Utility Transformer 13800 Primary

[rocket-electric]

Are utility company transformer's output calculated differently?

I have the following setup:
A pad mount liquid filled transformer 150KVA 13,800v primary, 208/120v 3 phase secondary
Doing the math on the secondary voltage, I get 150,000 / (208 x 1.73) = 417 amps

The service required for building #1 is 800A 208/120v 3 phase

The second setup is:
A pad mount liquid filled transformer 1000KVA 13,800v primary, 480/277v 3 phase secondary
Doing the math on the secondary voltage, I get 1,000,000 / (480 x 1.73) = 1200 amps

The service required for building #2 is 2000A 480/277v 3 phase

Please advise, Thanks

 

[GoldDigger]

Are utility company transformer's output calculated differently?

No, but they are allowed to apply their own diversity and permissible short term overload figures to the application. If the voltage drop is too great at the service or their transformer overheats, they will install a bigger transformer. Until then, they do not believe that the service will ever be loaded to its nominal capacity for any significant period of time, if ever. And they are usually right.

However, this does mean that the 800 or 2000A breakers at the service disconnect are all that is protecting the service wires against low level overload.

If you have an engineering calculation showing that the REAL load will be greater than the rating of the transformer, then POCO may or may not be interested in it.

 

[kingpb]

The only thing I would like to add is that the non-utility portion of the installation needs to be done according to calculations and meet at least minimum code requirements. What the utility does is their business.

Unless you want to drive yourself insane, don't ever try to understand how they select their transformer size. It is based on a large number of service installs and many, many years of historical data.

 

[ron]

If they bring service feeders to meet yours at a property line or similar, you will laugh to see that you might have many paralleled sets of wire and they will have ~1/2 of that. They don't need to follow NEC, so they can save cash.

 

[kwired]

If they bring service feeders to meet yours at a property line or similar, you will laugh to see that you might have many paralleled sets of wire and they will have ~1/2 of that. They don't need to follow NEC, so they can save cash.

Their equipment is also not inside any buildings, so if it overheats is not a risk of burning a building down.

They do take loading cycle times into consideration also. They figure that they can run certain equipment at a certain overload level if it has a regular cycle that allows a "cool down" time also.

I added some significant air conditioning load to a school building a few years back and informed the POCO of what was being added. We did upgrade service equipment to comply with NEC, but when they looked at their transformer and their secondary conductor size they took some of the following into consideration: existing load data including peak demand and peak demand time periods, expected increase in peak demand as well as duration, maybe the average ambient temperature or some value that is related to that, amount of time expected between peak demand time periods.

They increased the transformer size, but was still below the service calculation values, they changed their underground secondary conductors from aluminum to copper but kept same size - this allowed them to use same raceway where an increase in aluminum size would have possibly been too tight of a fit though they don't necessarily follow NEC for raceway fill either.

 

[iwire]

Are utility company transformer's output calculated differently?

I have the following setup:
A pad mount liquid filled transformer 150KVA 13,800v primary, 208/120v 3 phase secondary
Doing the math on the secondary voltage, I get 150,000 / (208 x 1.73) = 417 amps

The service required for building #1 is 800A 208/120v 3 phase

The second setup is:
A pad mount liquid filled transformer 1000KVA 13,800v primary, 480/277v 3 phase secondary
Doing the math on the secondary voltage, I get 1,000,000 / (480 x 1.73) = 1200 amps

The service required for building #2 is 2000A 480/277v 3 phase

Please advise, Thanks

To add to what the others have said.

The power company does not care what the NEC based load calculations are. The power company does not care what size main breaker is being supplied. They use their many years of data to determine what the building will actually use.

They are also not opposed to using a transformer above its labeled rating for short periods of time. They rely on the thermal mass of the transformer to keep it from getting overheated during the short periods of overload.

In short what they do puzzles us but works fine the vast majority of the time and saves all of us money.

 

[jim dungar]

The power company does not care what the NEC based load calculations are. The power company does not care what size main breaker is being supplied.

I would postulate that they do care. They just don't use the results the way we do.

They use their many years of data to determine what the building will actually use.

My point is they usually use the NEC load calcs as their starting point.


What data does the utility ask you for when you request a new service?

 

[iwire]

I would postulate that they do care. They just don't use the results the way we do.

My point is they usually use the NEC load calcs as their starting point.

We will disagree on that.

From what our moderator Charlie E had posted it did not sound like they have any interest in the NEC calcs. He mentioned their figures are often just 40 to 50% of the NEC.

What data does the utility ask you for when you request a new service?

http://www.nstar.com/docs3/misc/Work Order Application.doc?unique=20131121125941

They ask me all about the equipment, not NEC load calculations.

 

[Smart $]

...

What data does the utility ask you for when you request a new service?

https://www.aepohio.com/builders/PermanentBusiness.aspx
https://www.aepohio.com/builders/PermanentHome.aspx

 

[jim dungar]

We will disagree on that.

From what our moderator Charlie E had posted it did not sound like they have any interest in the NEC calcs. He mentioned their figures are often just 40 to 50% of the NEC.

So where do they get the NEC values in the first place?


In Wisconsin, for new services:
Alliant Energy wants to know "Total amperage rating of all service entrance equipment..."
Xcel Energy wants to know "Service Size* (amps)"
WE Energies spells it out even better "Service switch equipment shall be of a capacity not less than that required by the Wisconsin State Electrical Code or local ordinances, where more restrictive."

 

[jim dungar]

https://www.aepohio.com/builders/PermanentBusiness.aspx
https://www.aepohio.com/builders/PermanentHome.aspx

Which ask for the SERVICE SIZE in amps. Where do you get this if not from some type of NEC calcs?

 

[Smart $]

So where do they get the NEC values in the first place? ..."

IMO utilities don't seem to care about NEC calc's themselves... but when they ask for service equipment ratings they are in effect assuming those ratings meet or exceed values established through NEC load calculations.

 

[Smart $]

Which ask for the SERVICE SIZE in amps. Where do you get this if not from some type of NEC calcs?

I agree.... I was just responding to your question of what my POCO asks for.

 

[kingpb]

So where do they get the NEC values in the first place?


In Wisconsin, for new services:
Alliant Energy wants to know "Total amperage rating of all service entrance equipment..."
Xcel Energy wants to know "Service Size* (amps)"
WE Energies spells it out even better "Service switch equipment shall be of a capacity not less than that required by the Wisconsin State Electrical Code or local ordinances, where more restrictive."

But that's not a load calc, that's just what service size that has been decided. Then they look to see what type of facility it is, apply their historical data and that's what you'll get.

Heck, you can install a 2000A 208V switchboard for a 1000sqft storage unit, but you may get lucky to have a 75KVA transformer with a single set of 2/0 to connect your 6 sets of 500KCMIL too.:slaphead:

 

[iwire]

Which ask for the SERVICE SIZE in amps. Where do you get this if not from some type of NEC calcs?

The service size in amps tells them vitally nothing and I would be very surprised if that is even considered for anything beyond the meter type they will send out.

For example, an EC calculates a new homes service, he comes up with an NEC result of 90 amps. He says 'That is pretty close to a 100 so I am going to install a 200 amp service'. This is very common, the price difference is minimal and load centers with more circuit spaces are readily available in 200 amp sizes.

So you end up with a 200 amp breaker, on a NEC load of 90 at a home that will likely never draw more than 60 amps.

I doubt they are sizing transformers and conductors based on the service breaker size.

 

[jim dungar]

I doubt they are sizing transformers and conductors based on the service breaker size.

If they are not considering the breaker size, what are they using as a starting point, number of parking spots for the building?

I am simply saying, utilities use the Service Entrance Equipment size (per NEC 230.79) as one of the inputs to their calculations. After applying some type of multiplier (be it derating, or demand, or what they had for lunch) they decide on a transformer size range.

Ultimately, if your utility is like mine, they end up installing whatever standard size unit they have in the yard that day.

 

[GoldDigger]

And some POCOs, notably LADWP, calculate the required service size based on the bus rating of the service panel, regardless of the size breaker you put in it!
Just FWIW.

 

[kwired]

But that's not a load calc, that's just what service size that has been decided. Then they look to see what type of facility it is, apply their historical data and that's what you'll get.

Heck, you can install a 2000A 208V switchboard for a 1000sqft storage unit, but you may get lucky to have a 75KVA transformer with a single set of 2/0 to connect your 6 sets of 500KCMIL too.:slaphead:

lucky is correct for the 75 KVA. Unless there is more than just some general lighting that install is only getting a 10 or 15 kVA transformer max, they are not spending the money for a 75 unless the load is there.

 

[jim dungar]

But that's not a load calc, that's just what service size that has been decided. Then they look to see what type of facility it is, apply their historical data and that's what you'll get.

Heck, you can install a 2000A 208V switchboard for a 1000sqft storage unit, but you may get lucky to have a 75KVA transformer with a single set of 2/0 to connect your 6 sets of 500KCMIL too.:slaphead:

Xcel Energy has this clause in the service book"
"If the customer?s power requirement, within six months after the installation of transformers, proves to be less than shown in the application for service, the Company may make a reduction in the installed transformer capacity and the customer will be required to pay the Company the cost of making the change.

If the customer?s power requirement, within six months after the installation of transformers, proves to be more than the installed transformer capacity, the Company may make an increase in the installed transformer capacity and the customer will be required to pay to the Company the cost of making the change."

In their territory you would not want to 'mis-size' your service on your permit application, as you did in your example, it could come back and cost you.

Regardless the point still is, that utilities have their own methods for determining the size of their service equipment.

 

[robbietan]

here in my neck of the woods, the utility asks for the load schedule. then applies some load factors, utilization factors, diversity factors, availability of transformers, current mood of the approving authority, time of day, etc

in short, they get it right most of the time.