I have a situation where a customer wants to introduce/hookup an single phase 3 wire 18 kVA UPS to a 3 phase 4 wire panel and the UPS will be used to power a single phase 3 wire panel. The only thing the 3 phase panel will be used for is to power the single phase UPS. The entire building is 3 phase. The UPS single phase panel will be loaded to about 11 kVA to power sensitive computerized equipment. What problems will be experienced with this type of scenerio? Phase surges, spikes or suges? Neutral over loading problems? The UPS coming on prematurely?

18 kVA UPS is small, will not cause too much imbalance, esp if any of the equipment is 208-240 volts.

The neutral in the 3 ph panel should see no additional load, the feed to the UPS should be straight 208 volt.

Anything 20 or over should really be 3 phase thou.


The UPS output is now considered as a Separately Derived System, and neutral of the UPS output must be bonded to the equipment ground, and connect to grounding electrode, such as building steel, city water at the point of entrance, or the system ground for the building.

If the equipment is all in one room or closet, this is now considered as an IT room, and there must be a Emergency Power Off button at the door to kill the UPS system in the event of fire.

Check with local codes for fire suppression and detection system requirements.