unormal voltage drop!

I think it's quite normal.

At the start, the motor can take up to 6 times the In.

So In=6x450=2700 kW

You have 2000*cos φ (usually around 0.8) = 1600 kW available.

You can see why you have a voltage drop of 40 V.

Nothing to worry about.

Now, if you have a voltage drop of 40 V during established functioning, you can worry about.

but we have soft starter! we have 40 v drop till our MDB and another 40v drop till our motor terminal .40+40 for a system with a soft starter is very dangerous !

You didn't say if you are:

- Measuring the voltage drop during the start up stage or during the established functioning.

- Measuring with a RMS device.


A 40 V voltage drop is not usually dangerous for a soft starter, a 80 V drop is.

For power efficiency, the NEC standard of 5% maximum voltage drop is recommended.
From a safety perspective, because wiring connections in some homes deteriorate with time (particularly in homes that use aluminum wiring for power circuits), and do-it-yourself modifications may be less than professional, excess voltage drop is a concern because of the potential fire hazard at high resistance connections, particularly on circuits that power electric motors while occupants of the dwelling are asleep, e.g. air conditioners, refrigerators, furnace fans, exhaust fans, etc.
Some agencies arbitrarily set a maximum voltage drop criteria of 10% to be considered unacceptable and a hazard. The author believes that any voltage drop difference of >1% from an adjacent receptacle should be investigated, that any voltage drop difference of >2% from an adjacent receptacle should be considered a hazard, and that using a maximum voltage drop criteria of more than 8% (3% above the “efficiency” recommendation) is courting disaster. A 3% voltage drop (3.6 volts in a 120 volt circuit) at one connection with a 15 ampere flow develops 54 watts of heat - which can cause ignition under certain conditions.