't give any thought to. Would that also limit how much 240 volt load could be connected across one phase without some balancing load on the other phases? Seems like it would.

 

[jim dungar]

Would that also limit how much 240 volt load could be connected across one phase without some balancing load on the other phases? Seems like it would.

No.

 

[kwired]

No.

I'm not seeing why the coil should care if it is pulling x amps through half the coil or same x amps across the full coil.

 

[GoldDigger]

I'm not seeing why the coil should care if it is pulling x amps through half the coil or same x amps across the full coil.

The coil windings may not notice the difference (although the heating in the coil will differ by a factor of 2).
But the magnetic field pulling the armature in will certainly be different.

 

[kwired]

The coil windings may not notice the difference (although the heating in the coil will differ by a factor of 2).
But the magnetic field pulling the armature in will certainly be different.

What kind of transformer has an armature?

More heat when the entire coil is loaded - yes, but when only half the coil is loaded - only half the coil is heated, as long as it is only loaded to full rated amps it still is not overloaded, and in fact can probably depend on the other half as somewhat of a heat sink if the other half isn't producing any heat.

 

[Smart $]

What kind of transformer has an armature?

...

I was wondering the same thing when I read his post... hmy:

 

[Smart $]

...

More heat when the entire coil is loaded - yes, but when only half the coil is loaded - only half the coil is heated, as long as it is only loaded to full rated amps it still is not overloaded, and in fact can probably depend on the other half as somewhat of a heat sink if the other half isn't producing any heat.

How the heat is distributed is dependent on all load currents. For example, say the only load you had on the transformer was L1-N. The current actually runs both directions on the secondary windings, one-half winding in one direction, two and a half windings in the other direction, and divided by the impedance of the route (obviously the two and a half winding route has a higher impedance). When loads are added to the other conductors, the current then may be limited to only one direction, but the L1-N component still affects the current on the indirect winding route until you have zero net neutral current.

 

[kwired]

How the heat is distributed is dependent on all load currents. For example, say the only load you had on the transformer was L1-N. The current actually runs both directions on the secondary windings, one-half winding in one direction, two and a half windings in the other direction, and divided by the impedance of the route (obviously the two and a half winding route has a higher impedance). When loads are added to the other conductors, the current then may be limited to only one direction, but the L1-N component still affects the current on the indirect winding route until you have zero net neutral current.

I was asking why the transformer say the winding is rated for 100 amps at full load can not carry 100 amps of L1 to neutral? It is still 100 amps across the same (half) section of winding as it is if you were pulling 100 amps from L1 to L2. All that is different is it is half voltage and half Volt Amps but is half the rating of the full winding and is still full load rating for that half section of winding.

I understand if you throw other loads on the other portions of the system it will change but I'm only asking about a single 120 load on one half of the 120/240 phase (or similar for other voltages).