old 240 delta, new 480V

[mjmike]

Well, have an interesting project I am currently working on and just want to get some thoughts. The project I am working on is an existing manufacturering facility with a 204/120V 3-phase system and utilizes 2 utility transformers. This tells me I am dealing with an open delta center tapped system and that a high leg exists. Utility power goes into a main board then distributes out to serveral other panels, some 3-phase and some 1-phase. Regular receptacle / lighting loads are mixed throughtout all the panels.

We are designing an addition and I will be serving the addition with a new 480/277 service. I will then abandon the current 240 open delta service entrance and re-feed the existing main 240v 3-phase panel as a sub-feed from the 480V system via a step-down transformer. Now, here is where I am questioning the connection. I know the overall KW demand of the 240V system from past utility bills and will be metering in the future to determine KVA and Power factor. However, how do I go about determining the transformer size? I will not be able to determine the percentage of existing 1 pole loads verses the 3 phase loads and If I remember correctly, I am limited to 5% of the KVA rating for 1 pole loads because of the neutral. Maybe I am confusing the 5% with someting else.

That is my first question. My second question now involves the power in the addition. In the addition, I will have 480V equipment (no problem) and 240 3-phase equipment (problem). Since there is 240V 3-phase equipment, the obvious choice is to set a 240V 3-phase panel for the 3-phase equipment and receptacles / general 1-pole loads. But, now I am dealing with the same thing as I am dealing with on the existing system and the 5% rule. So, to combat that, what if I ran with a transformer to go from 480 to 240 closed delta, corner grounded. This will give me 240V power and I can come out of the 240V panel to another transformer and set a 240/120 1-phase panel for 1-pole loads? DOes this make sence?


Any thoughts would be great.

 

[jim dungar]

Well, have an interesting project I am currently working on and just want to get some thoughts. The project I am working on is an existing manufacturering facility with a 204/120V 3-phase system and utilizes 2 utility transformers. This tells me I am dealing with an open delta center tapped system and that a high leg exists. Utility power goes into a main board then distributes out to serveral other panels, some 3-phase and some 1-phase. Regular receptacle / lighting loads are mixed throughtout all the panels.

We are designing an addition and I will be serving the addition with a new 480/277 service. I will then abandon the current 240 open delta service entrance and re-feed the existing main 240v 3-phase panel as a sub-feed from the 480V system via a step-down transformer. Now, here is where I am questioning the connection. I know the overall KW demand of the 240V system from past utility bills and will be metering in the future to determine KVA and Power factor. However, how do I go about determining the transformer size? I will not be able to determine the percentage of existing 1 pole loads verses the 3 phase loads and If I remember correctly, I am limited to 5% of the KVA rating for 1 pole loads because of the neutral. Maybe I am confusing the 5% with someting else.

That is my first question. My second question now involves the power in the addition. In the addition, I will have 480V equipment (no problem) and 240 3-phase equipment (problem). Since there is 240V 3-phase equipment, the obvious choice is to set a 240V 3-phase panel for the 3-phase equipment and receptacles / general 1-pole loads. But, now I am dealing with the same thing as I am dealing with on the existing system and the 5% rule. So, to combat that, what if I ran with a transformer to go from 480 to 240 closed delta, corner grounded. This will give me 240V power and I can come out of the 240V panel to another transformer and set a 240/120 1-phase panel for 1-pole loads? DOes this make sence?


Any thoughts would be great.

The 5% limit of L-N loads is only associated with a single core transformer providing 240/120 3PH4W. Although in recent years there are newer designs that do not have this limit.

One solution is to get a 480-240 delta transformer for your 3PH 240 loads, and a second 480-120/240 1PH transfromer for your L-N loads.

A second solution is to duplicate the utility feed by creating your own open delta 240/120 3Ph 4W transfromer bank.

A third solution (which I hardley use) is a new design single core transformer.

Have you asked the utility what size transformers they are using?

 

[winnie]

I do not have direct experience with this, but here is my understanding:

'Open delta' high leg services were often used where the dominant load was single phase 120/240V, but a small amount of 3 phase power was needed. So a large single phase transformer would be combined with a small 'stinger' transformer to provide the third phase.

Most 'closed delta' dry type transformers are intended for use where the dominant load is three phase 240/120V, but can supply a small amount of single phase power.

So in answer to your first question: yes, you are remembering correctly about the 5% limit, however it is specific to common transformer designs; you can get transformers suitable for any combination of single and three phase loading. You can even 'bank' single phase transformers to get the desired combination of capabilities.

IMHO it does not make sense to put transformers in series as you describe. There is no benefit to going 480V 3 phase -> 240V 3 phase -> 240V single phase. However it does make sense to separate your 240V three phase loads and 240V single phase loads. I would suggest that if you are going to have separate transformers for 3 phase and 1 phase, that they both be 480V -> 240V transformers. I would also suggest that you not use corner grounding for the 240V three phase, but rather stick with the same center tap grounding of the current system.

-Jon

 

[mjmike]

The 5% limit of L-N loads is only associated with a single core transformer providing 240/120 3PH4W. Although in recent years there are newer designs that do not have this limit.

So for my existing feed, if I meter and get the KVA, that is the size transformer I need and do not need to worry about 1-phase verses 3-phase loads?

One solution is to get a 480-240 delta transformer for your 3PH 240 loads, and a second 480-120/240 1PH transfromer for your L-N loads.

This would work for my new system, but how about when I reconnect the existing system with a mixture of 1P and 3P loads? I have no way to separate them without a complete rewire.

A second solution is to duplicate the utility feed by creating your own open delta 240/120 3Ph 4W transfromer bank.

I would do this , but how do I determine the transformer sizes.

Have you asked the utility what size transformers they are using?

I would do this, but the 2 transformers feed other facilities. They are not dedicated to this one facility.

 

[jim dungar]

I would do this , but how do I determine the transformer sizes.

For an open delta bank where T = three phase KVA (balanced) and S = total single phase KVA:

Center tapped transformer for L-N loads kVA = .58T + S
"Stinger" transformer kVA= .58T

 

[LarryFine]

Any thoughts would be great.

My thought is to leave the existing installation intact, and install the new service independent of the existing one.

Because of the different voltages, there should be no issues with more than a single service, as per NEC 230.2(D).

You'll save a large amount in labor, materials, and down time. It'll be nice for you to have uninterrupted power, too.

Added: The above posts point out one of the biggest advantages. Look at the headaches you can avoid.

 

[jojo]

Hello good day! Just want to ask if the 5% limit of L-N loads applicable to 3 single-phase transformers which is banked to form three-phase power?

 

[jim dungar]

Hello good day! Just want to ask if the 5% limit of L-N loads applicable to 3 single-phase transformers which is banked to form three-phase power?

The 5% limit is only applicable to a single core three phase transformer.

 

[mjmike]

My thought is to leave the existing installation intact, and install the new service independent of the existing one.

Because of the different voltages, there should be no issues with more than a single service, as per NEC 230.2(D).

You'll save a large amount in labor, materials, and down time. It'll be nice for you to have uninterrupted power, too.

Added: The above posts point out one of the biggest advantages. Look at the headaches you can avoid.

Very good point. This is what I wanted to do, but I got a letter from the local POCO indicating a firewall would be required. I'll take a look at the nec, maybe some leverage.

 

[LarryFine]

. . . I got a letter from the local POCO indicating a firewall would be required.

Then the customer needs to compare the cost of a fire-rated dividing wall (if it really is necessary) to the cost of doing it as per your OP.

 

[jojo]

The 5% limit is only applicable to a single core three phase transformer.

Thanks jim for the info..

 

[jim dungar]

Thanks jim for the info..

While the 5% limit does not exist, the center tapped transformer must still be sized correctly for the combination of L-L and L-N loading.