Neutral Cable Size

You need this:

IS SP 30: National Electrical Code : Bureau of Indian Standards (BIS ...

Depends upon the system in use.

For example, in a 3 wire 120/240V residential service in Canada (and I am pretty sure in the US as well) the neutral needs only be sized to handle the maximum line to neutral load. If you have a 200 amp service and only 50 amps of line to neutral load on one line, then the neutral need only be sized to handle the 50 amps as it will never see more than that.

Remember that... neutral current is off set. If you see 50 amps on L1 to the neutral and 30 amps on L2 to the neutral, the neutral only sees 20 amps.

If 50A on L1 to neutral, 30A on L2 to neutral and 60A on L3 to neutral for a 4 wire 415V system, what would be the neutral current ? In designing a system, is there way to size the neutral cable without actual measuring on site ?

No!

The neutral wire gauge size is established by the local wiring code. It does not matter what load is presently on the three phases. This load will change over time. The neutral wire is part of the location infrastructure and must meet these local codes. Now if and only if the neutral wire in question is part of the short line cord going to this load and it was impossible for a failure mode of the load to exceed the stated currents then one could make an argument for under sizing a conductor from code.

The local code is not based on normal operating conditions, it is based on failure mode conditions that history has presented. The code prevents a failure from becoming a bigger disaster.

So why do you presume it is not ok for him to do what he has asked about? Do you know what code he operates under?

Code takes into consideration the design of the system at installation and assumes that if an when the load changes, the changes will force changes to the system. Codes are minimum based. Otherwise... we would be forcing people to install systems that far out weigh their needs.

Besides... I recommended to him that he go with a full size neutral and perhaps, may want to upsize based on the nature of the load.

I have absolutely no idea which local code the OP should be following and neither do you. So just because he might happen to be following code if he's lucky, should we encourage him to not bother knowing his local code? No. CR4 is not the relevant authority here and we should not pretend to be either.

If our OP identified the code that they thought was ambivalent for their application then we can advise.

You are correct that code is a minimum standard and not an optimum guide. You are also correct that it is usually best to size the neutral wire the same gauge and insulation as the phase wires. However, without knowing the relevant code and installation conditions then the correct wire cannot be known.

To your point... Yes... I agree he needs to undertand the rules around neutral sizing in his particular wiring regulations.

But... as to his 2nd question... I think you are missing the point. The OP asked about the "concept" behind downsizing neutrals, which is what I replied to.

I do not see any comments presented in this thread that appear authoritative in nature.

<...there way to size the neutral cable ...>

Every qualified Electrician in these islands uses British Standard 7671.

If the cable were to be sized on site, then every installation would be wrong. So it's best to do it beforehand.

What would be the direction flow of a neutral current ?

Downhill, half the time.

LOL...

Oh, it's simple. It depends upon the instantaneous loads on the phase currents, none of which can be seen from here, and all of which vary with time.

For normal resistive loads

In = √((Ia²+Ib²+Ic²)-((Ia*Ib)+(Ia*Ic)+(Ib*Ic)))

Once you start to introduce harmonics, screw that up and go for a full size neutral.

This is the correct method to calculate how much current will reside on the neutral for a particular load. Please see my previous comment.

Thanks all.

If the designer didn't know the nature of loads to be connected to the wiring, use a datalogger & record harmonics & neutral current and use an appropriate size of neutral conductor which may sometimes be as big as twice the phase conductor.