how can we calculate starting current of 20kw delta connection motor?

Why don't you look up the locked rotor current in the manufacturers data sheet. Every different brand is different.

most motors run with starting current ranging from 200-700% of full load... there's a table for the locked rotor current, just check for it like what Mr. Reg stressed... this can be lessen depending on the motor starter...

For simple use :

In = P/(1.73×Volt×efficiency×cosQ)

Result for In × 4-7 that is result for your question.

When electric motors start with delta connection (Direct On Line), then the maximum current occurs on a locked motor current, then it could reach 7 time the normal current.

In addition also To Clarify, please be advised that starting current should never be assumed. the "Code Letter not the FRAME TYPE found on the spec plate of all motors represents the valve of the starting current. starting current could be as high as 30 x the running current.

See page 605 NEC; Table 430.7(B) Locked-Rotor Indicating Code Letters.