Transformer and panel install questions

[jim dungar]

My concern with a 3 phase 480 to 240/120 comes from the neutral loading as expressed in this post from an earlier thread from Jim Dungar and from manufacturing data sheets I have reviewed:

• It really has to do with the laws of physics and things like magnetic fields and circulating currents. But, the fact of life is: a single core transformer connect in a 4 wire delta is limited to 5% neutral loading. Multi-core transformer banks (i.e. using single phase transformers) do not have this problem.
  ​

For the past 10 years or so, some manufacturers have lifted the 5% loading limitation, while others have not.

 

[florida-sparkey]

First let me check this before I continue???

Primary V*I = 480*135 = 64,800
Sec V*I = 240*270 = 64,800

Why such a big trafo 112,500 VA or is it you want to double it approx for expansion in future???
Is this a 3 phase delta to delta trafo??

Also could I request that you sketch a drawing with the details on it to keep it easier to follow......

Sorry but I do not have a sketch at this time.

I looked at your math and since you were willing to help me, I thought I would help you. In your calculations, I think you forgot the fact that this is 3 phase and you need multiply by the square root of 3 or 1.732

I*E*1.732 or 270*240*1.732= 112.234KVA

 

[florida-sparkey]

You appear to be new enough to the forum that you are not familiar with the three phase four wire high leg delta, with the center point of one delta winding connected to ground and used as the neutral for 120/240 circuits.
It is not uncommon, although not all POCOs will allow it for a new service.

It is very useful when you need a lot of 120 or 240 for lighting and general use but have one or more three phase motors, etc.
It is often implemented by POCO as an open delta with a large 120/240 single phase three wire pot and a smaller "stinger" pot sized just for the motor load(s).

One reason POCO may not like it is that it is not as inherently balanced across all three distribution phases as 208Y/120 three phase four wire.

GoldDigger,
FYI the 480 Volt service is a closed delta with 3 pots on the POCO's pole. It supplies only my customers unit as the other units are supplied by a 240 closed delta system mounted to another pole nearby.

 

[Fnewman]

You appear to be new enough to the forum that you are not familiar with the three phase four wire high leg delta, with the center point of one delta winding connected to ground and used as the neutral for 120/240 circuits.
It is not uncommon, although not all POCOs will allow it for a new service.

It is very useful when you need a lot of 120 or 240 for lighting and general use but have one or more three phase motors, etc.
It is often implemented by POCO as an open delta with a large 120/240 single phase three wire pot and a smaller "stinger" pot sized just for the motor load(s).

One reason POCO may not like it is that it is not as inherently balanced across all three distribution phases as 208Y/120 three phase four wire.

Yes, I am new to the forum but not the industry. The reason I did not mention the arrangement suggested is because the OP stated that he was using an existing 3 phase 480 v supply and because the high leg delta has very specific load limitations for the 120 v load.

OP: Yes, you will find a few delta secondary transformers at 240 v in the catalogs, some with a center tap as mentioned above, subject to the limitations mentioned. However, you need a Y connection to end up with a full size neutral (208/1.732=120). You should be ok for a 3 phase load at 208; I have seen very little equipment that actually required 240 v 3 phase.

 

[roger]

Do you think buck boost is the way to go long term?

If I only had a few pieces that truly needed 230V I wouldn't hesitate to go with a buck boost transformer(s). You could actually size a single buck boost transformer to feed a panel just for these few pieces of equipment.

Roger

 

[florida-sparkey]

Yes, I am new to the forum but not the industry. The reason I did not mention the arrangement suggested is because the OP stated that he was using an existing 3 phase 480 v supply and because the high leg delta has very specific load limitations for the 120 v load.

OP: Yes, you will find a few delta secondary transformers at 240 v in the catalogs, some with a center tap as mentioned above, subject to the limitations mentioned. However, you need a Y connection to end up with a full size neutral (208/1.732=120). You should be ok for a 3 phase load at 208; I have seen very little equipment that actually required 240 v 3 phase.

Thanks! I do not need a full size neutral so that is noted. Panel will be used for 240V 1 or 3 phase loads. I want the neutral for (if) when the equipment needs it. Example. I did a drill press that had an external drive unit attached that plugged into a 120V receptacle.

I am not experienced enough to know for sure if 208 will run equipment with no issue long term. I have read opposing views. Heating is the main motor issue. Electronic brains with low voltage are the other.

What I do know is that I do not have the ability to check with each individual equipment manufacturer and I am unwilling to take responsibility for the equipment if it dies or goes nuts. :happysad: Why should I assume such responsibility? As a licensed guy, connecting to the wrong voltage seems like a request for a lawsuit.

I absolutely have had a job that I was hired to deal with when correction was required and the CNC manufacture rep demanded the customer did so. This same customer had a unit failing. I am fuzzy on the details other than that all his equipment worked less the one that kept failing randomly. Power supplies and computer modules were going bad. As I remember, the voltage condition was reversed in that the voltage was high. At that time they said +-5% nominal was max so we installed a xformer for the two units and as far as I know all is well.

Please help me understand how I am foolish in going with the nameplate voltage +-5%
Example nameplate 230V:
230 + 5% = 241.5
230 - 5% = 218.5

 

[GoldDigger]

Yes, I am new to the forum but not the industry. The reason I did not mention the arrangement suggested is because the OP stated that he was using an existing 3 phase 480 v supply and because the high leg delta has very specific load limitations for the 120 v load.

Those limitations are specific to using a single core three phase transformer.
An open delta will always be made up of single phase pots, while closed delta high leg often uses individual pots too, so that there is no problem with a high single phase load and a smaller three phase load.

 

[florida-sparkey]

If I only had a few pieces that truly needed 230V I wouldn't hesitate to go with a buck boost transformer(s). You could actually size a single buck boost transformer to feed a panel just for these few pieces of equipment.

Roger

Buck boost to a panel? I had not thought of that.

This of course assumes that the small existing 45KVA 208 xformer can handle it.
I have not done the math (LOAD CALC) but it seems very unlikely that it can.

See if I am going in the right direction please:
If my rough math is correct then that is 125 FLA at 208 from xformer secondary.
If I do as many manufacturers say and consider 20% additional needed for motor starting and heat that reduces 125A. or 125*0.8 = 100A for all loads in the warehouse.

I am sure I need to connect at least three units with MOPDs at around 30A. So 3*30= 90A Ill take only 80% or 90*0.8 = 72 Amps estimated motorized equipment load. My math was based on the estimated current at 208 so If I step up voltage my current will also go up on the xformer. To much math at this time :happysad: I feel there is not enough capacity with that original system. Do you disagree?

I really do appreciate your input. I honestly had not considered your path till now.

My 2 choices are as I now see them:
Replace the 208/120 with larger and then step it up for the machines by buck boost at a panel or at each machine
OR
Add new 240Volt xformer for machines and use the 208 volt system for receptacles and lighting.

I am stupid :slaphead: but unless I am missing something I still think the latter is just as good or better then the first option. Obviously, you had no way of knowing any details that I did not state prior. But with the 45KVA 208V system and ballpark loads now known to you, do you still feel that option 1 or some variant is best for any reason?

 

[dionysius]

Sorry but I do not have a sketch at this time.

I looked at your math and since you were willing to help me, I thought I would help you. In your calculations, I think you forgot the fact that this is 3 phase and you need multiply by the square root of 3 or 1.732

I*E*1.732 or 270*240*1.732= 112.234KVA

Good. I was doing single phase so should have read more carefully.

Now I agree with some of the comments re 208V.

Here is what I suggest:


Use a 112 kVA dry-type 480 Delta to 208Y120 Y sec NEMA transformer.

Since this generates 208 VAC, you change the standard tap settings to boost the voltage to your need.

Select the primary tap connections as if your primary voltage is too low.

Your sec voltage will swing higher, so 208 VAC will become 230 or so.

I believe you will find this is the lowest cost and most effective solution you will find.


Also to keep everybody in sync please restate/update you current revised OP description with any changes to date you plan to incorporate.

These threads get very disorganized and tough to follow.

I will have a similar job coming up so I want to see where this goes.

You are doing good work.

 

[iwire]

Good. I was doing single phase so should have read more carefully.

Now I agree with some of the comments re 208V.

Here is what I suggest:


Use a 112 kVA dry-type 480 Delta to 208Y120 Y sec NEMA transformer.

Since this generates 208 VAC, you change the standard tap settings to boost the voltage to your need.

Select the primary tap connections as if your primary voltage is too low.

Your sec voltage will swing higher, so 208 VAC will become 230 or so.

I believe you will find this is the lowest cost and most effective solution you will find.

No, no, no.

Do not do this as that makes the 120 volt too high to use.

 

[roger]

My 2 choices are as I now see them:
Replace the 208/120 with larger and then step it up for the machines by buck boost at a panel or at each machine
OR
Add new 240Volt xformer for machines and use the 208 volt system for receptacles and lighting.

I am stupid :slaphead: but unless I am missing something I still think the latter is just as good or better then the first option. Obviously, you had no way of knowing any details that I did not state prior. But with the 45KVA 208V system and ballpark loads now known to you, do you still feel that option 1 or some variant is best for any reason?

You aren't stupid, it's easy to get tunnel vision until other eyes come to the game. I think I would let cost and ease of installation be the determining factors between the two options.

Roger

 

[Fnewman]

You aren't stupid, it's easy to get tunnel vision until other eyes come to the game. I think I would let cost and ease of installation be the determining factors between the two options.

Roger

Agreed - it is probably the best way to select based on info available. A couple of other considerations: if you go with 3 phase delta for the 240v loads, make sure the driven equipment does not need a neutral. To me I think it is a bit unreasonable to expect the OP to design an appropriate system without knowing more about the requirements of the driven equipment, how much capacity should be allowed for future needs (and at what voltage), etc. but that is just me, I guess. I have run into this exact issue before and when all the facts were known, it turned out that most of the loads were actually 3 phase 480!

 

[dionysius]

No, no, no.

Do not do this as that makes the 120 volt too high to use.

The OP is serving out 120V from existing system as he stated.

If the preponderance of the equipment from this new panel is 240 then my solution will give him the lowest cost trafo solution.

Should he need some 120 out in the future then use a buck-boost xfr for that to buck it down.

This is cost driven by manufacturing volume since the 208 is the popular heavy hitter sales-wise.

Correct me if I am wrong here. We want the optimal solution - cost (parts $ labor), serviceability, reliability, ease of installation.

 

[florida-sparkey]

Agreed - it is probably the best way to select based on info available. A couple of other considerations: if you go with 3 phase delta for the 240v loads, make sure the driven equipment does not need a neutral. To me I think it is a bit unreasonable to expect the OP to design an appropriate system without knowing more about the requirements of the driven equipment, how much capacity should be allowed for future needs (and at what voltage), etc. but that is just me, I guess. I have run into this exact issue before and when all the facts were known, it turned out that most of the loads were actually 3 phase 480!

"OP to design an appropriate system without knowing" I have to take issue with this statement. This is not a classroom question and to make such a statement when from the OP I have given a very detailed and specific design. Others can sometimes confuse the thread... possibly? But to say in effect that I do not know the requirements needed so I can detail a design and post it... is well... Lets just say I did not get my master's license in a cereal box.

"make sure the driven equipment does not need a neutral" I have already made the point (over and over and over) about wanting neutral available just in case. Others have pointed out not to overload it, but, that has never been my intention. Hence 240/120 on the secondary side in my OP

"without knowing more about the requirements of the driven equipment" I have all data tag info for on site equipment. Pics of them, taken while on site. I clearly stated a driven machine in my OP. I felt my OP was already a little to wordy and the extra bulk would have clouded the transformer design verbiage.

"future needs" That was also addressed and why I am installing a twice as large panel as is needed.

Thanks for the help

 

[florida-sparkey]

The OP is serving out 120V from existing system as he stated.

If the preponderance of the equipment from this new panel is 240 then my solution will give him the lowest cost trafo solution.

Should he need some 120 out in the future then use a buck-boost xfr for that to buck it down.

This is cost driven by manufacturing volume since the 208 is the popular heavy hitter sales-wise.

Correct me if I am wrong here. We want the optimal solution - cost (parts $ labor), serviceability, reliability, ease of installation.

Thanks so much but this seems overly complicated to me. The cost of the xformer is not that high to warrant not just getting what I need without the added cost of buck boost material and labor. I have bulk quotes (disco, xformer & panel) and they are on the shelf ready to ship from both ITE and Eaton.

 

[florida-sparkey]

You aren't stupid, it's easy to get tunnel vision until other eyes come to the game. I think I would let cost and ease of installation be the determining factors between the two options.

Roger

Thanks for your help Roger,
I am going in the direction of my OP. I have the design in my head and the major parts quoted, so, less a safety or serviceability issue (why I asked for other eyes like yours ) I think that's the path.

 

[florida-sparkey]

Please and thank you to everyone in the thread! The extra eyes have made me more confident in my design, its choice and identified a critical mistake in my electrode grounding conductor size.

Given all the back and forth, I am going with my design as in the OP (changing the GEC size of course.) I have to change or install a disconnect, transformer, panel and feeders to handle the estimated load so to me it's easier to keep what is there (208/120) and add the new 240/120 system.

I am asking a final follow up question and one that caused me to write the OP.

My concern is my calculation with AIC at the new panel main breaker lugs fed by xformer secondary. I need to confirm that I can use a 10K rated (most likely it will be a 22 or 24K, cause that's how most 3P panels come in the brands I use)

I have a couple bulk price quotes and they quoted a 240/120 high leg panelboard with 64K main and series rated breakers.

I think this is over kill (based on my math:blink: ) and the panel is the only item not ready to ship, so, if it's not needed then it would be better to go with a lesser unit that is also surely cheaper too.

 

[florida-sparkey]

The OP is serving out 120V from existing system as he stated.

If the preponderance of the equipment from this new panel is 240 then my solution will give him the lowest cost trafo solution.

Should he need some 120 out in the future then use a buck-boost xfr for that to buck it down.

This is cost driven by manufacturing volume since the 208 is the popular heavy hitter sales-wise.

Correct me if I am wrong here. We want the optimal solution - cost (parts $ labor), serviceability, reliability, ease of installation.

I do find myself still thinking about this idea. I reread your idea and see the IF NEEDED part on the buck boost. My concern and maybe not an issue at all is running a system at a voltage other than as stated on its data tag. If I looked at a 208V xformer, I would expect it to be connected at that. I guess a durable tag could fix that. I see your point on the availability of the 208Y gear.Both suppliers had to fish to get me the 240D gear. I am going to keep this in my pocket in case the lead time changes on the gear. Obviously the panel would remain at 240V but I think it will also state high leg as I think that was the only option. Another labeling concern for me.

 

[Fnewman]

A lot of interesting discussion about various approaches, any one of which would probably work out somehow. Once the job is complete, I hope you will come back and give us a "final report".

 

[dionysius]

This is a good thread and I wish most threads would be so well executed. The OP used unambiguous language to delineate his design in his OP and asked direct questions. He was open to looking at competing approaches and evaluate them within the context of his customer's requirements.

Please do post your final design and a diagram if you do produce one.

As always there are many ways to skin a cat and this website is the greatest IMHO.