can't find in nec , service entrance conductors shall be no longer than 6 ft ,in basement ,before entering service equipment

You won't find that because the code does not specify a maximum length of service conductors within a building. Look at 230.70(A)(1).
Don

I think that is a local law .The ahj in your area has probably adopted the"six-foot" ruleas a means to set the standard.The nec says that the main ocpd should be as "close as possible to the meter."you may find that in certain applications the inspector may allow a length longer than six feet.

THANKS FOR THE QUICK ANSWER ,HAVE A SAFE AND HAPPY HOLIDAY .

the purpose for the ahj to have a limit for se conductors is "overcurrent protection" we have a five ft rule in my area. if no rule was in place some electricians would have a hundred feet or more of un-protected se condutors without any type ocpd see section #230.70 2002 nec, also see sec #230.6, conductors considered outside the building.