Sizing Emergency Generators for Hospitals

[tennee2007]

I have a question regarding sizing a standby, diesel generator for a hospital.

Typically, I total up the total KVA load connected to the generator, add 20% to the load (assuming no future capacity needed) and size the generator to the next larger KVA size. For example, if the total load on the generator is 468 KVA, add 20% is 561.6 KVA, so I might pick a 500 KW/625 KVA generator. The voltage is 480/277 volt, 3 phase 4 wire.

Now, I KNOW that the actual load the generator will "see" is less than the 468 KVa, because of diversity, motors not running fully loaded, receptacles not being used, etc.

So, here is my question: Assuming the 468 KVa load is 150 KVa of motors, 100 KVa of lighting and the balance (218 KVa) is receptacles, misc loads, etc. how would you aize the generator? (Assume we are not in California, where you have to size to connected load).

Thx.

 

[itsmoked]

It can depend on a lot of things. The most important would be the chiller. If the hosp has a large A/C motor, that would be something that needs to be included in this discussion. You could load the gen with everything else and then have the chiller start-up. It would also depend on if it has a soft-starter or reduced voltage start of some kind. DO you know if a big motor is in the picture?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[dpc]

First, you need to tell us if this an Emergency (Life Safety) system or a Standby generator - these are not the same thing in the NEC and the requirements may be different.

 

[davidbeach]

I, for one, wouldn't try to nail it down any further than your initial cut. I might also go a size larger if that didn't push me up a block size. While you don't want a grossly over sized generator because you want the prime mover to get loaded up, you get much better response from a machine that isn't running close to its limits.

 

[subodhbhatnagar]

Your selection is very near to right selection subject to conditions that all motors do not need be started simulteneouly and maximum rating of individual motor is less than 100kVa.It is ensure that starting current does not exceed rated current of selected generator current rating. also ensure to segregate the single phase loads on on all the three phases

 

[tennee2007]

To add more info. No chiller on the project. It is a NEC 517 standby generator with the 3 branches (Life Safety, Critical and Equipment). Diesel powered with 3 Automatic Transfer Switches, one for each branch.

The largest load will be the DX cooling on 2 roof top units. There are 4 RTUs total, all with DX cooling. All RTUs will be on the generator, but only the cooling from 2 (that serve patient rooms) will be on the generator.

The largest motor HP will be approximately 30 HP.

Thanks for the feed back thus far.

 

[itsmoked]

30HP = 23kW

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[catserveng]

I'm from California, and I know it get's done differently here, but isn't NFPA 99 guidelines what you are supposed to use for sizing gensets in health care facilities as a general rule?

 

[tennee2007]

Yes, NFPA 99 is the guideline for generators and elec systems for hospitals. Also, NEC article 517, NFPA 110 and (to a lesser extent) NFPA 101 (Life safety code).

But, none of these dictate exactly how you have to size it. For example, NFPA 99, 2005 ed, chapter 4 (elec systems), 4.4.1.1.9 says "The generator set(s) shall have sufficient capacity and peoper rating to meet the maximum actual demand likely to be produced by the connected load of the essential electrical system(s) at any one time".

I was just curious as to how other EEs design/size the gensets they specify for hospitals.

thx.

 

[davidbeach]

I was just curious as to how other EEs design/size the gensets they specify for hospitals.

Generally with plenty of margin. The liability of under sizing the generator vastly out weighs the added cost of being a size or two too large.

 

[tennee2007]

David,

I hear ya! Thx.