Generator for the whole building, sized for service or load??

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I have a customer that wants everything covered by a generator. This is a medical office building with no existing generator. The service is a 120/208V, 800A, 3 ph, 4 wire service. My question is do I have to size the generator for the entire 800A capacity or can I size it per the know load (several years of electric bills)? If the know load is 200A and I size my generator at 85kw (300A) is this a code violation because it isn't sized for the 800A service? Also, because this is a MOB, if I put in a generator am I opening the door to issues of "Emergency" generator verses "Standby" generator?
 

charlie b

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If you are concerned about having to deal with article 700, then I suggest asking how the building is dealing with that article now. I suspect that the egress lighting is handled by batteries, and that there is nothing in the building that would bring the article 517 rules for the "essential electrical system" into play. If that is true, then the new generator is strictly an optional standby system. You can size that for whatever the owner is willing to pay for.
 

qcroanoke

Sometimes I don't know if I'm the boxer or the bag
Location
Roanoke, VA.
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Sorta retired........
I have a customer that wants everything covered by a generator. This is a medical office building with no existing generator. The service is a 120/208V, 800A, 3 ph, 4 wire service. My question is do I have to size the generator for the entire 800A capacity or can I size it per the know load (several years of electric bills)? If the know load is 200A and I size my generator at 85kw (300A) is this a code violation because it isn't sized for the 800A service? Also, because this is a MOB, if I put in a generator am I opening the door to issues of "Emergency" generator verses "Standby" generator?

How old is this building? If it is fairly new and doesn't already have a "legally required emergency system"
I doubt it would become one from just putting in an "optional standby system"
There is a huge difference in the requirements between the 2.
Optional standby allows you to size to the load.
 
Standby Generator

Standby Generator

The building isn't too old, say less than 15 years. The title of the building is "Wound Care" hyperbolic", and I there is a dentist office in one of the suites. The owner just doesn't want to have to separate loads and worry about running cords around if the power goes out. If this is truly a "Standby" generator, is there anything in the code that requires sizing for the full service load or changing the main disconnect down to the size of the generator feed? An electrical engineer is stating I need to change the 800 amp main to the 300 amp capacity of the generator and that I need to consider section 517? I appreciate any help anyone could offer.
 

GoldDigger

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If the generator is not required, then the most significant restriction is that IF you use an ATS either the generator must be able to supply the full calculated load or there must be automatic load sharing to protect the generator.
 

qcroanoke

Sometimes I don't know if I'm the boxer or the bag
Location
Roanoke, VA.
Occupation
Sorta retired........
If the generator is not required, then the most significant restriction is that IF you use an ATS either the generator must be able to supply the full calculated load or there must be automatic load sharing to protect the generator.

Agreed!
I don't have my code book with me but I think the reference is in optional standby systems.
Article 705?????
Keeping in mind the calculated load is usually nowhere near the service size. Also load shedding will greatly reduce it too.
 

qcroanoke

Sometimes I don't know if I'm the boxer or the bag
Location
Roanoke, VA.
Occupation
Sorta retired........
? An electrical engineer is stating I need to change the 800 amp main to the 300 amp capacity of the generator and that I need to consider section 517?
He's wrong. How would doing that possibly make it good?
I don't know about 517.
 

kwired

Electron manager
Location
NE Nebraska
You need to determine if a legally required standby system is required for this facility. Keep in mind once the proper authority finds out - if one is required the owner ultimately will be putting one in or will be closing the doors some time soon. This proper authority may be health care codes enforcement more so then electrical codes enforcement. Electrical inspectors will go by whether or not someone else requires the standby system to determine which portions of NEC apply.

If the building is fairly new and conditions of operations have not changed the chances are good that a legally required system would have been put in at initial construction if one was required.
 

Tony S

Senior Member
Most are OK with 110/115% for limited periods, it’s one to check with the manufacturers. CAT have a good site for information.

I learnt about diesels coking up the hard way. The one time the 1 MVA emergency set was needed in years it started first time. Once load hit it, it wasn’t happy and it stopped. I hadn’t been with the company long and asked when it was last load tested? I was met by a wall of blank faces.
A beautiful Rolls Royce V12 ruined due to the fitters running it unloaded for five minutes a week.
 

GoldDigger

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Location
Placerville, CA, USA
Occupation
Retired PV System Designer
You can size the generator for the load, but, if it is not equivalent to the service you cannot use an ATS, only an MTS.

I disagree:

From 702.5 (702= Optional Standby Systems, which we are discussing)

(2) Automatic Transfer Equipment. Where automatic transfer equipment is used, an optional standby system shall comply with (2)(a) or (2)(b).
(a) Full Load. The standby source shall be capable of supplying the full load that is transferred by the automatic transfer equipment.
IMHO that refers to the calculated load (not the connected load), and not the service size.
A small point but an important one!
 
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