75KVA Padmount transformer feeding 800A service

Johnny B.

Member
Location
Sandy Utah
Occupation
Electrical Engineer 1
Hey everyone,

I am doing a retrofit project out in Colorado I am trying to do a fault calculation required by city comments. Its a 200 amp service for the small store, but the disconnect going to the gutter is 800 amp disconnect. The transformer says 75 on it so they people out there told me its 75KVA. I know that does not make any sense 75KVA is way too low to power a 800A service. I called the power company Xcel and they said its 75KVA banked side. So I read this article: https://www.hammondpowersolutions.com/en/Resources/Connections/What-does-the-term-Banked-describe
I figure its a 225KVA transformer all together 75KVA for each single phase transformer. I just want to make sure I am correct on this since speaking to engineer representatives at the power company they made it sound like the transformer is just 75KVA which does not make any sense to me. Plus it says 75 on the transformer but it seems like it should say 225. I just want to make sure I do the fault calculation right.
 
Hey everyone,

I am doing a retrofit project out in Colorado I am trying to do a fault calculation required by city comments. Its a 200 amp service for the small store, but the disconnect going to the gutter is 800 amp disconnect. The transformer says 75 on it so they people out there told me its 75KVA. I know that does not make any sense 75KVA is way too low to power a 800A service. I called the power company Xcel and they said its 75KVA banked side. So I read this article: https://www.hammondpowersolutions.com/en/Resources/Connections/What-does-the-term-Banked-describe
I figure its a 225KVA transformer all together 75KVA for each single phase transformer. I just want to make sure I am correct on this since speaking to engineer representatives at the power company they made it sound like the transformer is just 75KVA which does not make any sense to me. Plus it says 75 on the transformer but it seems like it should say 225. I just want to make sure I do the fault calculation right.
Couple of things. First, it would not be unusual to have a 75KVA transformer (or bank total) feeding an 800A service. Power companies dont care about NEC service size, they go off what the actual load is. Most services are way oversized. Second, pad mounts will be a single unit for three phase.
Three phase banks made from pole mounts will be three single units. So something is off. You say padmount in the thread title, but then it sounds in the thread like this is a pole bank.
 

Johnny B.

Member
Location
Sandy Utah
Occupation
Electrical Engineer 1
Okay that makes sense.
It is a pad mount, its not coming from the pole. The people at the power company keep telling me that it is 75KVA bank side, but sometimes they just repeat things they are told. Or maybe they are actually looking at the pole feeding the underground service instead of the pad mount transformer I am not sure now.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector
Since it's their transformer they should give you the SCA avialable. I would not worry about the size transformer.
 

wbdvt

Senior Member
Location
Rutland, VT, USA
Occupation
Electrical Engineer, PE
Since it's their transformer they should give you the SCA avialable. I would not worry about the size transformer.
Most likely when you first ask for available short circuit current, you will be given the infinite bus short circuit current as your first encounter will be with a customer service rep who will look up the size of the transformer at your service address and then refer to a table of short circuit currents by transformer size and secondary voltage. This can easily be confirmed by a quick calculation. A customer service rep will not have the available short circuit current, you will need to get that from the engineering dept.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Seems to me that most utilities have charts that will tell you what the short circuit current will never exceed. This has nothing to do with infinite bus current. I don't know how they come up with it but it's probably based on what transformers they use and what they know the line impedance is behind the transformers.
 
Most likely when you first ask for available short circuit current, you will be given the infinite bus short circuit current as your first encounter will be with a customer service rep who will look up the size of the transformer at your service address and then refer to a table of short circuit currents by transformer size and secondary voltage. This can easily be confirmed by a quick calculation. A customer service rep will not have the available short circuit current, you will need to get that from the engineering dept.
In my experience it is very unlikely one would be able to get actual SCC. I can't see POCO investing engineering resources for most small projects, at least they never have for me.
 

kwired

Electron manager
Location
NE Nebraska
Is is just one padmount enclosure or multiple units. I have seen "banked" padmounts particularly for lower capacity open delta three phase service. Most were existing for many years and the POCO woudln't build new one like that today though.

Have also seen, though rarely, where they use pole top type transformers but put them inside an enclosure mounted on grade. Most those also been in place for years and POCO would not likely build it today.
 

wbdvt

Senior Member
Location
Rutland, VT, USA
Occupation
Electrical Engineer, PE
In my experience it is very unlikely one would be able to get actual SCC. I can't see POCO investing engineering resources for most small projects, at least they never have for me.
I have been doing studies since 2004 on a part time basis and full time since 2013 and have had very few problems in obtaining the available short circuit data at the point of connection. It can take some work sometimes with a utility and at one utility, I had to get the PUC involved and I did finally get the available short circuit data. I figured it was going to be an effort with this one utility when they said "the available short circuit data was proprietary information." That was a head scratch-er there, for sure.
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
In my experience it is very unlikely one would be able to get actual SCC. I can't see POCO investing engineering resources for most small projects, at least they never have for me.
I don’t understand why...
Our system, like all others is modeled on computer software like Windmill. The stations, wire, impedances and transformer information Is in the model.
All I have to do is click a transformer on the system and the Fault current comes right up. Takes me all of about 5 minutes to tell you what it is at our transformer. Heck, another 5 minutes with your information the model can spit it out at your switchgear..
 
I don’t understand why...
Our system, like all others is modeled on computer software like Windmill. The stations, wire, impedances and transformer information Is in the model.
All I have to do is click a transformer on the system and the Fault current comes right up. Takes me all of about 5 minutes to tell you what it is at our transformer. Heck, another 5 minutes with your information the model can spit it out at your switchgear..
Well please go over to Seattle city light and PSE and straighten them out.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
True story:
I once had almost the same situation for a pump station with 2 x 300HPpumps on VFDs and Soft Starters on a 1000A service. The VFDs started up fine, but when we attempted to start the 2nd on a soft starter, the transformer fuses blew. I went out to the utility cage and saw what I thought was too small of a pad mount transformer. Eventually the utility tech showed up to replace the fuses on the transformer and when the door was opened, I saw that it said 75kVA. I questioned it, he said "we have different rules". Although I know that's true, they do NOT have different physics and you cannot get 1000A from a75kVA transformer regardless of your rules. Long story shorter involving multiple arguments and trips to the site, eventually the utility admitted that it was supposed to be a 750kVA transformer...
 
I have been doing studies since 2004 on a part time basis and full time since 2013 and have had very few problems in obtaining the available short circuit data at the point of connection.

Well you and HV&LV can have a lunch date and then show SCL and PSE the ropes. Believe me I would love to get actual data rather than these ridiculous numbers that are 150-300% above infinite bus value.
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
Well you and HV&LV can have a lunch date and then show SCL and PSE the ropes. Believe me I would love to get actual data rather than these ridiculous numbers that are 150-300% above infinite bus value.
The biggest problem is they are scared of liability issues should they give you a number.
mine are always a little high just in case we change a transformer for the next size up...
But never 300%
 
The biggest problem is they are scared of liability issues should they give you a number.
mine are always a little high just in case we change a transformer for the next size up...
But never 300%
Here is one. This is the email from the POCO:

" The table I’m looking at shows a max short circuit current of 37,600 Amps at the transformer for a 3-phase 500kVA 480/277 V transformer. "

Calculated off dataplate using infinite buss: 11,558

Here is another one:

" The fault current at the secondary spades of the transformer is: 111,000a."

Calculated off date plate using infinite buss: 77,105.

Its just really annoying. Yeah I concur providing some extra for next size up or lower impedance down the road is prudent, but some of these values are out of control.
 

mbrooke

Batteries Not Included
Location
United States
Occupation
Electricity
The biggest problem is they are scared of liability issues should they give you a number.
mine are always a little high just in case we change a transformer for the next size up...
But never 300%

Right, but often you need to know the exact number. Higher than actual numbers can increase the arc flash energy.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I could see a POCO saying something along the lines of 'your service equipment is required to tolerate a short circuit current of xxx to permit us to change our equipment' even when xxx is greater than the available short circuit current of the existing utility transformer is lower than xxx.

I know that if the available short circuit current is reduced this can increase fault clearing time and increase the arc flash energy, but I am missing something on the physics of this. Does fault current into an 'arc event' always climb to the maximum available, or do different fault events have different impedance. If arc events can have different impedance, then fault current could be less than the maximum available, again delaying clearing time and increasing arc flash energy.

Thanks
Jon
 
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