Utility owned service transformer question

[netaguy]

Ok, I have seen this 3 times in one week so now I am questioning this.

I was called by a mechanical contractor to look into supplying 1000A of temporary power to a roll up chiller unit because an old gas absorber failed at a large department store.

The customer says I have a 4000A service, plenty of availibility. Sure enough, 4000A 480/277V switchboard, 4000A main, 1033 amps on the IQ meter.

In the parking lot sits the utility owned oil transformer on a pad. 1500KVA label right on the front of it. Sorry, not going to happen anytime soon. I tell them apparently the utility company knows that this store would never need a 4000A service and installed a 1500KVA.

Now, last weekend during a utility outage at an office building about 15 years old I see something typical. The incoming at the switchboard there are (10) sets of 4 - 600MCM Cu. The utility company owned oil transformer on the pad is opened and there are (6) sets of 500MCM AL. Once again, 1500KVA 13.8kV to 480/277V rated transformer.

In between there is a pad mount collector box, (never seen this before, but I hear it is common in this area) sort of an electric demarc. The (10) sets done by original contractor out to collector, the (6) sets to transformer done by utility.

Now I am curious. I check the main 4000A breaker and it is set at 100%.

My question is how are the transformer secondary conductors protected properly? They are in PVC underground duct bank to the switchboard but how are they protected if the main breaker is set over 2X the transformer output rating?

BTW - this is typical on a building further up the street with a 1000KVA transformer and a 3000A main breaker/switchboard.

Thanks for your help.

 

[nakulak]

they aren't protected by NEC rules

 

[Leestarkweather]

they aren't protected by NEC rules

The NEC doesn't apply to Utility Owned Transformers, Primaries and Secondaries. What you see is very common in many installations--the service is sized with NEC Calcs, plus maybe a good E.C. explained to the Owner the value of having some (or a lot) of spare capacity when the service was put in. The POCO gets the building load info before they hook up, and apply their own demand percentages to each type of load.

Remember, the POCO cannot bill the customer for the core losses of their xfmrs. that are oversized, so they undersize them, and only increase if the loads grows. We always notify the POCO if there is any significant (more than 10%) load being added after the first buildout.

I've only seen one job in 30 years that had the "elec. demac" point, and it was in an area where the POCO required the UG service laterals to be run and paid for by the customer to a certain point, then the POCO picked up from that point and ran their own (per their sizing) into their XFMR.

Makes a fella wonder--when only 5 or 6 laterals go from xfmr. to "demarc" and 10 go into UGPS at service--what is crimped to what/how many?

I often open an UGPS to verify number of laterals installed and find 4 of 6 or 6 out of 10 UG PVC runs with conductors, and the balance spares for load growth.

In response to the first question--he had better get the POCO involved, cause that 4000A service doesn't mean he has 4000A utility capacity to the building.

 

[imaee]

Remember, the POCO cannot bill the customer for the core losses of their xfmrs. that are oversized.

Yes they can. And do.

 

[augie47]

In our area, you can build as large a service as you desire, POCO will size their transformer & wiring according to LOAD, not service size. If you provide incorrect load data and they upsize accordingly, they will penalize you.
If and when they see their transformer reaching capacity they will discuss changing it.
I have often seen 1200-2000 amp services on a 500 kva or less.

 

[mivey]

Not usual to see transformers at 40% of service size here. Some also have a contract minimum demand.

 

[netaguy]

Thanks for the replies. Good info here. Makes perfect sense about the utility not being able to capture/bill for the transformer losses plus save on the cost of furnishing a 2500KVA.

I will relay this info to the owner of one of the buildings who is trying to lease out the entire bldg to one prospective bio-tech tenant with the understanding that the bldg has full 4000A capacity.

Thanks again!

 

[iwire]

I will relay this info to the owner of one of the buildings who is trying to lease out the entire bldg to one prospective bio-tech tenant with the understanding that the bldg has full 4000A capacity.

It does have 4000 amps of capacity, it is not just the fact the POCO bases the equipment on the actual load the power company will also often run their transformers above the rated capacity, they will often run the conductors at much higher temps then the NEC would allow us.

It just an entirely different way of doing things and it works for the power company. You can not in anyway compare how the utility sizes things to how we size things for NEC applications.

For instance the primary side overcurent protection of that transformer is probably many times above what the NEC would allows us to do.

If any of this fails they will be out to replace it, to the NEC you have a 4000 amp service.

 

[charlie]

I agree with just about everything said in the answers. We do size only for the actual projected load, we will replace the transformer or service lateral if they are undersized, we do not normally protect our transformers but we do protect the line that feeds the transformer (it keeps the rest of the neighborhood on), and you do have the full use of your service that is installed (we also want to know when you are adding anything major to it).

We do not charge directly for transformer losses but those losses are included in the electric bill. If we were to meter the primary instead of the secondary but charge the secondary rate, we have to give a credit back for the transformer losses. An example is where we installed a pole mounted transformer bank that was fed with underground cable to some pumps in a well field and was in a flood zone (it flooded at least once a year.) Instead of having meters in that area and sometimes under water, we installed metering on the primary that was located about ? mile away and out of the flood zone.

 

[quogueelectric]

Ok, I have seen this 3 times in one week so now I am questioning this.

I was called by a mechanical contractor to look into supplying 1000A of temporary power to a roll up chiller unit because an old gas absorber failed at a large department store.

The customer says I have a 4000A service, plenty of availibility. Sure enough, 4000A 480/277V switchboard, 4000A main, 1033 amps on the IQ meter.

In the parking lot sits the utility owned oil transformer on a pad. 1500KVA label right on the front of it. Sorry, not going to happen anytime soon. I tell them apparently the utility company knows that this store would never need a 4000A service and installed a 1500KVA.

Now, last weekend during a utility outage at an office building about 15 years old I see something typical. The incoming at the switchboard there are (10) sets of 4 - 600MCM Cu. The utility company owned oil transformer on the pad is opened and there are (6) sets of 500MCM AL. Once again, 1500KVA 13.8kV to 480/277V rated transformer.

In between there is a pad mount collector box, (never seen this before, but I hear it is common in this area) sort of an electric demarc. The (10) sets done by original contractor out to collector, the (6) sets to transformer done by utility.

Now I am curious. I check the main 4000A breaker and it is set at 100%.

My question is how are the transformer secondary conductors protected properly? They are in PVC underground duct bank to the switchboard but how are they protected if the main breaker is set over 2X the transformer output rating?

BTW - this is typical on a building further up the street with a 1000KVA transformer and a 3000A main breaker/switchboard.

Thanks for your help.

I have questioned similar yet smaller the poco guy said you just worry about your end and dont let our xformers worry that pretty little head of yours. It never blew but i was cringing about it for 6 years.