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

[Besoeker]

Tying the neutrals together would be a dead short circuit on all three windings.

There is only one neutral shown. What am I missing?

 

[kwired]

Tying the neutrals together would be a dead short circuit on all three windings.

There is only one neutral shown. What am I missing?

You missed the suggestion and question of if it would be possible to ground a midpoint on each coil presuming each one had an accessible midpoint. As mentioned you could, if you did not tie the corners of the "delta" together. Which is pretty much what one would have if they simply installed three separate single phase transformers and balanced them across all three input lines. But they would have to supply three separate single phase panels also.

 

[Besoeker]

You missed the suggestion and question of if it would be possible to ground a midpoint on each coil presuming each one had an accessible midpoint.

My question related to the one neutral, not grounds plural. Why can't you ground just that one neutral?

 

[kwired]

My question related to the one neutral, not grounds plural. Why can't you ground just that one neutral?

You can, and usually should.

Problem OP is having is he wants both 120 and 240 volts on the premises, and says he has no need for 208 volts.

You can get 120 volts on all three "phases" with a wye system, but the consequence of that is it is only 208 volts "phase to phase".

He can use a 4 wire delta and have both 120 and 240 but has one "phase" that is 208 volts to "neutral" and is pretty much useless for 120 volt loads. If 120 volt loads are significant enough he can't balance them across all three phases without additional transformation.

He could utilize a wye connected system that has 240 volts between "phases" but then gives up 120 volts to neutral as it will become about 138 volts in this setup.

Then he though why can't I use center tap of each coil on a delta - but that won't necessarily work either unless they truly are isolated single phase systems that are simply balanced across all three phases, doesn't leave you with a real delta but still balances loads plus there is no simple three phase three wire ability available.

No matter what he does he is looking at more then just one transformation to get what he wants overall.

 

[ActionDave]

There is only one neutral shown. What am I missing?

This

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

 

[Besoeker]

You can, and usually should.

Problem OP is having is he wants both 120 and 240 volts on the premises, and says he has no need for 208 volts.

Isn't the simple answer to that "then don't use it"?

 

[kwired]

Isn't the simple answer to that "then don't use it"?

The simple answer is he needs more then one separately derived system to fulfill all of his needs.

 

[Besoeker]

This

I didn't miss it. It shows just one neutral.
As has been pointed out, tying the three mid points together would be bad news.

 

[Jraef]

I would think it more cost effective to just do a 208Y120V transformer large enough for all of your >480V loads, then carefully investigate the specific machines that MUST have 240V and add Buck-Boost transformers for those only. I think you may find that people often say "230V" out of marketing laziness and the machines are perfectly fine with 208V. But 208V rarely exists outside of North America, so they don't bother mentioning it in basic advertising literature, then if you ask, their engineers will say it's fine.

 

[kwired]

I would think it more cost effective to just do a 208Y120V transformer large enough for all of your >480V loads, then carefully investigate the specific machines that MUST have 240V and add Buck-Boost transformers for those only. I think you may find that people often say "230V" out of marketing laziness and the machines are perfectly fine with 208V. But 208V rarely exists outside of North America, so they don't bother mentioning it in basic advertising literature, then if you ask, their engineers will say it's fine.

I agree that there is a lot of equipment only marked 240 (or 230) volts that will likely be fine on 208 volts.

 

[bryanbdp]

I agree that there is a lot of equipment only marked 240 (or 230) volts that will likely be fine on 208 volts.

I just don't see a reason to compromise on the 208 volt thing since this is a new installation. Most of the single phase equipment is really intended to use residential 240/120 split phase power, not 208v.
I really drilled down on the specifications on the equipment, and 208v is a compromise at best, from what I can tell. Some equipment can be rewired for 208, but most can not. Then they are derated at 208, which is less than optimal.

So, due to the complexity and issues with trying to derive everything from one transformer, I'll just do one 240v and one 208Y120v and call it a day.
That sounds like the general consensus.

Any input on the 120v calculations? If I remember correctly, you have to account for all the 180VA per strap at 100% for convenience outlets in the branch panel. How do you handle those same loads on the distribution panel? I will have several 208/120 subpanels connecting to one distribution panel next to the transformer. Do you still have to add them all up at 100%?

Thank you, everybody, for all your help!

 

[kwired]

I just don't see a reason to compromise on the 208 volt thing since this is a new installation. Most of the single phase equipment is really intended to use residential 240/120 split phase power, not 208v.
I really drilled down on the specifications on the equipment, and 208v is a compromise at best, from what I can tell. Some equipment can be rewired for 208, but most can not. Then they are derated at 208, which is less than optimal.

So, due to the complexity and issues with trying to derive everything from one transformer, I'll just do one 240v and one 208Y120v and call it a day.
That sounds like the general consensus.

Any input on the 120v calculations? If I remember correctly, you have to account for all the 180VA per strap at 100% for convenience outlets in the branch panel. How do you handle those same loads on the distribution panel? I will have several 208/120 subpanels connecting to one distribution panel next to the transformer. Do you still have to add them all up at 100%?

Thank you, everybody, for all your help!

180 VA is mostly for a general use receptacle outlet that doesn't have any specific load it is intended to supply. If you have a receptacle that is intended to supply say a 500 VA load then the load you should include in your calculations for that outlet is 500 VA.


Branch circuit load calculations are covered by 220 part II, Feeder/service calculations are covered by part III.

Feeder/service calculations may be able to apply demand factors, branch circuits generally must assume 100% of the load, unless something in chapters 5,6,7 allows otherwise for specific equipment.

 

[ActionDave]

I didn't miss it. It shows just one neutral.
As has been pointed out, tying the three mid points together would be bad news.

The diagram shows one neutral but the question was about adding two more to the other windings.

 

[bryanbdp]

180 VA is mostly for a general use receptacle outlet that doesn't have any specific load it is intended to supply. If you have a receptacle that is intended to supply say a 500 VA load then the load you should include in your calculations for that outlet is 500 VA.


Branch circuit load calculations are covered by 220 part II, Feeder/service calculations are covered by part III.

Feeder/service calculations may be able to apply demand factors, branch circuits generally must assume 100% of the load, unless something in chapters 5,6,7 allows otherwise for specific equipment.

OK, thank you, that's what I seem to remember from my last go around...
I am just talking convenience outlets, without any fixed load.

So if I have a 200A subpanel, which would be a maximum of 24,000VA, and each circuit of 6 quads is 2160VA, that would only allow 11 circuits on the panel? Or does the branch circuit refer to the max. number of convenience outlets on a 20a branch circuit, but allow more on the panel, since they aren't dedicated outlets?

 

[kwired]

I am just talking convenience outlets, without any fixed load.

180VA per outlet for branch circuit calculations.

Feeder/service calculations may be able to apply demand factors to them, not to individual outlets but to all the outlets on the feeder/service.

 

[bryanbdp]

OK, just reviewed 220.44 which allows a demand factor for non-dwelling receptacle loads

first 10kVA at 100%
remainder over 10kVA at 50%

When calculating sub-panel loads, do you have to calculate all the general purpose convenience outlets at 180VA per strap, and the totals of all circuits must be below the 24,000VA capacity of the panel, unlike residential panels?
So when using the demand factor above, can that be used to allow more branch circuits for outlets in the subpanel, or is that factor used at the service or distribution panel?

 

[Besoeker]

The diagram shows one neutral but the question was about adding two more to the other windings.

I think we already covered that.

 

[winnie]

As I understand it, at any level above the final branch circuit you can apply the demand factors.

Say you have a panel dedicated to general purpose receptacles, with circuits feeding a total of 400 general purpose duplex receptacles in it. (Not saying this is a good design!)

This is set up as 40 circuits with 10 receptacles per circuit. You have to calculate each branch circuit at 1800 VA.

But the calculated load for the feeder to the panel is:

400 * 180 = 72kVA Calculate at: 10 kVA + 0.5 * 62kVA = 41kVA

Now feed 3 of these panels from a single feeder.

You have to calculate the load on this feeder as:
1200 * 180 = 216 kVA Calculate at: 10kVA + 0.5 * 206kVA= 113kVA

Note that these numbers are in some way very conservative and in other ways very fictional. Depending on layout, some circuits will be overloaded, and others hardly used. Watch out for space heaters

-Jon

 

[kwired]

OK, just reviewed 220.44 which allows a demand factor for non-dwelling receptacle loads

first 10kVA at 100%
remainder over 10kVA at 50%

When calculating sub-panel loads, do you have to calculate all the general purpose convenience outlets at 180VA per strap, and the totals of all circuits must be below the 24,000VA capacity of the panel, unlike residential panels?
So when using the demand factor above, can that be used to allow more branch circuits for outlets in the subpanel, or is that factor used at the service or distribution panel?

A 200 amp feeder supplying a panelboard - the panelboard is an extension to the feeder.

The branch circuits start at the branch breakers.

 

[JP79]

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

Jon, I give kudos to your suggestion, stuck with 480 where possible and distribute with smaller conductors. If possible that is.

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