How do Utility Departments Size Transformers?

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CALIF_C10

Member
Location
N. California
Here's my situation:

I have been asked to install and wire two stand by generators for a 3000 amp existing service. I just happened to notice the Utility transformer is 1500KVA 480/277 volt. First thing I thought was this is not big enough to feed the service. If I calc'ed this correctly, I figured this transformer for around 1800 amps. The utility district told me they figured the size based on 277 volts. Which is the correct method?:-?

And FYI, this MSB has 800 & 600 amp breakers currently in use and a 1600 amp breaker which is going to be in use in about a month or so. The transformer at this time is warm to hot and humms like a bee farm. I'm just a little worried when that 1600 amp breaker is closed and the entire service is in full use. Anticipated full load amps is about 2400 amps.
 

iwire

Moderator
Staff member
Location
Massachusetts
The utility is not constrained by the NEC.

They size their transformers based on historical data, and that data shows that the NEC load calculations are very conservative.

The power company is rarely, if ever going to size the transformer based on the size of the customers NEC service size.

If they did it would be very wasteful as they would essentially have thousands of over sized transformers wasting both electricity and materials.

I think the best way to think about this is not to.

What the power company does or does not do simply has nothing to do with our work. :)
 

lowryder88h

Senior Member
Location
Massachusetts
trans

trans

As Bob has already stated the POCO in are area for upgrades is load history and your load calcs that you as the EC defines for them. In work work thay rely on again the load calcs you provide. Take a dep breath count to ten and go to bed.:D
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
Another thing that I like to do is to take copper conductors sized per the NEC from the panel to the PoCo transformer. That yields a little better voltage when the load gets to the high side since the PoCo sizes their conductors using the same data as they do to size their transformers.
 
What we do is ask the customer, with a load of 50 kw or greater, to give us a letter requesting what kw amount they want us to supply and at what voltage. Then we put in enough transformation to handle that plus a little extra. They sign a power contract on the amount they ask us for.
 

peter d

Senior Member
Location
New England
iwire said:
The power company is rarely, if ever going to size the transformer based on the size of the customers NEC service size.

Which begs the question, why does the NEC "overkill" everything?
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
peter d said:
Which begs the question, why does the NEC "overkill" everything?

It really doesn't.

The utility has a lot going for it that a residence, for instance, does not.

Their conductors and transformers are usually outside. So you do not have to worry so much about failures causing a fire in your house. If it starts a fire, it is not normally a life threatening issue, so they can reduce the amount of safeguards put in place, compared to your house.

They also have hundreds of years of experience in knowing about what they really need. In a house, you have to accomodate everything someone might do with what you put in place.
 

iwire

Moderator
Staff member
Location
Massachusetts
petersonra said:
It really doesn't.

I don't know why you say that.

If you forget about what may be added to a building the NEC calculations are overkill.

You could supply a dwelling unit with about 40% to 50& of what the NEC requires.

I do agree the power company does have a different set of circumstances to deal with.
 

CALIF_C10

Member
Location
N. California
Thanks for all the replies to my post, but I am still wondering how a "PO CO" calculates the loads to size a transformer for a particular service.

I called one of our local Electric Utility Designers, who is a very good friend, and told him of my situation and he told me he felt the existing transformer was not large enough to handle the future load. I am just wondering if there are any other Electric Utility Designers on board here that can verify this situation.

I am very aware that Utility Companies have free reign to do as they please. I found this out once when I had to install seven 5" conduits to feed a Hotel I was wiring and PG & E showed up and ran two runs of 1,000 mcm AL to feed a 1600 amp 3 phase 120/208Y service. I asked them why they only ran two runs of this after mandating I install 7 runs of 5" conduit. I was basically told, "None of your business neophyte". So yeah, I have tons of love for the PO CO guys.:mad:
 
CALIF_C10 said:
Thanks for all the replies to my post, but I am still wondering how a "PO CO" calculates the loads to size a transformer for a particular service.


Give me a service type and I will tell you how I do it.

residential,small commercial,industrial???? 100 amp,1000 amp,3000 amp????

Tell me what you want and I will give you the way that I do it, not that it is the right way but it works.
 

CALIF_C10

Member
Location
N. California
gobblerhuntr said:
CALIF_C10 said:
Thanks for all the replies to my post, but I am still wondering how a "PO CO" calculates the loads to size a transformer for a particular service.


Give me a service type and I will tell you how I do it.

residential,small commercial,industrial???? 100 amp,1000 amp,3000 amp????

Tell me what you want and I will give you the way that I do it, not that it is the right way but it works.


It is a commercial service. 480/277 Volt Wye System. Anticipated load is 2400 amps. I hope this is what you are looking for.
 
2400 amp 277/480

First I am going to ask someone that is responsible for payment of the monthly bill, owner preferrably, to submit me a letter requesting service for "x" amount of KW per month. With this letter I will do a couple of things, first I will set a power contract based on this amount which figures into the monthly bill each month, second I have just let you size what transformer/transformers go with your service. If you require the full 2400 amps which would be about 2000 kva you would request that in the letter and I would make sure you had the full 2000 at your disposal. I try and convince people to take a good look at their loads and figure diversity into their proposal cause the contract plays such a part in the bill especially if they don't use more that 30 per cent of the contracted amount. After that we go through the questions of what size wire and how many conduits will have to be run to the transformers.

A lot of the sizing just comes from plain ol experience, I have bout 20 years in this and the people that I worked with when I started had 35 years in and you can learn the most from experience. This is a quickie on how it is done here, any more questions just ask. It is basically simple but I didn't go into too much detail on specifics I just touched the outside.

Sizing other sizes and classes are different than this.
 

CALIF_C10

Member
Location
N. California
I was looking for something like this: A given utility transformer is 1500 KVA rated. Multiply 1500 X 1000=1,500,000 VA. Divide that by the voltage of 480. Then divide that by the square root of three. And that will give you what the approximate available amperage is of that particular transformer.

No offense, but I have no idea what mumbo jumbo you are talking about.:confused:
 
CALIF_C10 said:
I was looking for something like this: A given utility transformer is 1500 KVA rated. Multiply 1500 X 1000=1,500,000 VA. Divide that by the voltage of 480. Then divide that by the square root of three. And that will give you what the approximate available amperage is of that particular transformer.

No offense, but I have no idea what mumbo jumbo you are talking about.:confused:


No offense taken, I went back to your post 10 where you asked about the calculations the PO CO makes. I do just that but I do those calculations based on what the customer tells me they want. Most customers oversize their gear based on NEC requirements. We are NESC based and don't have to know what internal codes are that often. But if I sized all our services based on what the customer was putting in my company would go broke. We have people put in 2000 amp services and pull 40% of that, full load. There would be a lot of waste on my part to size the transformer for the total gear size. I would have larger loss numbers and waste money on all equipment involved.

Sorry I didn't catch what you wanted with the formula thing, but that formula will apply to any transformer not just the PO CO ones. I just thought you wanted to know the method the PO CO uses to size pots. All that mumbo jumbo I typed is pretty much restricked to our region of the country I guess.
 

e57

Senior Member
CALIF_C10,

Without angering yourself, you must always sit back and say this to yourself before any dealing with PG&E. "P stands for pemidonna."

Now that we have that out of the way... Sometimes its better to say it twice... :rolleyes:

The way I understand the mysterious working of thier beurocracy is something like this:

You do a load calc, come up with a certain KVA, submit your request for that service of that KVA. Then some rusty gears go into motion.....
  • Several branches of the same office pass your paperwork around.
  • One of them decides that they are responcible for it.
  • It ends up on the desk of some Engineer.
  • They send a Service Rep to see you, you discuss a few things about what they are requiring...
  • The Engineer rubber-stamps it.
  • An accounting office gets involved, and makes sure every i, and t is double crossed and dotted.
  • A few more lies from the Service Rep...
  • One of thier Field Supers comes out with an Inpector to make sure everything is to thier liking. They sign some papers that you'll never see.
  • Your side of the work is done.
  • PG&E's crew shows up, has no clue what any Engineer had to say, and ignore most all of the requirements they made you go through, and install what-ever was put on the truck for them to install. So long as it has the ability to install itself. They test A-B-C, they go home.
Now at this point, if it doesn't work, or explodes, its thier problem. Then the process starts all over again. But with a slightly different path of people.
 
e57 said:
CALIF_C10,

Without angering yourself, you must always sit back and say this to yourself before any dealing with PG&E. "P stands for pemidonna."

Now that we have that out of the way... Sometimes its better to say it twice... :rolleyes:

The way I understand the mysterious working of thier beurocracy is something like this:

You do a load calc, come up with a certain KVA, submit your request for that service of that KVA. Then some rusty gears go into motion.....
  • Several branches of the same office pass your paperwork around.
  • One of them decides that they are responcible for it.
  • It ends up on the desk of some Engineer.
  • They send a Service Rep to see you, you discuss a few things about what they are requiring...
  • The Engineer rubber-stamps it.
  • An accounting office gets involved, and makes sure every i, and t is double crossed and dotted.
  • A few more lies from the Service Rep...
  • One of thier Field Supers comes out with an Inpector to make sure everything is to thier liking. They sign some papers that you'll never see.
  • Your side of the work is done.
  • PG&E's crew shows up, has no clue what any Engineer had to say, and ignore most all of the requirements they made you go through, and install what-ever was put on the truck for them to install. So long as it has the ability to install itself. They test A-B-C, they go home.
Now at this point, if it doesn't work, or explodes, its thier problem. Then the process starts all over again. But with a slightly different path of people.



Pretty good description
 

bwyllie

Senior Member
Location
MA
gobbler,

is there any particular "magic number" such as a w/sf that you would use for a commercial property to size the transformer or is it typically a percentage number of the NEC calculated load, which we all know to be oversized?

Thanks
 
bwyllie said:
gobbler,

is there any particular "magic number" such as a w/sf that you would use for a commercial property to size the transformer or is it typically a percentage number of the NEC calculated load, which we all know to be oversized?

Thanks


w/fs??? No magic number most all cases are different dependant on what the load is i.e. grocery store, video store, McDonald's, or factory. Goes back to that experience thing and paying attention to the billed load even months after the service was hooked up.
 

bwyllie

Senior Member
Location
MA
w/sf is watts per square footage. That is a number that we typically express for cosumption or demand of a building, we use this in the beginning phases of a project when someone needs a quick size or if we review an existing building for an owner. Typically a DESIGN load is 12-15 w/sf for commercial space but the actual DEMAND load is 6w/sf.
 
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