size of neutral wire

[kimrichi]

when the neutral wire carry no current like in 4-wire multi-wire with equal load
what size we install?

 

[kbsparky]

At least as large as the required EGC.

 

[iMuse97]

when the neutral wire carry no current like in 4-wire multi-wire with equal load
what size we install?

Depends. What are you supplying? Voltage? Phases? Delta or Wye (Star)? Is it a motor, at 480V, then why would you need a neutral?

 

[infinity]

when the neutral wire carry no current like in 4-wire multi-wire with equal load
what size we install?

For a MWBC it would need to be the same size as the ungrounded conductors.

 

[Smart $]

when the neutral wire carry no current like in 4-wire multi-wire with equal load
what size we install?

As Rob said...

You have to consider the neutral current under all operating conditions. Say you had 16A lighting load on each line. Switch all but one line off. The neutral current would now be 16A.