Options for 480v transformer to supply 240v and 120v

[bryanbdp]

Hi. The new location for our Makerspace has 480v 3ph power available. I need to derive 240v single and 3phase power from it, as well as 120v power.
Before taking any diversity into account, we need 280 kVA of 240 3ph (includes single phase loads), 311 kVA of 120v, and 100 kVA of 480v.

A great deal of the 120v calculation is for non-continuous 120v convenience outlets.

My real question is what my transformer options are there to provide the 240 and 120v power.

I could use one transformer for 240v 1 and 3ph loads, and another for the 120v loads. A 208/120 transformer isn't much use, I don't need 208v at all. I think I could use a 240/120 split phase output on one or more of the 3 480v phases.
Is it possible to use a 3ph 480v input to create 3 240/120 volt split phase outputs?
And for that matter, can I just use one big 480 to 240/120 volt transformer with center taps on each coil? Which I believe would yield either 3ph 240v delta or 3ph 240/120v split phase power.

What is the best option in practice? Did I make any mistaken assumptions above, or miss any relevant ones?

Thank you.
Bryan

 

[infinity]

I didn't check your numbers but it seems like a 3 phase, 4 wire Delta secondary would work. That would yield 3 phase 240 volts and single phase 120 volts.

 

[bryanbdp]

This is the diagram for what you were talking about, correct?
Is it possible to tap all three coils, and tie the 3 neutrals together? That would make balancing the 120v on all three phases possible.
Thanks for the reply,
Bryan

 

[bryanbdp]

Or would a current flow on the three neutrals, since they're different phases?
But maybe it would work if you kept the 3 split phase 240/120 separated?

 

[ActionDave]

Is it possible to tap all three coils, and tie the 3 neutrals together? That would make balancing the 120v on all three phases possible.

I'm not smart enough to know why you don't want to do that but the fact that nobody does it is proof enough it's a bad idea. You'd have three high legs to deal with instead of one. That would be a nightmare right there.

It's not a big deal if the phases aren't balanced as long as they're not way out of whack. It's common to have a separate single phase panel for 120V loads so you don't have to worry about the high leg.

 

[infinity]

This is the diagram for what you were talking about, correct?
Is it possible to tap all three coils, and tie the 3 neutrals together? That would make balancing the 120v on all three phases possible.
Thanks for the reply,
Bryan View attachment 19708

I'm no transformer guru but I would say no, just size the 3Ø, 4W Delta large enough for the 120 volt loads and install only one transformer.

 

[bryanbdp]

OK, maybe by carefully balancing the loads I could make it work well.
I planned to separate the three phase and single/split phase into separate panels anyways.
If I decide to provide 240/120 with a separate transformer, what would the best option be? I don't really need the 2nd set of 240v power, so I guess maybe a 208Y120 would work, and they're common. I could either disregard the 208 or use it for lighting, I guess...
Lighting loads aren't all that great with LED lights!

 

[GoldDigger]

Or would a current flow on the three neutrals, since they're different phases?
But maybe it would work if you kept the 3 split phase 240/120 separated?

Tying the neutrals together would be a dead short circuit on all three windings. Not at all a good idea.
And keeping them separate would give you circuits with a neutral which is not at ground potential, in violation of NEC rules.

Sent from my XT1585 using Tapatalk

 

[bryanbdp]

Not sure I understand about the neutral.
In the diagram above, there is a single center tap labeled neutral.
Would it be different somehow if you had 3 center taps?
Or is neutral in this type of setup not really a neutral?
Thanks,
Bryan

 

[ActionDave]

Not sure I understand about the neutral.
In the diagram above, there is a single center tap labeled neutral.

That's a high leg delta and in a true sense it does not have a neutral but we call it a neutral.

Would it be different somehow if you had 3 center taps?....

Yes, you would have a 208V wye.

 

[jumper]

That's a high leg delta and in a true sense it does not have a neutral but we call it a neutral.

Yes, you would have a 208V wye.

The whole delta set up does not have a neutral like a wye, but the center tapped tranny has a neutral.

Center tapping the three delta trannies would not be the same as a wye.

As Goldy pointed out, you could not tie them together to a common GES, and voltages would be different.

 

[ActionDave]

The whole delta set up does not have a neutral like a wye, but the center tapped tranny has a neutral....

By strict definition it's not a neutral because it's not common to all three windings.

Heck, the NEC only started calling it a neutral a few cycles ago.

 

[winnie]

700 KVA for a makerspace? Are you using one of those new energy to mass conversion 3D printers? *grin*

The first thing that I'd suggest is double checking your load calculations. Just how large is the facility? Are you accepting memberships (I am located in Springfield, MA *grin* ) ?

It is quite possible to connect the neutrals of 3 single phase transformers together, to produce something called 'hexaphase'. What you cannot do is simultaneously connect both the neutrals and the hots of these transformers. As was already stated this is essentially a dead short between different voltage terminals of a transformer bank.

Hexaphase is not common. It has some benefits with respect to 'triplen' harmonics from switching power supplies, but otherwise a normal 120/208V system is much more common and balances the 3 phase loading. The hexaphase system also does not give you 240V three phase. It provides 240V single phase, 208V three phase, and also some unused 120V phase to phase connections.

If the facility is quite large, you might consider using smaller 120V transformers spread around the building, and distributing 480V, rather than dealing with the larger wires needed for 120V. This would let you use smaller wire, and would let you distribute your 120 and 240V single phase loads across all three phases, but at the cost of more (smaller) transformers.

You might want to see how many of your 240V three phase loads could be rewired for 480V (many motors are 'dual voltage').

-Jon

 

[jumper]

By strict definition it's not a neutral because it's not common to all three windings.

Heck, the NEC only started calling it a neutral a few cycles ago.

Well...... okay I cannot argue either, both are true.

 

[kwired]

Not sure I understand about the neutral.
In the diagram above, there is a single center tap labeled neutral.
Would it be different somehow if you had 3 center taps?
Or is neutral in this type of setup not really a neutral?
Thanks,
Bryan

there is voltage between each center tap (if there would be three of them) I think it should be 120 volts from one center tap to either of the other two, would still be 208 volts to the opposite corner of the delta, and would be so on all three center taps. You can not connect any two points along that delta together - it will result in short circuit current flowing. Wye connected winding are simply different. Reason you can't get both 120 and 240 out of a wye system is the ratio of line - line to line - neutral stays the same regardless what you make the main voltage. If you would increase number of turns on a wye secondary to achieve 240 volts line - line, line -neutral voltage also goes up and you end up with ~138 volts to neutral.

Tying the neutrals together would be a dead short circuit on all three windings. Not at all a good idea.
And keeping them separate would give you circuits with a neutral which is not at ground potential, in violation of NEC rules.

Sent from my XT1585 using Tapatalk

You could only ground one point of the system, then you would need overcurrent protection on the remaining seven "ungrounded" conducotrs.

 

[bryanbdp]

Thanks to all for the replies. 3phase theory makes my head spin pretty quickly.

Jon, yes, it's all for our fusion deposition printer, that MC^2 thing really soaks up the power.:blink:
Seriously though, we will have a 40hp waterjet cutter, a Haas milling machine center, electric kilns and ovens, a 7HP CNC router and table saw, welders, and a lot of other equipment. It all adds up.
I did all the calculations for the last space we were planning for, but it's been a while. I plan to sprinkle 120v quads around the space, and those 180VA/strap add up pretty quick. I think you only get 6 on a 20a circuit. 180x2x6=2160VA 20Ax120 = 2400VA
And although they aren't considered continuous loads, I also don't think you can derate them due to diversity in a commercial space. Someone tell me I'm wrong, please!!

I do plan to use 480v for the machines that support it, but there aren't that many. I did find that most machines are really designed for 240v, not 208, thus the 240v transformer. The items with switching power supplies don't seem to care that much, like the Haas and the welders/cutters. You can plug some of those welders and cutters into just about anything, although the max output and duty cycle may decline.

We have engineers going over everything, we may actually end up with an 800A service, although I think 600 would be OK. A Makerspace isn't a factory, even on a busy day!
We will definitely be accepting memberships, and would barter for electrical services! http://www.makerspacect.com

Kind of leaning towards using 2 transformers, one 240v 3 phase for single and 3 phase loads, and a 208Y120 for the 120V loads... Should be able to find one of those pretty cheaply.

Thanks,
Bryan

700 KVA for a makerspace? Are you using one of those new energy to mass conversion 3D printers? *grin*

The first thing that I'd suggest is double checking your load calculations. Just how large is the facility? Are you accepting memberships (I am located in Springfield, MA *grin* ) ?

It is quite possible to connect the neutrals of 3 single phase transformers together, to produce something called 'hexaphase'. What you cannot do is simultaneously connect both the neutrals and the hots of these transformers. As was already stated this is essentially a dead short between different voltage terminals of a transformer bank.

Hexaphase is not common. It has some benefits with respect to 'triplen' harmonics from switching power supplies, but otherwise a normal 120/208V system is much more common and balances the 3 phase loading. The hexaphase system also does not give you 240V three phase. It provides 240V single phase, 208V three phase, and also some unused 120V phase to phase connections.

If the facility is quite large, you might consider using smaller 120V transformers spread around the building, and distributing 480V, rather than dealing with the larger wires needed for 120V. This would let you use smaller wire, and would let you distribute your 120 and 240V single phase loads across all three phases, but at the cost of more (smaller) transformers.

You might want to see how many of your 240V three phase loads could be rewired for 480V (many motors are 'dual voltage').

-Jon

 

[kwired]

Kind of leaning towards using 2 transformers, one 240v 3 phase for single and 3 phase loads, and a 208Y120 for the 120V loads... Should be able to find one of those pretty cheaply.

Thanks,
Bryan

I was going to suggest doing something like that.

 

[LarryFine]

To add: The only way you can ground three separate neutrals from three 120/240v secondaries of a 3ph transformer would be to isolate every other conductor, so you would have to use three separate 120/240v 1ph panels. This is certainly doable, and could allow the three panels to be in separate locations.

If the 120v loads are all in proximity, however, I agree that the 208/120Y is the way to go. Many such panels never see 208v 3ph loads. The only decision you need to make is which conductor of your 240v 3ph system to ground.

 

[GoldDigger]

To add: The only way you can ground three separate neutrals from three 120/240v secondaries of a 3ph transformer would be to isolate every other conductor, so you would have to use three separate 120/240v 1ph panels. This is certainly doable, and could allow the three panels to be in separate locations.

If the 120v loads are all in proximity, however, I agree that the 208/120Y is the way to go. Many such panels never see 208v 3ph loads. The only decision you need to make is which conductor of your 240v 3ph system to ground.

Which requires that you have a nine wire connection available on the secondary side: six ends which are NOT connected together internally to make a delta, plus three neutrals. If the winding to winding delta bonds are internal to the transformer there is absolutely no way to ground the three center taps.

If instead of one three phase transformer you have three independent single phase transformers, it is easy to set up what Larry suggests. But I am not sure it would be a good idea.
Among other things if you are required to identify phases and voltages (more than one system present) then you will need to uniquely identify all six hot wires.

 

[kwired]

Which requires that you have a nine wire connection available on the secondary side: six ends which are NOT connected together internally to make a delta, plus three neutrals. If the winding to winding delta bonds are internal to the transformer there is absolutely no way to ground the three center taps.

If instead of one three phase transformer you have three independent single phase transformers, it is easy to set up what Larry suggests. But I am not sure it would be a good idea.
Among other things if you are required to identify phases and voltages (more than one system present) then you will need to uniquely identify all six hot wires.

Also would need to uniquely identify which neutral goes with each system.