Undersized service conductor for load

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gerardin

Member
In 1993 I installed an underground feed 800 amp three phase 4 wire 120/240 volt open delta service feeding a 10,000 sq. ft. commercial building.
Our utility installed pole mount transformers with wires coming down the pole to an underground splice box at the base of the pole. From this splice box I was required to install two 4" (PVC) conduits to feed the 800 amp main. The main has a Utility pull section, a section containing a meter with a 400 amp 3 phase main feeding (1) 200 amp 3 phase breaker and (1) 125 amp single phase breaker, and a section with (4) 200 amp 3 phase meters feeding breakers and panels within the building. This was the installation in 1993.
14 years later I am informed that the Utility conductors to the building are 350 MCM AL and are over heating due to load.
The Utility informs me that an 800 amp service does not mean that they provide the conductors for an 800 amp service.
I've talked to two electrical engineers, one says they should have pulled the correct conductors at first installation, the second says I should have known what wires they installed and it's original limitations.
I have a couple of questions...
(1) Is this common practice?
(2) The Utility informs me that they do not do 800 amp pole mount transformers and I will have to re-trench through a concrete parking lot to supply a pad mount transformer. Shouldn't this have been their engineering spec from day one?
(3) Who should pay for all this?

Thanks,
gs
 

Jljohnson

Senior Member
Location
Colorado
In the area that I live and work in, one-line diagrams would have been provided to the utility prior to permits being issued. The utility would do their own engineering and decide what conductors to install to the line side of the service. We would then do the installation of said conductors. Bottom line is the utility would be the responsible party for sizing the conductors on the LINE side of the metering equipment, I am responsible on the LOAD side. I have seen the utilty install 4/0 Aluminum conductors to the line side of a 600 amp service on a strip mall. The reply when questioned was " We are not under NEC rules. We are governed by our engineering staff and they say the conductor size is fine for the calculated load". I do not have a problem with this so long as they own up to it if there are ever problems. So far, I have never had an issue like the one you describe.
 

iwire

Moderator
Staff member
Location
Massachusetts
gerardin said:
The Utility informs me that an 800 amp service does not mean that they provide the conductors for an 800 amp service.

That is entirely correct, they are not bond by the NEC in anyway, not by NEC load calcs, not by NEC ampacity tables.

I've talked to two electrical engineers, one says they should have pulled the correct conductors at first installation,

They did by the engineering done at the time, it appears the actual connected load has increased since the time the engineering was done.

one says I should have known what wires they installed and it's original limitations.

In my area the size of the utility owned conductors are never my (the electricians) concern.

couple of questions...
(1) Is this common practice?

Yes.

(2) The Utility informs me that they do not do 800 amp pole mount transformers and I will have to re-trench through a concrete parking lot to supply a pad mount transformer. Shouldn't this have been their engineering spec from day one?

They do not do 800 pole mounts in my area either.....however in my opinion it was their mistake and additional costs should be theirs as well. (They may go with 800 on a pole if they are forced to pay for the pad and trench)

(3) Who should pay for all this?

In my area the customer would have paid for this from the get go had it been required originally but at this point in my opinion the utility needs to pay for the work.

They are not going to volunteer to do it, you should probably try to get utility commission involved.

You asked for an 800, your 800 is not tripping so I assume you are still under 800 amps. A recording meter may help your case but it could also hurt your case if you are actually pulling over 80% continuous or over 800 for any amount of time.

Good luck.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Was a load calc done at the time of the original construction?

If so, and the load has increased, it's the customer's problem.

If so, and the load has not increased, it's the POCO engineer's problem.

If not, who specified the 350MCM al sizing way back when?
 

mdshunk

Senior Member
Location
Right here.
As the good POCO engineers retire, and they're replaced with cheaper LED's ("designers"), this is going to be more and more of a problem. I don't know if any of you all read any magazines related to power transmission and distribution, but there's a serious engineer shortage and the bulk of them are due to retire within a single decade.
 

72.5kv

Senior Member
You could have not said that any louder Mdskunk. I am a recent graduate EE. One thing I notice is its true there is a shortage of Power engineers and the schools are cutting power classes. Poco are short staffed from engineers to linemen.

Yes it is common practice to Poco engineers to size transformers and conductors based on the load not the service size. I was told by a distribution engineer he would have no problem feeding with 200 amp residential with 10KVA transfomer, but to avoid excessive dimming when the AC kick in, he spec a 15KVA.
 
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iwire

Moderator
Staff member
Location
Massachusetts
72.5kv said:
I was told by a distribution engineer he would have no problem feeding with 200 amp residential with 10KVA transfomer, but to avoid excessive dimming when the AC kick in, he spec a 15KVA.

There are legitimate reasons for this.

The power company has a long history of what particular occupancies will actually draw for a load. The NEC load calcs result in a service that is usually much larger then needed.

Lets say the power company did base all their work on the total of the NEC load calcs. We would all end up paying much more as larger conductors and transformers would be installed when they are really not needed.
 

72.5kv

Senior Member
The rule of thumb an engineer told me is 30 percent is for calculated service for a residential and 50-60 percent for commercial. Not uncommon to see 8 homes on a 75KVA
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
What makes you think the utility conductors are overheating? And what problem is it causing?

The utility (not being bound by the NEC) may still think everything is OK the way it is.

Even if there is a problem, the utiltiy probably won't upgrade to teh full 800A. Can they get by with just increasing the wire size? Can they up the pole mounted transformer any at all.

This has worked for 14 years, sounds to me like the utility did OK. Not great, but OK.

Steve
 

gerardin

Member
The utility says they no longer have that load calc in their files, and I do not have it either. The Utility dictates what size conductors are installed.
 

gerardin

Member
LarryFine said:
Was a load calc done at the time of the original construction?
I don't have the original calc, and the Utility says they don't either.

LarryFine said:
If not, who specified the 350MCM al sizing way back when?
The Utility.
 
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gerardin

Member
steve66 said:
What makes you think the utility conductors are overheating? And what problem is it causing?

I smelled burning at the service. After checking all the "customer side" of the service, I called the Utility. The serviceman unlocked the underground pull section, with an infrared thermometer I checked the lugs (good) then aimed the beam down to the conduit/wires exiting the slab. 200 degrees. The conductors amped at 500-600-100 (stinger leg). The Utilities advice was to shed load.

The utility (not being bound by the NEC) may still think everything is OK the way it is.

Even if there is a problem, the utiltiy probably won't upgrade to teh full 800A. Can they get by with just increasing the wire size? Can they up the pole mounted transformer any at all.

As a solution, I requested to pull parallel 600 MCM CU through the existing conduit for the 800 amp service, but there seems to be pole transformer/pad transformer indecision with the Utility engineering.

This has worked for 14 years, sounds to me like the utility did OK. Not great, but OK.
 
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