Small (less than 150kW) EES systems

[mshields]

I’ve been asked to put a Type 1 EES (so with LS, Crit and Eq branches) for a very old facility serving as a dormitory for extremely disabled children some apparently dependent on power to stay alive. I don’t know the square footage of the building but less than 20K and it’s already been determined that all we need is an 80kW, 208Y.120V generator. I’m new to the job and so am lacking some of the history but I’m being told that a key person with the facility is insisting that we need three ATS’s. Obviously given that the EES system is comprised of a only 80kW genset, means that we ought to be able to do it with a single ATS. I’m just trying to anticipate what this person’s argument might be for insisting on 3. I’m particularly interested given how extremely tight space is in this building. Am I missing something?

 

[ron]

Since 517.31(B) allows one transfer switch if the ESS is large enough to handle the entire demand, is it possible he doesn't think the 80kW gen can handle the demand all at once and needs to be stepped on, such as with multiple transfer switches?

 

[mbrooke]

I'd insist on two ATSs. With one critical branch you also need normal power at the patient beds.

In your case you can legally get away with one ATS provided you split the branches up after.

 

[mshields]

You'd have to see this place to know why I'm willing to take full advantage of the 1 ATS rule; severe limitations on space PLUS I think it very questionable that they even need a Type 1 EES. This place just doesn't look like a hospital. I've yet to see where they require critical power. Thanks for the input and Ron - I don't think that's an issue but I'm still getting familiar. I'll be sure to look into that.

 

[mshields]

Relatedly, assuming I go with the single ATS solution, I want to do so without a breaker at the generator (i.e. making sure conductors are 115% rated. After the ATS, I'll go to a wireway where I'll tap off to the three branches. Would those be legitimate taps OR would that make them a tap of a tap and therefore not permissible. Or worse still, would this constitute a tap at the generator, a tap at the ATS and then a tap at the wireway and therefore definitely multiple taps of a tap and very much impermissible?

 

[mbrooke]

Why are you skimping on a breaker on the generator?

 

[mshields]

Why are you skimping on a breaker on the generator?

For selectivity - eliminating it eliminates the possibility of a fault on one branch shutting down the entire system.

 

[mbrooke]

For selectivity - eliminating it eliminates the possibility of a fault on one branch shutting down the entire system.

Not if you have it properly coordinated.

 

[Russs57]

I could only tell you what my argument would be in requesting three ATS's.

In a nutshell, redundancy. It offers me peace of mind and makes maintenance and servicing easier to accomplish.

Besides, if I want to spend more money on something better why would an engineer fight that notion? I ask because they seem to do just that.