480v 3 phase to 480/277v 3 phase

[LarryFine]

I would run the 480V heaters off the 480V supply and just size the transformer for the 277V loads.

Agreed.

I imagine one feeder large enough can supply everything using a sub-panel. The transformer would simply be one load in the panel.

 

[winnie]

As others have said, it would really really help if you could post a schematic of the system, since words leave lots of room for misunderstanding.

In post 13 iceworm has a link to a pdf, just a hand sketch of two possible connections for the 277V heaters. Is the top or the bottom what you mean by 'The heaters are hooked up with one line going to power the other line going to a neutral.' I would _hope_ that the press was designed for loading that was as balanced as possible, otherwise it forces you to increase the size of all the supply equipment for no good reason.

In the _ideal_ case you would only have to add a transformer to cover the line-neutral loads, and not the whole thing. Something to keep in mind, however, is that a delta:wye transformer introduces a phase shift from primary to secondary. You need to double check the wiring and control system to make sure that this phase shift doesn't introduce problems.

If the 277V portion of the load is divided into separate sections powered by separate phases (so that your 600A of 277V load shows up as 3 phases of 200A of load each), and the _maximum_ neutral load is 200A, and you can use a delta:wye transformer for this portion of the load, then that load will be a bit too much for a 150kVA transformer, so you would need the next size up of 225kVA.

I believe (but am not certain enough to confirm nor engineer) that you could use a bank of 3 25kVA 240:240V single phase transformers, connected 'zig-zag' to derive a neutral that could handle up to 200A of neutral load. This would have the benefit of not using as much material, and having lower losses during use.

-Jon

 

[iceworm]

The heaters are hooked up with one line going to power the other line going to a neutral. The press is set up for 480v 4 wire my building has 480 3 wire. I dont have the room to change the heaters over to 240, that would requaire double the relays and fuse(no room in the panel boxes for that. I know i can use a transformer to turn the 480 delta to 480/277y i'm just trying to figure out what size of transformer I need. Is it better to power the 480 heaters off the main power and use the transformer to power just the 277v heater? or do I run all the heaters (480 and 277) off the same transformer how do I size for that?

With the information we have, jumper, Larry, and Winnie pretty well have it. 225KVA, 480D to 480Y transformer to feed the 277 loads. The feed from the transformer will be 3ph, 271A, 480V, 4 wire

However, I can't tell what the 3ph delta load is. You said that the 480V heaters would bring the load to 1300A. Is that a "per phase" or adding all three phase currents together.

 

[kwired]

perhaps I am failing to understand a basic tenant of electricity here, but if the heating elements are in a wye configuration, connected to a 480 volt Delta supply, with no neutral connected at the center point, how does one get 277 volts across any one element? As Kwired wrote, from what I am seeing, the best you could do is operate two in series and have 240 volts across two elements as they would be supplied by what amounts to 480 volt single phase. Or am I way off?

Wye connected motors do exactly the same thing, three input, other end of winding is connected in a wye and left floating. As long as you are inputting all three phases each coil/element (with equal impedance) sees 277 volts across it. Open one input leg and now all you are supplying is 480 volts single phase to two coils/elements, at the 180 phase angle (because it now is only single phase) you will drop half the applied across each coil/element (240 volts). If you connected that midpoint to a neutral and only supplied two "phases" you now have the neutral to stabilize that midpoint to 277 each.

I would run the 480V heaters off the 480V supply and just size the transformer for the 277V loads.

Problem that creates is that the derived neutral is an ungrounded conductor and ungrounded conductors require overcurrent protection. You can't ground this neutral point if the supply would already have a grounded conductor, (corner grounded system) that would be big fault current situation.

If your supply is ungrounded you could use such an arrangement and that is basically called a grounding auto transformer.

You could still leave three equal 277 volt loads connected in wye and float their common point and they will operate at 277 volts though. Like I said above that is exactly how wye connected motors that don't have a neutral run to them operate.

 

[iceworm]

A couple of issues with some of proposals: I'm not pointing out that anybody is wrong, just suggesting there is a lot we don't know.

I don't think a z-z derived neutral is a good choice. There is no isolation, it will act as grounded neutral for the entire system. And we don't know what the existing is:

• 480 grounded Y, neutral brought to the first disconnect.
• 480 HRG
• 480 ungrounded D
• Corner grounded D

We have no clue what system faults we could induce. If we are to suggest a z-z derived neutral, it should come with a mandate to review the existing system. With 1300A of spare capacity this is a non-trivial system. Easily could be 2MVA or bigger.

Connecting the 277V heaters without a neutral:
This only works if the 277V heaters are switched in banks of three. Are they? Don't know. (

edit to add) kwired is discussing these.

Using two feeds, one 480/277 grounded Y and one 480V 3W unspecified grounding could easily require internal wiring changes.

SF - Your going to have to get the book/wiring diagrams out.

The worm

 

[kwired]

A couple of issues with some of proposals: I'm not pointing out that anybody is wrong, just suggesting there is a lot we don't know.

I don't think a z-z derived neutral is a good choice. There is no isolation, it will act as grounded neutral for the entire system. And we don't know what the existing is:

• 480 grounded Y, neutral brought to the first disconnect.
• 480 HRG
• 480 ungrounded D
• Corner grounded D

We have no clue what system faults we could induce. If we are to suggest a z-z derived neutral, it should come with a mandate to review the existing system. With 1300A of spare capacity this is a non-trivial system. Easily could be 2MVA or bigger.

Connecting the 277V heaters without a neutral:
This only works if the 277V heaters are switched in banks of three. Are they? Don't know. (

edit to add) kwired is discussing these.

Using two feeds, one 480/277 grounded Y and one 480V 3W unspecified grounding could easily require internal wiring changes.

SF - Your going to have to get the book/wiring diagrams out.

The worm

And some options involve modifying the equipment and not just the supply circuit. Maybe not the end of the world for some but something to take into consideration.

 

[LarryFine]

Problem that creates is that the derived neutral is an ungrounded conductor and ungrounded conductors require overcurrent protection. You can't ground this neutral point if the supply would already have a grounded conductor, (corner grounded system) that would be big fault current situation.

Not true. With no direct connection between primary and secondary, any secondary conductor can be grounded. An auto-transformer, on the other hand, is a different story.

 

[LarryFine]

And we don't know what the existing is:

• 480 grounded Y, neutral brought to the first disconnect.

If this is the case, adding a neutral to the feeder would be the way I'd go. A transformer neutral would require a GEC.

 

[kwired]

Not true. With no direct connection between primary and secondary, any secondary conductor can be grounded. An auto-transformer, on the other hand, is a different story.

I thought we were talking about an autotransformer, with no voltage change other than what is created when deriving a neutral.

 

[JFletcher]

If you supply three identical 277v loads from a 480/277v supply, with the center point of the loads connected to the supply neutral, there would be zero neutral current. Opening the neutral would have no effect, and you would read 0v between the supply neutral and the load center point.

It's actually the same as a perfectly-balanced 120/240v 1ph system. If you have a pair of perfectly balanced 120v loads wired to a 120/240v supply, there is zero neutral current, and you could disconnect the neutral and still measure 0v between the supply neutral and the load center point.

Opening the neutral would make no change in either case. Remember, now, we're talking theoretically, but if the loads are truly equal, the theory works in reality. If the loads are imbalanced, the center-point voltage will vary from zero. Most balanced loads like this are close enough to work in reality.

my initial presumption was that the 277 volt heaters had to run completely independently of one another. If this were the case, and this press only required say one 277 volt heating element, it could not be supplied from a 480-volt Delta power source, correct?

I get what you and Kwired have written about equal loads and motors, however I am specifically referring to one single 277 volt heating element having to be on by itself.

~~~~~~~~~~~~~~~~~~~~

To the original poster: please post a picture of the name plate or at least accurately copy the information from it. the community here will be more than willing to help you however at this point all we have is conjecture about the electrical demands of your installation. that the replies here have indicated your Transformer size needs to be somewhere between 225 KVA and 2 MVA, almost a tenfold difference, is indication to me that the parameters of your initial post are at best unclear. It is also very easy to err in calculating loads for three phase equipment, do not take that as an insult, rather is better for others to check your work, especially when something this size is involved.

 

[kwired]

[/LIST]
If this is the case, adding a neutral to the feeder would be the way I'd go. A transformer neutral would require a GEC.

Which may or may not be simple to install. Building steel nearby - might be easy.

Though the size of transformer likely in question is probably fairly pricey compared to just installing a neutral if not too long or no room in raceway. Under a certain distance probably even less cost to run larger raceway/proper conductors than to purchase the transformer.

 

[jumper]

I am getting tired...

Jumping to conclusions, grasping at straws, running in circles, and chasing my tail is wearing me out.

OPs info is minimal at best.

 

[winnie]

A couple of issues with some of proposals: I'm not pointing out that anybody is wrong, just suggesting there is a lot we don't know.

I don't think a z-z derived neutral is a good choice. There is no isolation, it will act as grounded neutral for the entire system. And we don't know what the existing is:

• 480 grounded Y, neutral brought to the first disconnect.
• 480 HRG
• 480 ungrounded D
• Corner grounded D

We have no clue what system faults we could induce. If we are to suggest a z-z derived neutral, it should come with a mandate to review the existing system. With 1300A of spare capacity this is a non-trivial system. Easily could be 2MVA or bigger.

I absolutely agree that we don't know what is existing, and the OP is going to have to share more information if they want a better answer. Of course they may have a full engineering department just looking for ideas which will all be evaluated correctly...in which case it would be nice if they report back what the final answer is.

iceworm, one question for you: if a zig-zag derived neutral is used to feed this single machine, is it necessary that the neutral be grounded? Could it simply be treated as a 4th 'hot' conductor and brought to the machine? I know that in most situations if a neutral conductor is present it must be grounded, but is this all cases? If the transformers were somehow made 'part of the machine'?

Thanks
Jon

 

[SFMaintenance]

Sorry for all the confusion. I tried to upload pictures of the wiring diagram but for some reason. My phone what upload them. I appreciate all the post even though most of the answers I do not understand. I was give a task that is way over my head. I'm going to go with a 300kva transformer that will change the 480 delta to 480/277y and use it to just run the 277 volt heaters the 480 volt heaters I will power off the main 480 delta supply. The neutral is only used for the 277 volt heaters so I think it will be fine or I hope. I will keep trying to upload pictures of the wiring

 

[kwired]

I absolutely agree that we don't know what is existing, and the OP is going to have to share more information if they want a better answer. Of course they may have a full engineering department just looking for ideas which will all be evaluated correctly...in which case it would be nice if they report back what the final answer is.

iceworm, one question for you: if a zig-zag derived neutral is used to feed this single machine, is it necessary that the neutral be grounded? Could it simply be treated as a 4th 'hot' conductor and brought to the machine? I know that in most situations if a neutral conductor is present it must be grounded, but is this all cases? If the transformers were somehow made 'part of the machine'?

Thanks
Jon

An autotransformer no it would not, and if one of the supply conductors is already grounded, you can't ground another conductor, that would create a fault between the two grounded points. A separately derived system that will be grounded, the neutral is the conductor that must be grounded per NEC.

 

[winnie]

Sorry for all the confusion. I tried to upload pictures of the wiring diagram but for some reason. My phone what upload them. I appreciate all the post even though most of the answers I do not understand. I was give a task that is way over my head. I'm going to go with a 300kva transformer that will change the 480 delta to 480/277y and use it to just run the 277 volt heaters the 480 volt heaters I will power off the main 480 delta supply. The neutral is only used for the 277 volt heaters so I think it will be fine or I hope. I will keep trying to upload pictures of the wiring

If you feel that this task is 'over your head', do you have anyone available who can check your design before money gets invested?

You are talking about a system that could easily use $1000 of electricity in a single day. Easily 10K for the transformer. The press itself? I presume it wasn't free.

-Jon

 

[JFletcher]

Sorry for all the confusion. I tried to upload pictures of the wiring diagram but for some reason. My phone what upload them. I appreciate all the post even though most of the answers I do not understand. I was give a task that is way over my head. I'm going to go with a 300kva transformer that will change the 480 delta to 480/277y and use it to just run the 277 volt heaters the 480 volt heaters I will power off the main 480 delta supply. The neutral is only used for the 277 volt heaters so I think it will be fine or I hope. I will keep trying to upload pictures of the wiring

you will have to check the wiring in the machine to see if this is feasible... You will also want to check to see if your facility's main transformer and switchgear can handle the additional load; 700 amps of 480 volt three phase (to say nothing of the 277 volt heaters) is a very significant load. There is also the not so minor matter of getting rid of all of that heat.

 

[iceworm]

... iceworm, one question for you: if a zig-zag derived neutral is used to feed this single machine, is it necessary that the neutral be grounded? Could it simply be treated as a 4th 'hot' conductor and brought to the machine? ...

Well, there is a code issue, 250.20.B.2. And that would trump any physics issue.

 

[iceworm]

.... I'm going to go with a 300kva transformer that will change the 480 delta to 480/277y and use it to just run the 277 volt heaters the 480 volt heaters I will power off the main 480 delta supply. The neutral is only used for the 277 volt heaters so I think it will be fine or I hope. ...

With the information we have, that sounds like a good choice.

You likely know all of this, so just as a reminder:
(edit to add) Adding to kwired's post, as for the 480/277 transformer grounding/bonding, it will be a separately derived system. See 250.30.A.5, 250.104.D

The transformer needs primary and secondary overcurrent protection.
The secondary conductors need overcurrent protection.
And the equipment may need overcurrent protection.
Careful selection of the overcurrent devices can bundle these. You could end up with an xfm primary CB, and one CB for xfm secondary, secondary conductors, and equipment.

Let us know how it turns out. We will be interested.

the worm

Oh yah - Welcome to being the Engineer of Record. :thumbsup::thumbsup:
yep - two thumbs up

 

[iceworm]

... the 480 volt heaters I will power off the main 480 delta supply.

Following is strictly opinion - based on four words from the OP
And I recognize the OP may be well aware of all of this

Suggest you check you incoming service. Considering your service is big enough to accommodate this additional load:
480 delta corner grounded is uncommon
480 delta ungrounded is more uncommon

More common is 480Y with the neutral brought to the first disconnect - and no further.
And common for me, but perhaps no one else here, is 480Y, High Resistance Grounded
These last two could look like a delta - but they are not.

Knowing the service details may not change the feeder design. But it will certainly help to know the connection you are making won't screw up something else.

(The Law of Unintended Consequences)
yep - valid engineering just as important as the Laws of Thermodynamics, or Ohm's Law)

the worm