Transposition

I don't understand what you mean by "transposition" in this application. Are you talking about switching the alternator cable polarity?

By transposition i want to mean "cable crossing" i.e. each phase will take every other position after two crossing.

For example, say the original phase position on one structure is A-B-C. At the next structure the positions would be transposed to B-C-A.

Then I would have to say no. I cannot see how changing the phasing of the cables like this would ever be a good idea (or even legal) in this application.

Why do you think you need to, it will have no effect on the circuit breaker? Could you explain your reasoning behind why you thing you need to please.

I am working in a generator distributor company.Transposition in power cables is recomended by our principal.

I think you are referring to changing the physical position of the line cables at regular intervals along the whole length to get rid of mutual induction and EMI related problems. If I am right in my assumption of the question, then the matter would be solved by using sets of 3-phase cables which are already transpositioned anyway during manufacturing. For your amperage level,you want to avoid the issues with mutual induction on adjacent phases, so yes transpositioning is necessary.

I coulg be wrong in my assumptions of the question, though.

No. You do not need to transpose. The only reason to transpose between the generator leads and the main CB would be to match phase rotation; i.e. connect ABC generator to CBA system.

If the generator and system are both ABC, I certainly wouldn't want to be the one to try and transpose buswork over a distance of only 20meters. If you transpose once, you will have to do it atleast one more time to get rotation correct before connecting to the system. Of course, you could elect to then transpose again after the CB or at the transformer or whatever.

Transpositioning is not related to the actual connections but to the physical laying of cables. So ABC or ACB reconnection is not the question. Should there be a straight parallel run of phase cable from the generator to the breaker, or should there be a relative regular physical repositioning of the cable run between the generator and breaker? Isn't that transpositioning?

Apparently I did not make my answer clear enough.

There is no reason to try to balance mutual inductance by transposing. 20 meters is not enough distance for there to be any significant effects.

I mentioned phase rotation because it is the only reason I can think of that one would even consider transposing the generator bus.