Generator Sizing Per 220.87

[elecshop]

I have an existing building with peak recorded demand at the utility meter 700kW in June 2022. 400kW of load will be added to the facility including heat pumps, boilers, new medical equipment, etc. (this is a hospital in WI).

1. Per NEC 220.87 (2), to size a generator that backs up the entire facility, should I calculate the existing demand x1.25 then add the new 700kW, or multiply the sum (400+700)x1.25? It would differ by a 100kW.
2. The facility has 2 existing parallel generators, rated at 500kW each, which back up all electrical loads except for a normal distribution panel and a 1000A chiller panel that powers 3 chillers, each rated at 350kW and has a 500A breaker in the chiller panel. If I were to size the generator such that one chiller is supported, I am assuming that half of the 700kW is actually cooling load (reported in June), so the three chillers are functioning at 1/3 capacity, but to backup one of them I would assume full capacity of one chiller?

 

[JoeStillman]

If you don't have electric heat, the difference between your winter peak and summer peak will be the cooling load. Remember that chillers have significant peripheral loads like cooling towers, chilled water pumps and condenser water pumps. Just backing up a chiller isn't enough.

 

[elecshop]

If you don't have electric heat, the difference between your winter peak and summer peak will be the cooling load. Remember that chillers have significant peripheral loads like cooling towers, chilled water pumps and condenser water pumps. Just backing up a chiller isn't enough.

The winter peak is 400kW, but the nameplate rating of one chiller is 350kW. So even in winter I think the chiller is partially operational or in "standby" mode thereby consuming some power, do you agree? Note that the associated equipment are backup up by the generator including chilled water pumps are connected to the generator through a separate panel.

Heating was non electric when the utility reported 700kW peak demand, but a new electric heating plant will be installed, which should also be backed up by the generator.
Final total electric additions 400kW, do I add the 400 to the 700 then account for 20% capacity? Or 20% capacity with the 700 and add to that 400? Doesn't look like the NEC specifies that in 517.31(D).

Is anyone aware of a code requirement for chillers to be on generator power in healthcare facilities?

 

[JoeStillman]

Are you replacing the 2x500kW plant or adding to it?

If there is a requirement for cooling on emergency, it would be in NFPA 99. I think cooling is essential in an operating room. They need to keep the patient's body temperature way down for some procedures.

 

[elecshop]

Are you replacing the 2x500kW plant or adding to it?

If there is a requirement for cooling on emergency, it would be in NFPA 99. I think cooling is essential in an operating room. They need to keep the patient's body temperature way down for some procedures.

2x500kW are old gensets (30 years old), although functional, installed in a generator room that cannot fit larger units. My plan is to install new larger enclosed gensets outside (maybe 2x700kW) with new accessories and a controller.

If I reuse existing units, I am not sure their piping system is code compliant - they both take fuel from a 100gal day tank through one pump, and the day tank has supply and return fuel lines to an outdoor main fuel source tank that also provides fuel to mechanical equipment. Do you know of a code section that does not allow this configuration?

Another question, if three chillers operating at 1/3 capacity consume as one chiller ~350kW, to bring one chiller on an ATS to a generator I shall assume this one chiller consumes 350kW, right?

 

[JoeStillman]

2x500kW are old gensets (30 years old), although functional, installed in a generator room that cannot fit larger units. My plan is to install new larger enclosed gensets outside (maybe 2x700kW) with new accessories and a controller.

If I reuse existing units, I am not sure their piping system is code compliant - they both take fuel from a 100gal day tank through one pump, and the day tank has supply and return fuel lines to an outdoor main fuel source tank that also provides fuel to mechanical equipment. Do you know of a code section that does not allow this configuration?

Another question, if three chillers operating at 1/3 capacity consume as one chiller ~350kW, to bring one chiller on an ATS to a generator I shall assume this one chiller consumes 350kW, right?

IMHO, the backup power systems for a hospital ought to be the same as a Tier IV data center - full, maintainable backup with N+1 redundancy. Why are we pursuing code-minimum where people's lives are on the line while the Second Life game servers on AWS are pursuing maximum reliability? (Is Second Life still a thing?) You should be able to fully support hospital operations while one of your generators is out of commission for maintenance.

I agree with your assessment of the existing generators - you have too many potential single-points-of-failure, and they are due for replacement. New engines will probably be unable to share the boiler fuel supply and still meet the emissions requirements.

You are probably right about the chillers. But don't just provide emergency backup for one of them. If you are using the utility peak demand as your base load, wouldn't that include all the chillers they need? I suggest you provide backup power to all of them and if the generator load is tight, use lockout controls that allow them to select which is the "emergency" chiller. Chillers are manually started anyway. It's probably a matter of writing some lines of code into the ATC system.