# Help understanding commercial "line drop"



## carter840 (Jul 1, 2014)

Hi all, 

I am somewhat new to electric work, but have a degree in Mechanical Engineer and certainly have quite a background in the science of electricity. I was wondering if any of you could help explain how service starts in a commercial building. I assume that most commercial buildings do not have the typical line drop after a transformer that feeds in 240V. I am guessing that commercial buildings require transformers either supplied by the utility company or themselves (not sure which) and then a bunch of medium voltage switchgear, that likely leaves some voltage high for heavy equipment and sends others to separate transformers. 

Would anyone mind walking me through how electricity enters a commercial building and how it is distributed once inside? I know it will vary based on the building's load, but assuming a medium sized office building with a few elevators and large HVAC unit. Any idea on what voltage enters such a building and how it is passed?

Also don't worry I am not about to undertake any of this work, I'm just trying to learn about an area of building facilities that I currently don't understand. 

Thank you


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## wildleg (Apr 12, 2009)

1) the building is engineered (new or retrofit) and the engineer does a loadcalc, or if simple the electrician does a loadcalc and submits to the power company and ahj for permits, usually the owner submits the application for service to the power company (because fees are involved).  If the engineer or the electrician are smart, they contact the power company 1st to see what's readily available.

2) the power company lets the customer know if what they want is available, or what they can give them, and what it will cost, and what the customer needs to build. Often the power company will provide exactly what's needed, other times not. the customer needs to provide the rest. For simple commercial office/retail buildings in my area, the customer provides primary and secondary duct banks, power company provides up to one transformer and usually feeders to customer's gear. For large commercial and industrial, the poco just provides medium primary and the customer does all the rest. It's really up to the customer to figure out (using a good engineer/electrician) what will end up being cheaper for the customer, and going in that direction.

3) build it

hope this helps


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## carter840 (Jul 1, 2014)

wildleg said:


> 1) the building is engineered (new or retrofit) and the engineer does a loadcalc, or if simple the electrician does a loadcalc and submits to the power company and ahj for permits, usually the owner submits the application for service to the power company (because fees are involved). If the engineer or the electrician are smart, they contact the power company 1st to see what's readily available.
> 
> 2) the power company lets the customer know if what they want is available, or what they can give them, and what it will cost, and what the customer needs to build. Often the power company will provide exactly what's needed, other times not. the customer needs to provide the rest. For simple commercial office/retail buildings in my area, the customer provides primary and secondary duct banks, power company provides up to one transformer and usually feeders to customer's gear. For large commercial and industrial, the poco just provides medium primary and the customer does all the rest. It's really up to the customer to figure out (using a good engineer/electrician) what will end up being cheaper for the customer, and going in that direction.
> 
> ...


That does help, but I'm hoping to dig a bit deeper. I understand that scope of supply might vary and that input voltage might vary but I guess I am trying to understand what equipment is typical in a commercial building. So when you get the higher voltage in a commercial building how does it travel to it's end uses? Does the power coming in first go through a bunch of medium voltage switchgear? Then various transformers change voltage to meet the needs of certain equipment? Essentially I'm looking to understand what the components are in layman's terms to a commercial buildings power supply. 
Thanks for the help


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## macmikeman (Jan 23, 2007)

Carter when feet meet the pavement , understanding comes up thru the soles . 



go out and look. You will learn plenty on the first day.


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## carter840 (Jul 1, 2014)

macmikeman said:


> Carter when feet meet the pavement , understanding comes up thru the soles .
> 
> 
> 
> go out and look. You will learn plenty on the first day.



I'll actually be doing just that tomorrow morning, but since I don't have any building owner contacts I will not have much access to see what equipment is needed in a commercial building. I was just hoping to get a bit of a primer on here so I can continue to learn. I completely agree that in the field is the best place to further my understanding.


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## wildleg (Apr 12, 2009)

unfortunately, there are so many variations of what you ask, that answering your question is impossible. Even though there are an enormous number of "typical" installations, every building is essentially a build to suit custom installation. Pocos have a green book for the most common of their services, but customers all want different stuff, and services are sized to suit. Building components meet the requirements of the customers.


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## Nobaddaysinak (Jan 17, 2012)

What size building are you fishing for?


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## carter840 (Jul 1, 2014)

I have no particular size, I'm really just looking for a high level explanation of the type of equipment required in a larger building. So for example a 15 story commercial office building with 4 elevators. I'm just trying to get an understanding of how many stages of step down there tend to be (how many large electric products do you need and what are they). I know that voltage to a building like this (and supplied amperage) will vary, but is there a typical range for the type of building I'm describing. Say 240-600 or maybe it's more like 600-7,200 volts. Sorry to be dense, but I am just looking for a basic understanding.


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## CopperSlave (Feb 9, 2012)

Around here, most commercial buildings you will encounter will have a 480/277V service at the most. Usually the HVAC units (and any other larger loads, such as elevators) are 480V and lighting will be 277V.

From there, any step-downs would be done for general-purpose receptacles and things like cash registers, vending, etc. 

Most of the time, you will only encounter medium voltage in an industrial setting.


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## carter840 (Jul 1, 2014)

Nobaddaysinak said:


> What size building are you fishing for?





CopperSlave said:


> Around here, most commercial buildings you will encounter will have a 480/277V service at the most. Usually the HVAC units (and any other larger loads, such as elevators) are 480V and lighting will be 277V.
> 
> From there, any step-downs would be done for general-purpose receptacles and things like cash registers, vending, etc.
> 
> Most of the time, you will only encounter medium voltage in an industrial setting.


This is exactly the type of information I was looking for. Thank you! 
I noticed that there are a ton of companies who make products for commercial buildings of this size, Eaton, Legrand, Siemens, GE, and Schneider is there a reputation associated with each of these brands? What do you use and why? I assume that you all mix and match brands, but is there any reputation associated within industry on these brands?


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## Ultrafault (Dec 16, 2012)

That same Comercial building would have y208/120 step down transformers to feed general purpose recpts and 120 volt loads. Your lighting would probably be fed by 277 to ground of of the 480 panels. You would have high current switch gear with ground fault protection on a y system that big.

I have never wired a 15 story building but I would think that 5k distribution would be appropriate hopefully someone else will chime in. Maybe they will even have a single line diagram or two of some large buildings they would share.


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## hardworkingstiff (Jan 22, 2007)

I think the OP would benefit from buying an EE a few drinks and dinner.


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## hardworkingstiff (Jan 22, 2007)

How hard would it be for you to get your hands on a bid package for the kind of building you are talking about? That might answer a lot of your questions (the drawings and specifications).


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## theloop82 (Aug 18, 2011)

Usually a normal office setting would just have a large 480v (800-5000A) service with utility owned transformers in their vault (usually 12,470-13,800v) around here at least. Another scheme for buildings with larger loads or campuses would be customer owned vaults with medium voltage distribution throughout the building and multiple transformers. The upside of that type of system is you can cover a long distance with much smaller wire and have the 480v runs start closer to the point of utilization. The downside is you have to buy, test, and maintain medium voltage equipment which is a bit more specialized than some maintenance electricians are comfy with. 

Id say about 80 percent of large buildings i have worked in have utility owned transformers but the larger you get the more likely it is you may have customer owned vaults.


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## Big John (May 23, 2010)

theloop82 said:


> ...The downside is you have to buy, test, and maintain medium voltage equipment....


 Not true! They don't have to test or maintain it at all! 

They can just leave it in some dark hole somewhere that they can't even find the keys to the door and the fire department has to ax it open when the whole neglected mess finally explodes at 3AM on Tuesday and as that horrible burned-winding smell just coats everything in the building they can stand there and yell that this has to be repaired by eight o'clock because a very important client is showing up and this is just totally unacceptable and you want _how much_ to fix it!?!

That's a very popular option.


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## hardworkingstiff (Jan 22, 2007)

Big John said:


> Not true! They don't have to test or maintain it at all!
> 
> They can just leave it in some dark hole somewhere that they can't even find the keys to the door and the fire department has to ax it open when the whole neglected mess finally explodes at 3AM on Tuesday and as that horrible burned-winding smell just coats everything in the building they can stand there and yell that this has to be repaired by eight o'clock because a very important client is showing up and this is just totally unacceptable and you want _how much_ to fix it!?!
> 
> That's a very popular option.


Sounds like the voice of experience. :blink:


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## theloop82 (Aug 18, 2011)

Big John said:


> Not true! They don't have to test or maintain it at all!
> 
> They can just leave it in some dark hole somewhere that they can't even find the keys to the door and the fire department has to ax it open when the whole neglected mess finally explodes at 3AM on Tuesday and as that horrible burned-winding smell just coats everything in the building they can stand there and yell that this has to be repaired by eight o'clock because a very important client is showing up and this is just totally unacceptable and you want _how much_ to fix it!?!
> 
> That's a very popular option.


Okay they *should* maintain their equipment. Then again you should maintain 480v equipment too and that is even more rarely done


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## Wirenuting (Sep 12, 2010)

theloop82 said:


> Okay they *should* maintain their equipment. Then again you should maintain 480v equipment too and that is even more rarely done


If they maintained the gear then the EC won't get the premium pay on Super Bowl Sunday.


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## AK_sparky (Aug 13, 2013)

What is this "maintenance" word you guys speak of?...I have never seen it in the field....


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## carter840 (Jul 1, 2014)

So yesterday I got to tour a large university building that was newly constructed. I saw most of the high level switchgear and distribution gear and certainly have a better understanding of things. I guess one thing that confused me is the difference between switchgear, circuit breakers, and distribution panels. In some way they all seem to do that same thing, but just on different scales. Is that somewhat true. Would it be fair to say that distribution panels are essentially just larger circuit breakers?

Thank you


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## Big John (May 23, 2010)

Switchgear and switchboards and panelboards are all different enclosures for overcurrent protection.

Each one has different testing and listing standards. For something to be switchgear it must be compartmentalized and reenforced enough to withstand full short-circuit-current for 30 cycles without failing. 

You can often tell something is a switchboard because you can open covers and look from one end to the other, it doesn't have the partitions and re-enforcement of switchgear because it's only required to withstand a fault for (I think) 3 cycles.

This is also the reason switchboards can be protected by breakers without instantaneous (magnetic) trip, but switchboards cannot.

Panelboards are the wall-mounted guys and would fall under rules similar to switchboards.


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## carter840 (Jul 1, 2014)

Big John said:


> Switchgear and switchboards and panelboards are all different enclosures for overcurrent protection.
> 
> Each one has different testing and listing standards. For something to be switchgear it must be compartmentalized and reenforced enough to withstand full short-circuit-current for 30 cycles without failing.
> 
> ...



Great info could you clarify that last part a bit? You compared switchboards to switchboards? I'm guessing it is switchgear (not switchboards) that do not need to be connected with instantaneous trip. Can you confirm?
Thank you


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## Big John (May 23, 2010)

See, this is why it would've been nice if the post "Edit" feature hadn't been limited for absolutely no good reason.

Yes, switch_gear_ does not need instantaneous protection because it will withstand a fault long enough for the OCPD to open on "short time." 

Switch_boards_ do need instantaneous protection because they only have that 3 cycle withstand time.


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## 480sparky (Sep 20, 2007)

Big John said:


> See, this is why it would've been nice if the post "Edit" feature hadn't been limited for absolutely no good reason...


The 'good reason' is to prevent posts from being changed [or even deleted] and making the responses to it non-sensical. If one needs to be edited, just PM a mod.


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## wildleg (Apr 12, 2009)

480sparky said:


> jibba jabba. jabba jibba jibba jibbity joojoo. jawanna ?.


fixed it for ya.

I like nonsense ! I love the old threads where the posts were deleted . . . they are hilarious !


rant over


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## carter840 (Jul 1, 2014)

So what about circuit breakers? Is that just an improper name for distribution panels or is there a difference (again likely in code requirements) between a common household circuit breaker and a distribution panel? I would think that a distribution panel is filled with circuit breakers, but there also seems to be a big difference in the size of distribution panels and the common "circuit breaker" boxes so I'm guessing there is a difference in build spec based on higher load values. 

From my limited knowledge distribution panels exist more in commercial type settings, typically one in each equipment room and maybe one on each floor. The commonly called circuit breaker boxes are more often found in an apartment (maybe after a larger distribution panel).


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