Design input - Revised

[Alwayslearningelec]

They closed the other thread so I can not respond. Looks like your getting a handle on It now.

Yes I am. The only thing I need to confirm is whether a 45kva xfrm will work in this scheme.
I did post a pic of one of the trailers( 125A) panel schedule.

 

[Alwayslearningelec]

Anyone able to help as to whether a 45lva 3P xfrmr would provide enough power for the 3 trailer panels? I was about to post a pic of one of the trailer panel schedules....the 125A panel.

 

[augie47]

The problemis without an exact load there is no definite. Based on panel sizes, no; but based on comments you made and logical experience form some here, the 45 should be enough, I feel the 45 will more than likely work and from posts here and in your earlier thread it appears infinity and Tulsaelectrican agree. If you want 100% assurance you would need to go with the 75 but if cost is a concern I would go with the 45 especially since it's a "temporary" set up and the loads are not likely to increase over time.

 

[infinity]

Anyone able to help as to whether a 45lva 3P xfrmr would provide enough power for the 3 trailer panels? I was about to post a pic of one of the trailer panel schedules....the 125A panel.

The amount of load is unknown so lets say that your 3 panels with 60, 60, and 125 amp mains each use 80% of their rating. Convert that to 1Ø VA for each panel and figure out the maximum transformer load on any one phase. If it's greater than 45 kva then you'll need a 75 kva transformer.

 

[Alwayslearningelec]

The problemis without an exact load there is no definite. Based on panel sizes, no; but based on comments you made and logical experience form some here, the 45 should be enough, I feel the 45 will more than likely work and from posts here and in your earlier thread it appears infinity and Tulsaelectrican agree. If you want 100% assurance you would need to go with the 75 but if cost is a concern I would go with the 45 especially since it's a "temporary" set up and the loads are not likely to increase over time.

Thank you.

 

[Alwayslearningelec]

The amount of load is unknown so lets say that your 3 panels with 60, 60, and 125 amp mains each use 80% of their rating. Convert that to 1Ø VA for each panel and figure out the maximum transformer load on any one phase. If it's greater than 45 kva then you'll need a 75 kva transformer.

Thank you. What would the line to line voltage be 120 or 208v? Trying to do the calc.
Once I get the VA then how exactly do I apply that to figure max load on any one phase of xfrmr? Thanks

 

[Tulsa Electrician]

Thank you. What would the line to line voltage be 120 or 208v? Trying to do the calc.
Once I get the VA then how exactly do I apply that to figure max load on any one phase of xfrmr? Thanks

For this base calc use single phase line to line for each single phase panel.

Example: 125 amp main breaker panel.
125 x .80. = x amps
X amps x line to line (208 v )= x VA
x VA/ 1000 = x KVA.
For this panel using 80% of the main rating method.

Now assign linea to this single phase load.

Example 125 amp panel on lines A and B.

The KVA on line ( phase A) would be x KVA and then the same for Line ( phase B) . Your line to line is 208 volt.

Now do the same for the other two.
Then you simple add up all on each line ( phase) in Descending order.

This would be the KVA for each Line or phase.
Do this first before doing any thing else.

This would be step one the way you working the issue.

Once you post those we move forward if you want to learn how to do from a field perspective.
At this point you can either say move forward for the 45 or step up to a 75.

This is a key step so the one line and panel sch lay out has all information for the field installer to follow. If not you can easily overload the transformer. A simple pic can be used as a inspection tool to sign off on task. Load checks on start up can identify any issue which we will get into later based on your panle pic.

So if you want an easy answer add all panel mains up single phase and size the trans to that . Then no one line will be over loaded. Max KVA on one line and round up on transformer size.
At the end of the day it's your choice. Your name is on it.

 

[Alwayslearningelec]

For this base calc use single phase line to line for each single phase panel.

Example: 125 amp main breaker panel.
125 x .80. = x amps
X amps x line to line (208 v )= x VA
x VA/ 1000 = x KVA.
For this panel using 80% of the main rating method.

Now assign linea to this single phase load.

Example 125 amp panel on lines A and B.

The KVA on line ( phase A) would be x KVA and then the same for Line ( phase B) . Your line to line is 208 volt.

Now do the same for the other two.
Then you simple add up all on each line ( phase) in Descending order.

This would be the KVA for each Line or phase.
Do this first before doing any thing else.

This would be step one the way you working the issue.

Once you post those we move forward if you want to learn how to do from a field perspective.
At this point you can either say move forward for the 45 or step up to a 75.

This is a key step so the one line and panel sch lay out has all information for the field installer to follow. If not you can easily overload the transformer. A simple pic can be used as a inspection tool to sign off on task. Load checks on start up can identify any issue which we will get into later based on your panle pic.

So if you want an easy answer add all panel mains up single phase and size the trans to that . Then no one line will be over loaded. Max KVA on one line and round up on transformer size.
At the end of the day it's your choice. Your name is on it.

for the 125A panel I come up with 20.8 kva and each of the 60A panels I come up with 9.98 kva.
So the sum of these is 40.76.

 

[Tulsa Electrician]

That is what I come up with as well based on all line to line (L-L).
Now let's look at line to netural loads (L-N).
I worked up some schedules to view as we go.

So for the 125 we have a picture of the panel schedule. The only 208v load would be the HVAC and all other are 120v (L-N) load's
What need to take place is some how with knowing exactly what the loads are.
Then we make a panel schedule with load to see what the L-N might be.

Her some common sense and assumptions need to take place. As well as review of Artical 550.

The 60 amp two pole HVAC is an easy one. If you follow the code for such units. We do not have name plate so following code 48 amp max. For an electric heat set up. 60*.80= 48 amps. 48*208= x VA/1000= x KVA. Now what Kva for line A and Line B ?
This need to be put on the connected load KVA for L-L. Then work on L-N for the rest. As you do this you will see what I see in way of load and a change in the circuitry to help balance the assumption of load. Remember this needs to fall with in the 80% of the 125 amp main if you can and no more than 100% of the 125 amp main L-N. If you don't than one L-N will be high.

Again a field perspective based on past experiences.
If your wondering why the space between breaker. I like to space them so the heat from has less effect on the next.
I also stack them to reduce load on the interior buss per line. At site I would move the first 60 across. For sheet I stacked them to make it easier to follow.

 

[infinity]

for the 125A panel I come up with 20.8 kva and each of the 60A panels I come up with 9.98 kva.
So the sum of these is 40.76.

What the cost difference between the 45 kva setup and the 75 kva setup?

 

[Alwayslearningelec]

What the cost difference between the 45 kva setup and the 75 kva setup?

It is a probbly about $8K we can save. Which is about 25-30% of price.

 

[Alwayslearningelec]

That is what I come up with as well based on all line to line (L-L).
Now let's look at line to netural loads (L-N).
I worked up some schedules to view as we go.

So for the 125 we have a picture of the panel schedule. The only 208v load would be the HVAC and all other are 120v (L-N) load's
What need to take place is some how with knowing exactly what the loads are.
Then we make a panel schedule with load to see what the L-N might be.

Her some common sense and assumptions need to take place. As well as review of Artical 550.

The 60 amp two pole HVAC is an easy one. If you follow the code for such units. We do not have name plate so following code 48 amp max. For an electric heat set up. 60*.80= 48 amps. 48*208= x VA/1000= x KVA. Now what Kva for line A and Line B ?
This need to be put on the connected load KVA for L-L. Then work on L-N for the rest. As you do this you will see what I see in way of load and a change in the circuitry to help balance the assumption of load. Remember this needs to fall with in the 80% of the 125 amp main if you can and no more than 100% of the 125 amp main L-N. If you don't than one L-N will be high.

Again a field perspective based on past experiences.
If your wondering why the space between breaker. I like to space them so the heat from has less effect on the next.
I also stack them to reduce load on the interior buss per line. At site I would move the first 60 across. For sheet I stacked them to make it easier to follow.

Your one smart guy. I sincerely thank you. I have to buy this out tomorrow. It seems highly likely, to me, that a 45kvs xfrmr will work. Even if I had a 150A main instead of the 125A it seems it would fall right under the 45kva.

 

[Alwayslearningelec]

That is what I come up with as well based on all line to line (L-L).
Now let's look at line to netural loads (L-N).
I worked up some schedules to view as we go.

So for the 125 we have a picture of the panel schedule. The only 208v load would be the HVAC and all other are 120v (L-N) load's
What need to take place is some how with knowing exactly what the loads are.
Then we make a panel schedule with load to see what the L-N might be.

Her some common sense and assumptions need to take place. As well as review of Artical 550.

The 60 amp two pole HVAC is an easy one. If you follow the code for such units. We do not have name plate so following code 48 amp max. For an electric heat set up. 60*.80= 48 amps. 48*208= x VA/1000= x KVA. Now what Kva for line A and Line B ?
This need to be put on the connected load KVA for L-L. Then work on L-N for the rest. As you do this you will see what I see in way of load and a change in the circuitry to help balance the assumption of load. Remember this needs to fall with in the 80% of the 125 amp main if you can and no more than 100% of the 125 amp main L-N. If you don't than one L-N will be high.

Again a field perspective based on past experiences.
If your wondering why the space between breaker. I like to space them so the heat from has less effect on the next.
I also stack them to reduce load on the interior buss per line. At site I would move the first 60 across. For sheet I stacked them to make it easier to follow.

How does one determine L to L loads vs. L to N loads?

 

[Tulsa Electrician]

You have done the math if all loads are line to line or 208 volt only loads.
In this case you a combination of 120 volt L-N and 208 L-L.
For each panel you will need to do a KVA assessment at 120 volts for each Line. Any connected load that is Line 1 to netural goes in one column and then the same for Line 2 in the panel or your panel schedule.

Then add up descending to get a total for that line. This KVA is at 120v.
This KVA can not or should not be greater than the OCPD rating.
What this will do is also determine if any load should be relocated to help balance the line to line KVA.
For now leave out any demand factors we can discuss later and will as it relates to the application.
Once that is all done we can move on.
I have already done so with panel schedules so we will see how it compares.

This will help you make a final decision. Slow going however worth it in the end.

 

[Alwayslearningelec]

You have done the math if all loads are line to line or 208 volt only loads.
In this case you a combination of 120 volt L-N and 208 L-L.
For each panel you will need to do a KVA assessment at 120 volts for each Line. Any connected load that is Line 1 to netural goes in one column and then the same for Line 2 in the panel or your panel schedule.

Then add up descending to get a total for that line. This KVA is at 120v.
This KVA can not or should not be greater than the OCPD rating.
What this will do is also determine if any load should be relocated to help balance the line to line KVA.
For now leave out any demand factors we can discuss later and will as it relates to the application.
Once that is all done we can move on.
I have already done so with panel schedules so we will see how it compares.

This will help you make a final decision. Slow going however worth it in the end.

Thank you very much. I'm under the gun here with so many things. I think I'll be ok with the 45kva. If you have any thoughts about my opinion let me know.

 

[infinity]

Thank you very much. I'm under the gun here with so many things. I think I'll be ok with the 45kva. If you have any thoughts about my opinion let me know.

I think that you'll be fine with that extra $8k in your pocket.

 

[Tulsa Electrician]

As you do this pay attention to the branch circuit OCPD's (15/ 20 amp breakers).
An example would be the KVA load for line 1 lite/ fan. 15 amp breaker, 1.4 kva L-N.
15*.80=12 continues load lighting.
12*120= 1440 VA
1440/1000= 1.44
So I would use 1.4 KVA for this L-N.
In reality this would be. A lot less more like 50 % depending what you see when your there or do a load check, field work. Got a good field foreman.

Now you can do the rest and see how it comes out.

Use common sense on all others. You would not always need to use 80% of the branch

 

[Tulsa Electrician]

Ok, save for later if you want to learn how it all works. Beat of luck to you.

 

[Alwayslearningelec]

Ok, save for later if you want to learn how it all works. Beat of luck to you.

Again, thanks a lot.

 

[Alwayslearningelec]

Ok a little potential change.

Say I wanted to eliminate the 225A panel( with the 125A, and (2) 60A branch breakers) and use (3) disconnect switches instead could I doo that.
I'd think so but then how would I feed (3) disconnects from a transformer. Run one feed from transformer to a trough, splice, then feed the discos?

Would the disconnects have to be fused? Also 2P3W disconnects?