I am helping out with the construction of a 28 room/unit rental building. It will have 3-phase supply at the main breaker from the electric company distributing to safety-cuts at each unit. Are there folks on this board who could answer a couple of questions on the premises distribution network?

You can send me a pm with a few details and I'll see if I can help you out.

Cheers Jorgo

The business is a room-for-rent type; rent payable by the month. Tenants are responsible for paying for their utility usage.
The safety-cuts are rated at 45A: 3 breakers: A/C 16A, Lighting, outlets, etc. 20A, Water Heater 32A.
There will be 28 of them. I have been told that we may need a separate transformer from the electric company. I hope not, as that would be expensive.
The safety-cuts are made by:
C.S. International Electronics Co.
http://www.safe-t-cut.com/en/products/c ... -unit.html

The only reason I can see for needing a transformer would be for a neutral connection which you would take off the center tap point. The Electricity company should give you a 3 phase supply & neutral.

I think Ray is talking about a transformer located on the utility pole outside. The supply grid from the electric company may not be sufficient to handle the load without the voltage dipping down low especially at night and during heavy usage. Three phase supply will provide 2 positve and 1 neutral but that may may not be enough hence the suggestion to have a transformer to boost the power installed. I would think a good electrician (hard to find) working in conjunction with the electric company would be able to determine what's needed. Additionally, a 3 phase power electrical meter comes in a variety of capacities so he will need to determine what size (capacity) to purchase.

A 3 phase supply is exactly what is says. You get 220V between each phase & neutral & 440V between 2 phases so basically you have 3 separate supplies at 220V. The transformer could be to get 220V between 2 phases if a neutral is not supplied. If you look at any overhead electric lines along the street you will see 3 insulated wires. These are the phases & each house comes off one phase. The neutral is the top bare wire which is basically the earth connection. I agree that he will have to check his loads if there is going to be air con in each room as he may need his own transformer like you said. Which is very expensive.

Pal

You are correct about the number of "hot" leads. My apologies for the mistatement.

An update: the electrician said nothing about a transformer. When my lady went to the electric company office by herself, the counter person mentioned the possible need of a transformer. I expect more posts from me.

Much depends on your start up wattage. It's my understanding that you will need more power available than the total of all your appliances starting at the same time. If you have an A/C unit in each room and a hot water heater and possibly a small frisge you will need to add that all up and then add in the lights. There is a formula for determing your minimum power availability based on all factors. Additionally, some A/C units may require less start up power than others. ie. i believe an A/C inverter unit will require less start up power than a standard unit. I believe Daikin inverters should use substantially less power than a non inverter unit. This may allow you to down size the amount of supply coming in to the apartment. Maybe pal can explain this further.