Two emergency generators, one facility, not paralleled

[paulternstrom]

I am working on engineering an upgrade to an existing skilled nursing facility. They have an existing 30KVA generator to handle the existing Life Safety and Critical Branch requirements, but it has no additional capacity to handle their new requirements. They would like to add a separate, second generator to handle additional Critical Branch load.

It seems to me that the old and new generators should be paralleled together to still maintain one "essential electrical system" for the facility, but I am struggling to back that up that per NEC requirements. Has anyone seen this arrangement, or have any thoughts, pro or con, to having two separate emergency systems at one facility? Can anyone point to a code section that says it must be one system, or that says two systems are allowable?

 

[charlie b]

I don't have my code book handy, but my suspicion is that the code will allow it. And I see no reason that it should not allow it. But it might be one of those things that, since the code does not prohibit it, we can infer that it is OK.

 

[Dennis Alwon]

Would art. 225.30 come into play or perhaps the exceptions in (A)

 

[Dennis Alwon]

Here is the article

225.30 Number of Supplies.
Where more than one building or other structure is on the same property and under single management, each additional building or other structure that is served by a branch circuit or feeder on the load side of the service disconnecting means shall be supplied by only one feeder or branch circuit unless permitted in 225.30(A) through (E). For the purpose of this section, a multiwire branch circuit shall be considered a single circuit.
(A) Special Conditions. Additional feeders or branch circuits shall be permitted to supply the following:
(1) Fire pumps
(2) Emergency systems
(3) Legally required standby systems
(4) Optional standby systems
(5) Parallel power production systems
(6) Systems designed for connection to multiple sources of supply for the purpose of enhanced reliability

 

[iwire]

Where are the generators located?

 

[paulternstrom]

I guess my biggest concern is the reliability of the overall system if there are two separate systems vs. one integrated (parallel) system. I may be reading too much into NEC 517, but I think it says that when the gensets fire up, if there is any sort of problem, you should have the capability to shed load down as needed (meaning that the Life Safety system is the last thing to go) so everyone can at least "safely" exit the building. If you have two separate systems, you could have the new Critical Branch genset still providing power, but the original genset struggling and/or failing, and people could suddenly find out they are trapped in the building with no Life Safety system to guide them out. If the systems were paralleled, they would "cover" for each other, and the system would be MORE reliable.

iwire, what are you thinking about with your question regarding the location of the generators? They are outside the building, in the maintenance area out back.

 

[ncwirenut]

In NC if we are adding another service they allow us to have another generator, but if it is the same service they frown on it. NFPA 99 stresses reliability but if you configure it correctly, I don't think a second generator violates the NEC. 517.44 actually says that the alternate power source can be a generator(S).

 

[hillbilly1]

The problem with paralleling, is if one fails the other would not be able to sustain the load. Since all of the loads are life safety, you cannot shed any of them.