277/480V 200A supply stepped down to 120/240V...

[EEEC]

Theoretically, a 277/480V 200A service can be stepped down (via transformer) to supply a 120/240V 400A system... correct?

 

[kwired]

Theoretically, a 277/480V 200A service can be stepped down (via transformer) to supply a 120/240V 400A system... correct?

Yes. Is the 120/240 single phase or three phase? You could supply either, but the single phase would only load two primary phases (to their capacity) and leave the other phase sort of unusable, unless you can find a way to load it to no more then 200 amps with 277 volt loads, assuming you are using full 200 amp capacity of two phases supplying the transformer.

 

[EEEC]

Thank you! Yes, it is a 3ph system. So I assume that I'd use the following formula to calculate the size of the transformer:

kilovolt-amps = ?3 ? amps ? volts / 1000

or
kVA = ?3 ? A ? V / 1000


Therefore:

kVA = ?3 ? 400 ? 120 / 1000 = 83.14kVA

Correct?

 

[kwired]

Thank you! Yes, it is a 3ph system. So I assume that I'd use the following formula to calculate the size of the transformer:

kilovolt-amps = ?3 ? amps ? volts / 1000

or
kVA = ?3 ? A ? V / 1000


Therefore:

kVA = ?3 ? 400 ? 120 / 1000 = 83.14kVA

Correct?

Correct formula but you need to use 240 volts instead of 120 if your figuring a 240 volt system, which will double your result. Do the same for the primary voltage and current and your result should be same as secondary.

Should you actually have a 208/120 secondary your secondary amps will be a little higher at the same KVA, which if all the KVA capacity is necessary you may need more then a 400 amp overcurrent device on the secondary (full load @ 208 volts would be about 462 amps if we have 200 amps @ 480 volts primary) more important is load calculations though instead of matching an overcurrent device to the transformer rating

 

[iceworm]

Theoretically, a 277/480V 200A service can be stepped down (via transformer) to supply a 120/240V 400A system... correct?

Thank you! Yes, it is a 3ph system. So I assume that I'd use the following formula to calculate the size of the transformer:

kVA = ?3 ? 400 ? 120 / 1000 = 83.14kVA

I'm a little lost. Here is my translation of what you are after - just listing the issues that affect the xfm size:

Secondary is 240V 3ph delta - likely one phase centertapped (and grounded). But there are no single phase 120V or 240V loads.

Xfm is sized to deliver 400A, 3ph, 240V, delta.

If this is true:
xfm KVA = V x I x sqrt(3)/1000 = 240 x 400 x 1.732/1000 = 166KVA. You will pick either a 150kVA or a 225kVA

For a 480V primary, 150kVA xfm, primary CB will be:
KVA x 1000 /480/Sqrt(3) = 225A

480V, 3ph, 200A feeder is a bit short to feed a 240V 400A delta.

ice

 

[iceworm]

kilovolt-amps = ?3 ? amps ? volts / 1000 ...

Correct formula but you need to use 240 volts instead of 120

AARRGgg- i just figured out you were using the "?" symbol as "sqrt()".

Yes, as kwired said, your formula is correct except for the 120 - 240 issue.

However, you will still have an issue with the 480V feeder being too small.

ice.

 

[kwired]

AARRGgg- i just figured out you were using the "?" symbol as "sqrt()".

Yes, as kwired said, your formula is correct except for the 120 - 240 issue.

However, you will still have an issue with the 480V feeder being too small.

ice.

Understood if we go with actual transformer full load ratings, but going back to what was asked in OP if we have 400 amp 240 out we will have 200 amp 480 in, disregarding minimal losses. As you more less said this value is in between standard size transformers so if full 200/400 amps is needed then we must use the larger standard size transformer, but the smaller 150KVA unit would be acceptable as long as load calculations do not exceed 150KVA and would be common to use such size if those conditions are met.

 

[iceworm]

..., but the smaller 150KVA unit would be acceptable as long as load calculations do not exceed 150KVA and would be common to use such size if those conditions are met.

Yes - But you got to get it energized. And the minimum CB I would use for a 150kva is 225A. I'm not commenting on if one could find NEC justification for feeding this with a 200A feeder. Rather a 200A feeder is a really poor design practice - perhaps value engineered unto project death.

ice

 

[kwired]

Yes - But you got to get it energized. And the minimum CB I would use for a 150kva is 225A. I'm not commenting on if one could find NEC justification for feeding this with a 200A feeder. Rather a 200A feeder is a really poor design practice - perhaps value engineered unto project death.

ice

I have pretty fair confidence this would not be much of a tripping issue in most circumstances - even with 150 or 175 amp overccurrent protection.

I have used transformers for deriving temp power before and only supplied them with a circuit of about 25 to 40 percent of their rating and never had any issues. In each case the transformer just happened to be on hand but was of a larger capacity then what was needed, like a 15 or 25 kVA being used in a place where maybe a 5 or 7.5 kVA would work.

 

[iceworm]

completed a couple of sentences

completed a couple of sentences

I have pretty fair confidence this would not be much of a tripping issue in most circumstances - even with 150 or 175 amp overccurrent protection.

I have used transformers for deriving temp power before and only supplied them with a circuit of about 25 to 40 percent of their rating and never had any issues. In each case the transformer just happened to be on hand but was of a larger capacity then what was needed, like a 15 or 25 kVA being used in a place where maybe a 5 or 7.5 kVA would work.

With all due respect I don't think there is a good comparison between 5kva - 25kva xfm on temporary feeders and a permanent installation of a 150kVA xfm.

I'm pretty sure my customers would have a freeking fit if a transformer tripped on energization and they could still remember the last time that had happened. Or, if it was a critical circuit, if it ever happened.

My current design spec is a 10x FLA inrush will happen 1 out of 6 energizations. That puts a 150kva at 1800A - which is in the instantaneous range on a typical 150A CB (I looked at a GE TED curve)

As for a 225A (GE TFx) - 1800A is right on the edge of the instantaneous. So for this case, I'd likely pick a 250A CB.

I would not have any confidence a 150A CB or a 200A CB would not occasionally trip on energization.

However, I suspect this does not have anything to do with the OP question - which I am guessing is about transformer physics. On that note:

Yes, following kwired's statements, considering a 480/240D transformer, 200A in will give you 400A out. And 200A of 480v, 3ph, is 166kva. Reliably feeding it from a 480V, 200A service is a diferent question.

ice

 

[EEEC]

Some very good info here... Thanks guys!

Here's the scoop... A long time customer of mine owns a printing shop. He currently has a 120/240V 200A service (all of his equipment works just fine). He is looking into moving into a larger shop and has asked me what type of power should be available while looking around at properties. I said that we are at the load threshold with the present equipment count, and told him if he was planning on getting a bigger shop for the purpose of adding more equipment, he'd be wise to look for a shop that will accommodate 400A. Last night he asked me if a 277/480V 200A system equates to a 120/240V 400A system. I told him it did in theory, but I wanted to make sure it could happen, hence my questions. I consider myself a good electrical contractor with quite a bit of knowledge, but I am certainly not an engineer. It is one thing to know how to install what is specified in drawings, but quite another to actually draw the drawings!

So, that being said, if the primary breaker size is 200A, would it be safe to install a 150kVA 480V primary, 120/240V 3ph secondary xfmr to supply a 400A subpanel? I could always pull an amp reading each time he adds equipment to see when he's near his limit. Also, it is crucial that phase to phase is 240V because these big printers are very finicky and need at least 230V minimum and I'd like to avoid using buck boosters.

 

[GoldDigger]

Some very good info here... Thanks guys!

Here's the scoop... A long time customer of mine owns a printing shop. He currently has a 120/240V 200A service (all of his equipment works just fine). He is looking into moving into a larger shop and has asked me what type of power should be available while looking around at properties. I said that we are at the load threshold with the present equipment count, and told him if he was planning on getting a bigger shop for the purpose of adding more equipment, he'd be wise to look for a shop that will accommodate 400A. Last night he asked me if a 277/480V 200A system equates to a 120/240V 400A system. I told him it did in theory, but I wanted to make sure it could happen, hence my questions. I consider myself a good electrical contractor with quite a bit of knowledge, but I am certainly not an engineer. It is one thing to know how to install what is specified in drawings, but quite another to actually draw the drawings!

So, that being said, if the primary breaker size is 200A, would it be safe to install a 150kVA 480V primary, 120/240V 3ph secondary xfmr to supply a 400A subpanel? I could always pull an amp reading each time he adds equipment to see when he's near his limit. Also, it is crucial that phase to phase is 240V because these big printers are very finicky and need at least 230V minimum and I'd like to avoid using buck boosters.

You are getting a bit confused.
A single phase 480V 200A will have the same power capacity as a 240V 400A system.
But you are looking at a 480V three-phase system, which will give you an additional factor of 1.732 IF you can use it.
But since none of his machines are currently three phase, you will instead end up with an interesting load mismatch which will require a transformer, as you noted.
But if he has 120V loads too, then a 208Y/120 system or a 240Y/139 system are the two simple choices.
You could instead create on the secondary a 240/120 three phase four wire system (called a high-leg system) to provide for the 120V loads.
But make sure that his 240V machines (including the controls) do not also require 120V to ground/neutral or all bets are off and it gets a lot more complicated.

 

[EEEC]

My apologies. Reading back through my last post I see I left out some important information....

The current system is 120/240V 3ph 4-wire.
At least one of the printers is 240V 3ph.
At least a half dozen 120V computers.

Does this make a difference GoldDigger?

 

[iceworm]

... customer of mine owns a printing shop. He currently has a 120/240V 200A service (all of his equipment works just fine).

... Last night he asked me if a 277/480V 200A system equates to a 120/240V 400A system.

... Also, it is crucial that phase to phase is 240V because these big printers are very finicky and need at least 230V minimum

Adding to DG post. We would all feel better if you would use different nomenclature.

If the existing service is single phase 120/240, then use "120/240"

If the existing service is 240V 3ph delta, with one of the phases centertapped for 120V loads, then use "240/120. All will understand this is a 240V delta with a center tap, or call it a stinger of hi-leg.

So, is the existing service single phase or 3 phase? Usually one can tell from context, but now I not so sure. And it matters.

... if the primary breaker size is 200A, would it be safe to install a 150kVA 480V primary, 120/240V 3ph secondary xfmr to supply a 400A subpanel? ....

I wouldn't do it. Too close to nuisance trips on energization. Unless the customer understands and is okay with getting the occasional trip on energization.

Maybe look at a 112.5kVA with a fairly low impedance. That would give you a rock solid 300A, 240/120D, panel. Depending on the service transformer size and impedance - that may be all you can get anyway. If the printers are not happy with voltage sags.

As for engineering - You are the Engineer of Record.

ice

 

[kwired]

With all due respect I don't think there is a good comparison between 5kva - 25kva xfm on temporary feeders and a permanent installation of a 150kVA xfm.

I'm pretty sure my customers would have a freeking fit if a transformer tripped on energization and they could still remember the last time that had happened. Or, if it was a critical circuit, if it ever happened.

My current design spec is a 10x FLA inrush will happen 1 out of 6 energizations. That puts a 150kva at 1800A - which is in the instantaneous range on a typical 150A CB (I looked at a GE TED curve)

As for a 225A (GE TFx) - 1800A is right on the edge of the instantaneous. So for this case, I'd likely pick a 250A CB.

I would not have any confidence a 150A CB or a 200A CB would not occasionally trip on energization.

However, I suspect this does not have anything to do with the OP question - which I am guessing is about transformer physics. On that note:

Yes, following kwired's statements, considering a 480/240D transformer, 200A in will give you 400A out. And 200A of 480v, 3ph, is 166kva. Reliably feeding it from a 480V, 200A service is a diferent question.

ice

I will not disagree with you, but will add that the amount of current can be limited depending on size and impedance of the source, as well as impedance of the conductors on the supply side. If the facility has a 200 amp 480 volt service and the load is not all that high POCO may very well only have a 75 kVA transformer if moderately loaded maybe only a 112.5 kVA.

Best advice for the OP is to figure out what you have for machines that need voltage changed as you likely only want to supply what you need plus some room for growth, but consider that if the facility already has 480/277 supply that some of your existing general lighting and other general loads are likely already taken care of at the new site and all you need to consider is the equipment you bring in. You possibly even have some equipment that could be dual voltage and could run on 480 volts, or possibly any new equipment maybe can run on 480 volts.

 

[Smart $]

...
I wouldn't do it. Too close to nuisance trips on energization. Unless the customer understands and is okay with getting the occasional trip on energization.

Maybe look at a 112.5kVA with a fairly low impedance. That would give you a rock solid 300A, 240/120D, panel. Depending on the service transformer size and impedance - that may be all you can get anyway. If the printers are not happy with voltage sags.

As for engineering - You are the Engineer of Record.

ice

I agree regarding potential nuisance trips.

An option to get a full 200A to 400A conversion is to go with two transformers. Nuisance trip issue may still exist on power up after an outage, but this method would provide the ability to do one-at-a-time reboot. May have to unload the system until getting both energized. Refer to 450.6 regarding secondary ties if combining transformer secondary outputs is desired.

 

[EEEC]

Forgive my ignorance ice, and thank you for the clarification. What is the term you use for an individual who calculates, designs, and drafts electrical systems, but does not implement the design? I don't want to embarrass myself again by using the wrong terminology. As I said before, I am no (fill in the blank), I am usually a person who implements the (fill in the blank)'s design, so I am not abreast with all of the proper nomenclature. Thank you for your understanding and patience...

So, is the existing service single phase or 3 phase? Usually one can tell from context, but now I not so sure. And it matters.

My apologies. Reading back through my last post I see I left out some important information....

The current system is [edited] 240/120 3ph 4-wire.
At least one of the printers is 240V 3ph.
At least a half dozen 120V computers.

 

[EEEC]

I will not disagree with you, but will add that the amount of current can be limited depending on size and impedance of the source, as well as impedance of the conductors on the supply side. If the facility has a 200 amp 480 volt service and the load is not all that high POCO may very well only have a 75 kVA transformer if moderately loaded maybe only a 112.5 kVA.

Best advice for the OP is to figure out what you have for machines that need voltage changed as you likely only want to supply what you need plus some room for growth, but consider that if the facility already has 480/277 supply that some of your existing general lighting and other general loads are likely already taken care of at the new site and all you need to consider is the equipment you bring in. You possibly even have some equipment that could be dual voltage and could run on 480 volts, or possibly any new equipment maybe can run on 480 volts.

I like where you are going with this and it has sparked an idea...

What if I install a 75kVA xfm (50kVA would be too small, right?) fed by a 125A CB on primary, with a 200A MCB sub on the secondary to feed his existing equipment. Then, as you said, he can purchase equipment that will run on 480V, which can be fed directly from the existing panel. Worst case scenario being that I'd have to install smaller xfms per instance thereafter. Thoughts on this?

 

[kwired]

Forgive my ignorance ice, and thank you for the clarification. What is the term you use for an individual who calculates, designs, and drafts electrical systems, but does not implement the design? I don't want to embarrass myself again by using the wrong terminology. As I said before, I am no (fill in the blank), I am usually a person who implements the (fill in the blank)'s design, so I am not abreast with all of the proper nomenclature. Thank you for your understanding and patience...

The answer to that question could vary from one place to another.

Some places it must definitely be a professional engineer, others it may be someone at some lower level - this is based on possible local regulations. I am in no way anywhere close to a legal PE, but I do design a lot of things that I install, and something like you have in this thread is certainly something I very well may get involved with at times, yet other times maybe all I have to do is follow a plan designed by others.

 

[EEEC]

I like where you are going with this and it has sparked an idea...

What if I install a 75kVA xfm (50kVA would be too small, right?) fed by a 125A CB on primary, with a 200A MCB sub on the secondary to feed his existing equipment. Then, as you said, he can purchase equipment that will run on 480V, which can be fed directly from the existing panel. Worst case scenario being that I'd have to install smaller xfms per instance thereafter. Thoughts on this?

Anyone?