3 phase generator sizing

[hounddog05]

Have a project to back up a 100A, 3-phase 120-208 panel for a data room. not sure what size kva generator to have. as far as load I have 45A on A phase, 36A on B phase and 15A on C phase. do I just add the total amperage for all 3 phases and multiply by 120 to find the kva? Was thinking of using a portable 3 phase generator.

 

[kingpb]

Generators are not rated based on Amps, they are rated based on KW and power factor.

To serve the basic load, you would take your largest amps measured, 45A, and multiply by 120V = 5.4KVA, then multiply 5.4KVA x 3 = 16.2 KVA; now step up to the next standard size. This would be the smallest conceivable generator to use.

However, what has not been considered is the power factor of the loads, and what order in which they come on-line in the event of a normal source failure. If it comes on all at once than that's what the generator has to be able to handle without going into load rejection.

 

[Phil Corso]

HoundDog,

Before you consider using a Genset, you'd better work on equalizing the load! The current-values cited yield an Ampere imbalance which could have a deleterious impact on Genset performance!

Regards, Phil Corso

 

[under8ed]

I believe the project warrants further investigation. Your requester may not have all the information to provide the most practical solution. The data room may not even require 3ph., it may have been installed because it was available, or the room was placed where it had existed. Or, it may be that the critical loads are only single phase, either way, a 1ph. sub panel with 1ph. generator back up may offer some cost savings.
If 3ph. is required, the panel may be feed from a dedicated transformer with a 480v. primary; this emergency generator set up is common & therefore may be cost effective. This could also provide an alternate, or possibly better location for a generator.
Many rooms used for data systems only require emergency power to provide an organized shutdown; in these cases, a battery UPS often provides the most cost effective solution.