" I know for fact, we use I=kva/1.73 x V."

You had better check your facts, or your typing.

I'm confused. Is it "Electrical gernrator cable" or "cable size of transformer"?

What you seem to be asking about is the relationship between line to line and line to neutral. In single phase, it is 1/2, or 120/240. In 3 phase, 1/1.73 or 1/√3 or 120/208 or 277/480 etc.

You do not have to size a load side wire to match the total capacity of a transformer, the main breaker protects the wire. A transformer often supplies more than one house, or several 200 amp services.

Hi,
the fact was for 3phase system.

Lets assume there is a transformer supplying a single house with 120/240 v.

What voltage should i consider when i want to run cable from transformer to the house?

i= kva/120? or i=kva/240

thank you

What cable are you talking about. If you are talking the service drop, you size it based on what the power company tells you that they want typically. If you're talking about from your generator to the panel, then it's 140 KVA/240 if it's a 240V generator. 140 KVA is huge, that's a 580A service. I've got an 80KVA generator which is overkill (that will run the entire 400A service). You're talking serious wire sizes once you're on your side of the disconnect (750 kcmil range).

I know you were trying to write a formula for 3 phase, but what you wrote was wrong. "we use I=kva/1.73 x V" would mean that on a calculator you would punch buttons for; kVA ÷ 1.73 × V = ? =(V²A÷1.73) What you probably meant to write is I=kVA/(1.73×V) which on a calculator you would punch kVA ÷ 1.73 ÷ V = A or I

This would give you a line to neutral load, without figuring in other factors such as the length of the cable, duty rating of the transformer, power factors, how many loads are supplied by the transformer,and at what rate you want the power company to charge you for the electricity, etc. If you are really talking about a transformer, then you have not given enough information, which is why the power company determines this.

If you use I = kVA / (1.732 × V) for a 120/208 3 Phase transformer, which do you use, 120V or 208V ?

You use the line to line voltage.

1.732 is the adjustment for the phase angle (voltage difference), not the amperage going through the wires

With single phase, the equation can be done with 120V or 240V, but it is a slightly different equation since the VA is for the TOTAL capacity of the transformer.

I = kVA ÷ V where volts is measured line to line @240V

Or

I = (kVA ÷ V) ÷ 2 where volts are measured line to neutral @120V and remembering to take into account both sides of the center neutral tap.

In other words, it is customary to use the line to line voltage on both the 3 phase and single phase transformers, and the equation;

I = kVA ÷ V and then ÷ 1.732 if it is 3 phase.