In a three phase circuit theoritically there will be Zero
current in Neutral. But due to un-balancing of phases
practically phase voltage becomes unequal with the result
some current flows in neutral.That is why neutral is of
half rating only.
Any switch mode power supply (computer, UPS, AC/DC
transformer) has some leakage current and imbalance. In
data centres they are starting to use full size neutral
cables to account for this. Basically only need 1/2 size in
motors, saves money on expensive copper
Generally it depend on load, If cable is used for inductive
load e.g.motor, if there is any fault then 1/2 core is
sufficient to carry fault current due to lagging power
factor. However, in resistive load we are using 4 core
cable since there is no power factor.
Neutral conductor must be 1/2 the size of other
conductor.It is theoraticaly expalin that algebric sum of
all the phases when in balannce condition then neutral
current will be zero.
Now suppose one phase fail i.e total unbalace condition its
neutral conductor will provide a return path and its
current will be half the load on other phases.so by this
result on neutral conductor for every unbalance condition
its current will not exceed the rated than other phases.
Normally Neutral conductor is 1/2 the size of other
conductor.It is theoraticaly expalin that In a three phase
circuit ( Equal load in all three phasse) there will be
Zero current in Neutral, Ex. 3phase indusction load (Motor)
(NO NEUTRAL CONNECTED T OANY MOTORS AS THE RESISTANCE OF
THE THREE WINDINGS ARE EQAL).
But in a three phase to single phase distribution
(Resistive loads) it is not possible to maintain the equal
load in all three phases hence the unbalance current has to
be taken away from the circuit for the healthy opertion of
Thearitically and also practically it is prooved that the
flow of unbalance current in the 3 phase resistive load
distribution is always less than 1/2 of the phase current.
So 1/2 caore has been taken as a neutral
The choice of whether to use a neutral of equal size to the
phase conductors depends on your design criteria and load
equipment. The difference of the load being delta, floating
star or star with a neutral point
A 3 phase circuit in balance will produce little or no
neutral current contribution.
However a 3 phase circuit imbalance will produce neutral
currents as per the following formula
A^2 + B^2 + C^2 - AB - AC - BC).
A three phase circuit wired in star with the neutral at the
star point will operate with null balance if the loads are
equal, it is possible (and I have seen this) for 2 phases
to fail and leave one phase connected to the load and
neutral. The neutral current becomes equal to the phase
current, the designer of the circuit failed to account for
this, the undersized 1/2 neutral overheated and caught fire
destroying the switchgear and damaging the furnace.
I would urge anybody to carfully consider all possible
outcomes when choosing neutral cable sizes as flames coming
out of a switchboard often alarms the unwary.