Boost Transformer Wire Size

[BigBillEQ]

We are reconnecting 3 - boost transformers for a 3 phase machine that was relocated from another building, and had been installed by some other contractor originally. The source voltage is 120/208v 3 phase 4 wire and we are boosting to 230 volt 3 phase 3 wire. The 3 transformers are to be connected in a wye configuration with H4 from each xfmr connected to the source neutral. Previously, the neutrals were connected using a #10 wire size which I believe should be the same size as the 3 phase conductors (1/0). Although the load is 3-phase and there may not be much load on the neutral wires, I can find no exceptions in the code to allow the reduction in wire size. Therefore, I think we should change those # 10's to 1/0.
Any Opinions???

 

[LarryFine]

Mine: only the max imbalance need be supplied by neutral capacity; no neutral load, no neutral needed. In a perfect world, no neutral back to the source would be needed, but if you're matching the transformer lead size, I can't see a problem with the #10. In fact, I'd use a single neutral conductor if the x-formers are remote from the panel.

Anyone else?

 

[ptonsparky]

Would this be better to connect in a delta - delta configuration?

 

[steve66]

IMO, 220.61 allows the neutral wire to be reduced.

I'm not sure how you would calculate the load on the neutral. (like Larry said, in a perfect world the load should be zero on the neutral, but nothing is perfect.)

I would probably use something larger than a #10. I think you still want a short to the neutral to be able to trip the OCP. With 1/0, you probably have 150A or higher OCP, and I'm not sure a very long piece of #10 would trip that.

Steve

 

[BigBillEQ]

Thanks for the input

Thanks for the input

Thanks for the responses! I contacted tech support at Acme transformer and he said the neutral should be the same as the phase conductors. I don't believe there will be any load on that neutral but it would be difficult to caculate the neutral currents, if any.

To play it safe, I'll change them to 1/0.
As for the delta - delta, I need the 3 transformers to meet load capacity and these 3 came with the equipment. An open delta would work if they were bigger units. Acme recommends connecting the neutral on a 3 phase 4 wire system, if you use 3 transformers.

Thanks again. It's nice to have someone to discuss grey areas with!

 

[ptonsparky]

I was thinking closed delta, but am mostly curious, thanks

 

[dsteves]

It sounds like your'e going wye to delta in the boost. On the primary side, even if the neutral current vector sum is zero, each jumper from transformer H4 leg will carry the full phase current on it's H4 leg. The H4 lugs would all have to be connected together with fully-sized conductors to form the "neutral" supernode.

If you're wye-delta, you wouldn't even have to tie that node to your primary side neutral if you didn't want to. If you did, the jumper from the H4 supernode could be pretty small, but from the supernode to the individual windings you've got the same current as the line side of the winding. Either way, you'd better have #1/0 AWG on your H4(A) <-> H4(B) <-> H4(C) jumpers.

The little jumper would want to be a big jumper (full-sized) in wye-wye.

Dan

 

[BigBillEQ]

Thanks, Dan. Sounds like I made the right choice to use the 1/0. I've never heard of the term supernode though. What is the definition?

 

[jtester]

There is a huge difference between a booster transformer installation and a wye-delta or delta delta transformer installation. You can't confuse the two without getting in trouble.

Jim T

 

[dsteves]

There is a huge difference between a booster transformer installation and a wye-delta or delta delta transformer installation. You can't confuse the two without getting in trouble.

Jim T

OK, let's see if we can deal with it. Buck/Boost is typically an autotransformer, right? Autotransformers are never wired delta/wye or wye/delta. They are always delta/delta or wye/wye.

There is no evidence that the OP is wiring buck/boost or autotransformers. If he is, my statement about D/D and Y/Y applies.

Dan

 

[W6SJK]

If he is installing 3 of them for one machine, they are most likey boost autoxfmrs. Otherwise they are step-up transformers.

The 3 neutral leads of the boost xfmrs that are tied together will carry FLA. The single conductor connection from them to the source neutral will carry unbalanced current, if any. Yes?

 

[dsteves]

If he is installing 3 of them for one machine, they are most likey boost autoxfmrs. Otherwise they are step-up transformers.

The 3 neutral leads of the boost xfmrs that are tied together will carry FLA. The single conductor connection from them to the source neutral will carry unbalanced current, if any. Yes?

Hi, sparky.
On point one, I say No, they are not necessarily boost autotransformers, but even if they are, they would be single-winding devices with a voltage ratio equal to the turns tap ratio. Let's assume for the moment that you are right, and that the transformers are autotransformers.

If the bottom of the transformer is H4, as the OP has stated, and the "primary" tap is H2, then H1 would be the high side of the autotransformer, and connected to the load, per phase. In other words, each winding would consist of H4 at the bottom, H2 somewhere close to the top, and H1 slightly above that.

The primary circuit of the transformer, in that case, would be the connection between H2 and H4. Since there is no separate secondary winding, per se, then the virtual secondary of the transformer would be the section of winding between H4 (on the low side) and H1 (on the high side).

Take that connection and multiply the scenario times three for all three phases, and you end up with three single-winding autotransformers. They are connected as follows:

The supply-side connections of cores A, B and C, respectively, will be connected to H2 of each of the three cores. H4 of each core will be connected together to form the common supernode.

The "secondary" of the system will be derived by connecting to the "H1" terminals of each of the windings. If a neutral is needed, it will be made by connecting to the H4 supernode.

On your point two, I say Yes. That is exactly right. If I understand the application, the only current flowing on the primary "neutral" leg will be any imbalance in magnetization current for the three transformer cores, whether they be single or double winding devices.

One of these days I'll figure out how to draw a schematic and post it to this forum. That would make it so much easier to 'splain what I'm trying to say.

Am I making sense, or would you like me to take another run at it?

BigBillEQ, I offer this as an explanation of a supernode. In this instance, the supernode also works as the reference node.

regards,
Dan

 

[BigBillEQ]

Sorry for the confusion - The 3 - transformers are all Acme Buck/Boost Transformers cat# T-1-11686. The connection diagram is Diagram AA and
I have attached a copy (if I did it correctly).

 

[kspifldorf]

Thanks for clearing that up Dan

 

[jtester]

Sorry for the confusion - The 3 - transformers are all Acme Buck/Boost Transformers cat# T-1-11686. The connection diagram is Diagram AA and
I have attached a copy (if I did it correctly).

The connection shown boosts the L-N voltage. The neutral connection wouldn't seem to carry much current normally, but in the case of a L-G fault, it probably would carry nearly line current. I can see why the manufacturer suggested the neutral be the same size as a phase wire.

Jim T

 

[dsteves]

After seeing the diagram, a couple of things are apparent.

1. I was wrong about the single winding with tap statement. This diagram shows 4 windings per core, with two LV and two HV configurations possible. This is a slightly fancier autotransformer than I expected it to be.

2. It looks like the three transformers BigBillEQ references are actually inside a single transformer housing.

That said, the diagram clearly shows the H4 supernode in the middle of the diagram. The vertical conductor in the diagram from the H4 supernode to the "Neutral" bus also appears to be the intended EGC. That's kinda scary.

That clump of H4's is either a factory-assembled single lug, or the end user has to build it. What's it look like, BigBillEQ? Is it a single enclosure factory-built or are you really putting three separate units together?

kspifldorf, I'm not sure what I cleared up yet...

Dan

 

[jtester]

After seeing the diagram, a couple of things are apparent.

1. I was wrong about the single winding with tap statement. This diagram shows 4 windings per core, with two LV and two HV configurations possible. This is a slightly fancier autotransformer than I expected it to be.

2. It looks like the three transformers BigBillEQ references are actually inside a single transformer housing.

That said, the diagram clearly shows the H4 supernode in the middle of the diagram. The vertical conductor in the diagram from the H4 supernode to the "Neutral" bus also appears to be the intended EGC. That's kinda scary.

That clump of H4's is either a factory-assembled single lug, or the end user has to build it. What's it look like, BigBillEQ? Is it a single enclosure factory-built or are you really putting three separate units together?

kspifldorf, I'm not sure what I cleared up yet...

Dan

1. High and low side coils in series is what makes up a basic autotransformer, I don't know what you mean about a fancier one.
2. Nearly every multi phase transformer sold is three transformers in a single housing. Sometimes they are wound on a connected core, but they are truely three separate transformers.
3. I would be more scared if there wasn't a connection from the super note to the neutral-ground bus. The connection is for ground faults.

Jim T

 

[dsteves]

My concern about the diagram is the explicit earth ground symbol connected to the neutral. Personally, I don't think the Code should permit the bonding of the neutral and the frame of the transformer. That's what the EGC is for. I should have expanded on that thought before. Reference 450.10.

The OP indicates three transformers - I'm trying to clear up the semantics here. Is it a single package with a 3 phase autotransformer, or three packages? I know what I expect the answer to be, but I want to hear it from the OP.

If the current is on the neutral and not the EGC, it isn't considered a ground fault, is it?

Dan

 

[LarryFine]

My concern about the diagram is the explicit earth ground symbol connected to the neutral. Personally, I don't think the Code should permit the bonding of the neutral and the frame of the transformer. That's what the EGC is for.

I don't think that diagram intended to show the actual grounding of the neutral to be at the transformer(s), just that it was the grounded conductor, or neutral as we like to call it. Schematics are usually not intended to show physical locations, just electrical pathways and connections.

 

[dsteves]

I don't think that diagram intended to show the actual grounding of the neutral to be at the transformer(s), just that it was the grounded conductor, or neutral as we like to call it. Schematics are usually not intended to show physical locations, just electrical pathways and connections.

I draw schematics as part of a living (actually, only a small part of a living). I'll be danged if I show an earth connection where it doesn't belong, but you're right. My issue here, Larry, is that the schematic implies a connection which should *not* be made, but if you are not certain of the application you could be coerced to do something not quite right.

Dan

PS - Isn't your last name actually Fine?

Quote : "Dr. Howard! Dr. Fine! Dr. Howard!" (from the PA system)
Moe: "I'm Howard!"
Larry: "I'm Fine!"
Curly: "So am I!"