Does anyone know of any guidelines used by utilities for sizing their distribution transformers? When sizing a transformer per the NEC it is straight forward but utilities tend to overload their transformers. I am trying to get some information as to how they decide on the size of the transformer. I know it will depend on the type of load - residential, commercial, industrial.
transformer sizing
- Thread starter bsh
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ohmhead
Senior Member
- Location
- ORLANDO FLA
Well on most jobs we do the electrical engineer designs the project on the oneline riser the size of the utility transformer is by electrical plans thats on new construction and the power company installs per our plans .
Heres a example of a common riser 4000 amp SWBD fed by a 2500 kva utility trans.
Thats normally what we see on a electrical plan .
When we start a new project we have a sit down meeting with the power company they take our electronic files of our oneline and there field engineer uses it to size the project the only thing they do is up size never down size and use our plans for location so they can plan there work to us .
Then they send us all there pad layouts and give us there rules on how many stub ups are allowed in one transformer what type of lugs they need and copper or aluminum wire what size the transformer pad is to be and clearance and space per power company .
We get a oneline from them and run there primary conduits and our secondarys we install per there plans they pull there wire only and make up the transformer primary grounding on primary/ sec .
We do the rest
Heres a example of a common riser 4000 amp SWBD fed by a 2500 kva utility trans.
Thats normally what we see on a electrical plan .
When we start a new project we have a sit down meeting with the power company they take our electronic files of our oneline and there field engineer uses it to size the project the only thing they do is up size never down size and use our plans for location so they can plan there work to us .
Then they send us all there pad layouts and give us there rules on how many stub ups are allowed in one transformer what type of lugs they need and copper or aluminum wire what size the transformer pad is to be and clearance and space per power company .
We get a oneline from them and run there primary conduits and our secondarys we install per there plans they pull there wire only and make up the transformer primary grounding on primary/ sec .
We do the rest
Last edited:
- Location
- Illinois
- Occupation
- retired electrician
The utilities know that the load calcs in Article 220 are very conservative and size the transformer on what they expect the load to actually be. it doesn't matter what size transformer they use as they are responsible for providing power to the building. Our service conductors have to be sized based on the code load calculations and will often have an ampacity of 2 to 3 times the rated current of the utility transformer.
mivey
Senior Member
For common loads, we build data tables that tell us about how much demand we can expect. These tables account for load type, AC size, diversity, varying weather zones, etc. The results are roughly 40-60% of the size we see at most customer disconnects.
We may run a transformer up to 110-115% or more of its rating for peak load, depending on the loading cycle. If we expect growth, the initial loading may be more like 70%. We usually don't peak load more than about 150% of the transformer rating. They can handle 200% for short periods (even 300% for very short periods), depending on the pre-loading.
We then use our loading data to generally keep the drop less than about 5% for starting and 3% for continuous when combined with the drop.
We have noticed that the older transformers were able to handle a LOT more overload.
We may run a transformer up to 110-115% or more of its rating for peak load, depending on the loading cycle. If we expect growth, the initial loading may be more like 70%. We usually don't peak load more than about 150% of the transformer rating. They can handle 200% for short periods (even 300% for very short periods), depending on the pre-loading.
We then use our loading data to generally keep the drop less than about 5% for starting and 3% for continuous when combined with the drop.
We have noticed that the older transformers were able to handle a LOT more overload.
- Location
- Massachusetts
Mivey already covered it but I was going to point out that transformers do not instantly catch fire if you pass the rated capacity. 
If the power company had to size all their equipment under the NEC rules everyone of us would be paying higher electric rates.
No surprise there, just the cheapening of everything. I bet the weight has dropped as well.
If the power company had to size all their equipment under the NEC rules everyone of us would be paying higher electric rates.No surprise there, just the cheapening of everything. I bet the weight has dropped as well.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Gee, doing electrical work in compliance with the NEC costs more.
mayanees
Senior Member
- Location
- Westminster, MD
- Occupation
- Electrical Engineer and Master Electrician
As an engineer, if I do a design for, say a pumping station, I will perform a Transient Motor Starting (TMS) Analysis on the system in its worst-case scenario to determine the worst-case voltage drop (VD) using SKM PowerTools software.
I will then indicate on the print what I recommend as the minimum size transformer to adequately serve the load.
But, as mentioned in the earlier posts, the Utility provider will install whatever they see fit, knowing that if there ends up being a problem, they can just replace it with the next size up.
I did a pumpstation once that had two (2) 480 V, 75 HP across-the-line started motors. The secondary cable was approximately 250 feet long. I recommended a minimum size of 500 kVA, based on one motor running, and the other one coming on-line.
The Utility installed a 300 kVA transformer, which in the words of the Utility engineer, has been what he's done for years. In this particular case, the motor circuit protectors tripped on startup. I was called out as having undersized the secondary feeder.
So I re-ran my SKM TMS analysis, paranoid as hell that I had screwed up and would be liable for an upsize of the secondary feeder. I increased the number of conductors by a factor of 10, which of course is easy to do in the program. The starting VD was only negligibly improved, so I rested easy that the Utility transformer was undersized.
I was called to task by the Civil engineering firm that I did the design for, so I faxed 10 pages of my TMS analysis to the Utility, defending the selection of a 500 KVA unit. They reluctantly increased the size of the transformer , not to 500 kVA but to 750 kVA since that's all they had in stock and apparently the footprint for the 300 is the same as the 500 and 750.
It resolved the problem.
A conservative criteria to use for the VD is the article 695 fire pump limits which, off the top of my head, is a maximum 15% VD on startup.
A rule of thumb I developed from that experience is that the transformer shouldn't be saturated much past maybe 150% of it's rating during startup. In this case that means 75kVA (running motor) plus 6-8 times 75kVA (starting motor) = 525-675 kVa.
John M
I will then indicate on the print what I recommend as the minimum size transformer to adequately serve the load.
But, as mentioned in the earlier posts, the Utility provider will install whatever they see fit, knowing that if there ends up being a problem, they can just replace it with the next size up.
I did a pumpstation once that had two (2) 480 V, 75 HP across-the-line started motors. The secondary cable was approximately 250 feet long. I recommended a minimum size of 500 kVA, based on one motor running, and the other one coming on-line.
The Utility installed a 300 kVA transformer, which in the words of the Utility engineer, has been what he's done for years. In this particular case, the motor circuit protectors tripped on startup. I was called out as having undersized the secondary feeder.
So I re-ran my SKM TMS analysis, paranoid as hell that I had screwed up and would be liable for an upsize of the secondary feeder. I increased the number of conductors by a factor of 10, which of course is easy to do in the program. The starting VD was only negligibly improved, so I rested easy that the Utility transformer was undersized.
I was called to task by the Civil engineering firm that I did the design for, so I faxed 10 pages of my TMS analysis to the Utility, defending the selection of a 500 KVA unit. They reluctantly increased the size of the transformer , not to 500 kVA but to 750 kVA since that's all they had in stock and apparently the footprint for the 300 is the same as the 500 and 750.
It resolved the problem.
A conservative criteria to use for the VD is the article 695 fire pump limits which, off the top of my head, is a maximum 15% VD on startup.
A rule of thumb I developed from that experience is that the transformer shouldn't be saturated much past maybe 150% of it's rating during startup. In this case that means 75kVA (running motor) plus 6-8 times 75kVA (starting motor) = 525-675 kVa.
John M
mivey
Senior Member
You better believe it. An old 15 kVA might be the size of a modern 150 kVA.
Have you ever noticed an old 1 HP motor might need a replacement bigger than 1 HP?
or they may tell you to soft start or put in a bigger feeder.
mayanees
Senior Member
- Location
- Westminster, MD
- Occupation
- Electrical Engineer and Master Electrician
true, true.
In fact, what I should have mentioned is that Utilities will have limits as to what size motor can be started across-the-line (ATL). 75 HP was a large motor to be permitted for ATL starting.
But in this case the Utility provider was also the owner of the pumpstation, and there wasn't enough room in the building for anything but ATL. It was an upgrade to a 50 year-old pumpstation.
John M
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